JP2002103072A - Lap laser welding method for aluminum plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide lap laser welding for aluminum plated steel sheets by which reduction in the tensile strength of a weld zone caused by the intrusion of aluminum in a plated layer into weld metal and deterioration in its fatigue strength caused by the coarsening of ferritic grains can be prevented. SOLUTION: In this lap laser welding method for aluminum plated steel sheet, as an assist gas, a gaseous mixture containing >=50% vol.% gaseous nitrogen, and the balance one or more kinds of gases selected from gaseous argon, gaseous helium and carbon dioxide is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for welding a plated steel sheet having excellent corrosion resistance in the production of parts for automobiles and motorcycles, home electric appliances, industrial equipment, and the like. About.

[0002]

2. Description of the Related Art Conventionally, lead-plated steel sheets have been used as members requiring rust prevention, such as fuel tanks for automobiles and motorcycles. However, in recent years, aluminum plating has been used as an alternative material due to environmental problems. Steel sheets have been used, and lap welding has been started using laser welding in which the plating layer is less deteriorated.

Conventionally, in lap laser welding of aluminum-plated steel sheets, a large amount of aluminum in a plating layer is mixed into a weld metal to form a locally concentrated portion of aluminum or an intermetallic compound with iron, There is a problem that mechanical properties such as tensile strength of the metal part are reduced.

In order to solve this problem, for example, Japanese Patent Laid-Open No. 9-1
In Japanese Patent No. 55575, laser welding is performed by providing a gap in a superposed portion around a laser welded portion of an aluminum-plated steel sheet,
There is disclosed a method in which aluminum melted at the time of laser welding is allowed to escape to obtain good laser weld strength.

In Japanese Patent Application Laid-Open No. Hei 10-71480, when superposition laser welding of a plated steel plate such as zinc or aluminum is performed, a laser beam is scanned within a specific scanning range on a two-dimensional locus to remove molten metal. A method of suppressing welding defects in a laser welded portion by stirring is disclosed.

In Japanese Patent Application Laid-Open No. Hei 10-296472, when a superposed aluminum-plated steel sheet is overlapped and laser-welded, a shielding gas containing a large amount of oxygen is used.
By discharging unnecessary components such as molten aluminum and silicon in the molten metal as oxides during welding,
A method for increasing the strength of a joint is disclosed.

However, in these conventional methods, although the enrichment of aluminum in the weld metal during laser welding can be suppressed and the tensile strength of the weld metal can be improved, the weld metal has a coarse ferrite structure and has a fatigue strength. Cannot be suppressed. For example, when lap laser welding of aluminum-plated steel sheets is applied when manufacturing automobiles and motorcycle fuel tanks used in environments with vibration, the improvement of the fatigue strength characteristics of the laser welded joints is required. Required.

In Japanese Patent Application Laid-Open No. 6-79484,
In a lap welding of a plate material having a coating layer in which at least one material is made of a substance having a lower melting point than the base material, oxygen gas having a volume ratio of 5 to 30% to the whole is added to an inert gas or a nitrogen gas as an assist gas. A laser welding method using a mixed gas mixture is described. However, this welding method is substantially intended for laser welding of a galvanized steel sheet, and is characterized by containing oxygen gas in the assist gas. In other words, the oxygen in the assist gas oxidizes and burns the plating layer to prevent explosive phenomena caused by vaporization of the plating layer due to heat conduction from the laser beam irradiation part. It is not a technique aimed at improving tensile strength and fatigue strength.

Therefore, there is a demand for a laser welding technique capable of improving the fatigue strength characteristics as well as the tensile strength of the weld metal at the laser weld joint in lap laser welding of an aluminum-plated steel sheet.

[0010]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention relates to a laser welding method for lapping aluminum-plated steel sheets. An object of the present invention is to provide superposition laser welding of an aluminum-plated steel sheet which can prevent a decrease in tensile strength and a deterioration in fatigue strength due to coarsening of ferrite grains.

[0011]

SUMMARY OF THE INVENTION The present invention is to solve the above-mentioned problems, and the gist thereof is as follows.

(1) In the lap laser welding method for an aluminum-plated steel sheet, nitrogen gas is contained as an assist gas in an amount of 50 vol% or more, and at least one of argon gas, helium gas and carbon dioxide gas is used as an assist gas.
A superposition laser welding method for aluminum-plated steel sheets, comprising using one or more kinds of mixed gases.

(2) In the lap laser welding method for aluminum-plated steel sheets, a one-dimensional scanning of a laser beam in a welding progress direction or a direction perpendicular thereto, or a two-dimensional scanning in a welding progress direction and a direction perpendicular thereto. The aluminum-plated steel sheet according to the above (1), wherein scanning is performed and laser welding is performed in the welding progress direction so that the scanning range is 0.5 to 3 times the focused spot diameter of the laser beam. Laser welding method.

(3) In the above-mentioned laser welding method for lapping aluminum-plated steel sheets, a gap of 0.1 to 0.5 times the sheet thickness is further provided in the aluminum-plated steel sheet around the welded part. (1) The method of superposition laser welding of an aluminum-plated steel sheet according to any of (1) and (2).

