JPH0441089A - Method for welding precoated steel plate - Google Patents

Abstract

PURPOSE:To improve the joint strength of the precoated steel plate by irradiating the tight contact part of the single Vee groove formed between the non- coating surface of the precoated steel plate bent to a U shape and the surface of the metal to be welded with a laser beam. CONSTITUTION:The end side is bent to the counter end side to the U shape in such a manner that the non-coating surface 4 is positioned on the outer side in welding of the steel plate 2 precoated on one surface with an insulatable high-polymer film 3. The point to be welded is then brought into tight contact with the point to be welded on the metallic surface 5 of the steel plate 1 to form the single Vee groove. The tight contact part in the deep part of this groove is irradiated with the laser beam 6 from the inlet side of the groove. The high-strength joining is executed in this way without removing the coating film, i.e., without giving thermal damages to the coating film near the weld zone.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for welding prepainted steel sheets used in automobiles, home appliances, etc.

[Prior art] Pre-coated steel sheets with a polymer film on the surface can omit complicated processing steps such as degreasing, chemical conversion, and painting, have excellent surface smoothness, have excellent coating film properties, and can be press-formed. For these reasons, demand has increased in recent years for use as exterior steel sheets in home appliances.

Since the coating film on the surface of this pre-coated steel sheet is insulating, it can be used for spot i-contact, which is normally used for joining thin steel sheets. Bonding is not possible. Therefore, when applying spot I welding or other electric resistance welding, it is necessary to remove the coating film at the welded location or to form protrusions on the mating steel plate.

In addition to electric resistance welding, mechanical fitting, adhesion, etc. may also be used.

[Problem to be solved by the invention]

However, with electric resistance welding such as spot welding, not only does it take extra time to remove the coating film, but also the amount of heat input is large, so the surrounding area of the weld is affected by heat, which can cause thermal damage to the coating film. However, the appearance of the product will be significantly impaired. If it is desired to make the thermally damaged part invisible, it must be folded back, which further increases the number of man-hours and limits the design.

In addition, mechanical fitting, adhesion, etc. cannot provide any joint strength with welding.

An object of the present invention is to provide a welding method by which prepainted steel plates can be easily joined with high strength without impairing the appearance.

[Means for Solving the Problems] Incidentally, in the field of welding thin steel plates, there has been an increasing trend in recent years to apply laser beams and electron beams. Laser beam welding has extremely little thermal effect on the surrounding area, and when applied to pre-coated steel sheets, the temperature rise of the coating film due to heat conduction can be suppressed to several tens of degrees at most. However, in resistance welding, the parts to be welded are overlapped and welded, so the part to be welded is not exposed to the outside, but in laser beam welding, the part to be welded is directly irradiated with a laser beam, so the part to be welded is exposed to the outside. Must be exposed. Furthermore, if the laser beam is directly irradiated onto the coating surface, the coating material will burn and gasify, making it impossible to obtain a sound weld.

In laser beam welding of pre-coated steel sheets, a practical welding method cannot be expected unless these two problems are solved.

Regarding the laser beam irradiation method for welding thin steel plates, for example, when stacked strips are continuously rolled down with rolls, the strips are opened in a V shape on the entrance side of the rolls, and the laser beam is irradiated deep inside the strips. A method has been developed for continuous welding of strips without penetration melting (Proceedings of LAMP).
'87.0saka (May, 1987) E.

If this method is applied to a single-sided pre-coated steel plate, it becomes possible to irradiate the welded part with the beam and prevent thermal damage to the coating film. However, since the method is only applicable to joining strips together, pre-painted steel sheets are mainly used and cannot be applied to the exterior assembly of home appliances. When assembling the exterior of a home appliance, it is necessary to weld pre-formed members easily and freely, such as through-hole welding.

In the present invention, by slightly processing the vicinity of the part to be welded of a single-sided pre-coated steel plate, the part to be welded can be directly irradiated with a laser beam, and the coating film can be prevented from being irradiated with the laser beam. This allows for welding with minimal mobility.

That is, the welding method of the present invention is a welding method for a pre-coated steel plate, in which the non-coated surface of a single-sided pre-coated steel plate coated with an insulating polymer film on one side is welded to a metal surface, and the welding method comprises: The part to be welded on the metal surface is brought into close contact with the part to be welded on the metal surface, and the pre-coated steel plate is bent into a substantially U-shape so that the non-coated surface is on the outside in the part that is in contact with the part that is in contact with the part to form a ■-shaped groove, The feature is that the laser beam is irradiated toward the close contact portion located deep inside the groove.

Figure 1 shows one implementation of the welding method of the present invention! j! This is a schematic diagram showing four types, and shows a case where a pre-coated steel plate 2 is welded to a thin steel plate l.

The pre-coated steel plate 2 is coated with an insulating polymer film 3 on one side, with the non-coated surface 4 not covered with the polymer film 3 facing outward, and the end side opposite to the welding area. It is bent in a U-shape toward the side. In this state, a ■-shaped groove is formed by bringing the welded area into close contact with the welded area on the metal surface 5 of the steel plate l, and the groove is opened toward the close contact area located at the deep end of the groove. A laser beam 6 is irradiated from the entry side.

At this time, it is necessary to avoid the situation where the laser beam 6 hits the bent part of the pre-painted steel plate 2 (see Figure 2).
. For this purpose, if necessary, for example, the material to be welded may be tilted relative to the laser beam axis, or an optical fiber may be used.

Since the ends of the pre-coated steel plate 2 are bent with the non-coated surface facing outward, the metal exposed surfaces are in close contact with each other, and the laser beam 6 reaches the close contact portion and melts the close contact portion. As a result, the parts to be welded can be joined by spot welding without removing the polymer film 3, and
It is possible to avoid thermal damage to the polymer film 3 in the vicinity of the welded area. Furthermore, even if the polymer film 3 is damaged by welding heat, the damaged part is covered by the bent part of the pre-coated E plate 2, so there is no risk that the appearance will be impaired.

Furthermore, since the laser beam is irradiated parallel to the contact surface, penetration welding will not occur.

Note that the welding method of the present invention can be applied in exactly the same way to joining pre-coated steel sheets.

〔Example〕

Embodiments of the present invention will be described below with reference to FIGS. 3 and 4.

Drill a round hole in the center of a disc-shaped pre-coated steel plate 2 made of JIS 5PCD steel plate coated with polyester paint on one side and coated with polyester paint on one side. It was folded outward around the circumference. The thickness of JIS G3193 hot-rolled steel sheet is 2.
The pre-painted steel plate 2 is stacked concentrically on a thin steel plate 1 which has a disc-like shape and has a round hole smaller than the round hole of the pre-painted steel plate 2 in the center, and the thin steel plate 1 is placed around the entire circumference near the bent part of the pre-painted steel plate 2. While in close contact with the metal surface 5 of 1, the adhered portion was irradiated with a laser beam from a direction of approximately 45°.

Laser beam irradiation is performed using a high-speed axial flow type 1. Irradiation was performed for 1 second at an output of 500 W using a kW carbon dioxide laser oscillator and a GaAs lens with a focal length of 126 mm.

When the workpiece was rotated and welded at 16 locations in the circumferential direction, no discoloration of the coating film was observed. In addition, after joining, the pre-coated steel sheet 2 and the thin steel sheet 1 were each joined to a round bar with adhesive, and the peel strength was measured using an Instron type tensile tester, and it was confirmed that the joint had a sufficient joint strength of 120 kgf. Ta.

For comparison, when the same material was irradiated with a laser beam from directly above, as shown in Figure 2, the laser beam did not bend to the welded parts that were in close contact with each other, and the peeling strength was extremely high at 3 kgf. It was small.

Furthermore, when the same material was joined by projection welding, the base material fractured in a peel test, but the appearance was significantly damaged due to burning of the coating film.

[Effects of the Invention] As is clear from the above explanation, the welding method of the present invention has the following effects:
Can join pre-painted steel plates with high strength.

Moreover, in the joining process, there is no need to remove the coating film, and there is no risk of thermal damage to the coating film in the vicinity of the welded portion. Furthermore, in the welding, the mobility of the welded part (7) is exhibited. Therefore, even when applied to the exterior assembly of home appliances, the number of steps is small, excellent assembly strength can be obtained, and there is no risk of deteriorating the appearance.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing one embodiment of the method of the present invention, Fig. 2 is a schematic diagram when there is a problem with beam irradiation, and Figs. 3 and 4 are workpieces used in the embodiment of the method of the present invention. FIG. l: Thin steel plate, 2: Precor) fl plate, 3: Polymer film, 4
: non-coated surface, 5: metal surface, 6: laser beam. Applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] (1) A welding method for a pre-coated steel plate, in which the non-coated surface of a single-sided pre-coated steel plate coated with an insulating polymer film is welded to a metal surface, the welding point being welded on the non-coated surface of the single-sided pre-coated steel plate and the metal The pre-coated steel plate is brought into close contact with the area to be welded on the surface, and a V-shaped groove is formed by bending the pre-coated steel plate in a substantially U-shape so that the non-coated surface in contact with the adhered area is on the outside. A method for welding pre-painted steel plates, which is characterized by irradiating a laser beam to the closely-attached parts located at the parts.