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below.

FIG. 1 shows an example of an embodiment in which laser welding of aluminum-plated steel sheets of the present invention is performed by superposition.
FIG. 2 shows an overlap laser welding of a flange portion when a tank is manufactured by using an aluminum-plated steel plate as an example of another embodiment of the present invention.

The two aluminum-plated steel sheets 1 and 2 which are the materials to be welded are restrained by the restraints 3a, 3b and 4 in a state where a part of them is in close contact with each other or overlaps with a predetermined gap 8.
The overlapped portions of the aluminum-plated steel plates 1 and 2 are welded by irradiation of the laser beam 9 while injecting the assist gas 7 by the laser irradiation torch 6.

Conventionally, Ar or He has been generally used as the assist gas 7 shown in FIGS. Further, as described above, Japanese Patent Application Laid-Open No. 10-296472 and the like use an assist gas containing a large amount of oxygen for the purpose of reducing the strength of a welded portion.

In contrast to these conventional techniques, the first feature of the present invention is that nitrogen gas is used as an assist gas at 50 vol.
% Gas or more.

Conventionally, although nitrogen gas is very inexpensive, in the case of laser welding of a low carbon steel sheet, nitrogen dissociated or ionized by a laser dissolves in molten steel to generate blowhole defects. , Was not commonly used.

However, as a result of examinations by the present inventors through experiments and the like, it was found that a CO ₂ laser or Y
In superposition laser welding using an AG laser, etc., the use of an assist gas containing a large amount of nitrogen not only eliminates the problem of blowhole defects in the molten metal, but also prevents the conventional superposition laser welding of aluminum-plated steel sheets. It was found that the reduction of the weld strength and the fatigue strength due to the incorporation of aluminum into the weld metal can be prevented.

Conventionally, when aluminum-plated steel sheets having aluminum-plated layers on the front and back layers are closely adhered to each other and laser-welded, the aluminum in the plated layer is locally concentrated in the weld metal, or the aluminum between the iron and the metal is It is known that a compound is formed to lower the tensile strength of a weld. Also, according to the experimental results of the inventors, aluminum dissolved in the weld metal is Al
Is a ferrite-forming element, so that the weld metal is formed into coarse ferrite grains and the fatigue strength of the weld metal is also reduced.

The reason why the tensile strength and fatigue strength of the weld metal can be improved by using an assist gas containing a large amount of nitrogen in the lap laser welding of the aluminized steel sheet of the present invention is as follows. Conceivable.

By superposing laser welding of aluminum-plated steel sheets while injecting a gas containing a large amount of nitrogen, the fluidity of molten steel is improved, local concentration of aluminum is suppressed, and the molten steel melts into the molten metal. It is thought that the aluminum in the plating layer was reacted with nitrogen supplied from the assist gas to form aluminum nitride, thereby losing the ability of aluminum to produce ferrite, and consequently improving the tensile strength of the weld metal. Further, since a large amount of nitrogen dissolved in the molten steel acts as an austenite-forming element, it is considered that by suppressing the formation and coarsening of ferrite grains in the weld metal, the fatigue strength of the weld metal is improved as a result.

From the results of examinations by the inventors, such as experimental results, the above-described effects of improving the tensile strength and fatigue strength of the weld metal are due to the fact that the nitrogen gas concentration in the assist gas used in the lap laser welding of the aluminum-plated steel sheet is reduced. It is known that the effect is sufficiently exerted when the content is 50 vol% or more.

As described above, in the present invention, as an assist gas used in lap laser welding of an aluminum-plated steel sheet, nitrogen gas is contained at 50 vol% or more, preferably 65 vol% or more, and the other gas is argon gas. Helium gas or carbon dioxide gas
It is necessary to use a species or a mixture of two or more species. If the nitrogen concentration in the assist gas is less than 50 vol%, the above-described effects of improving the tensile strength and fatigue strength of the weld metal cannot be sufficiently exhibited.

Further, air can be used as a nitrogen source in the assist gas at the time of laser welding used in the present invention. Air typically has about 80 vol.
1%, which satisfies the above-mentioned nitrogen concentration range of the present invention, and by using air as an assist gas, the above-mentioned effects of improving the tensile strength and weld strength of the weld metal can be sufficiently obtained. When air is used as an assist gas by mixing it with at least one or more of an argon gas, a helium gas, and a carbon dioxide gas, the tensile strength and welding strength of the above weld metal due to nitrogen in the assist gas are determined. In order to sufficiently obtain the effect of improving the strength of the part,
The content of air in the assist gas must be 70 vol% or more, preferably 85 vol% or more.

Further, in the present invention, the above-mentioned nitrogen gas
While using an assist gas containing at least vol%,
Further, the laser beam is subjected to one-dimensional scanning in the welding traveling direction or a direction perpendicular thereto, or two-dimensional scanning in the welding traveling direction and a direction perpendicular thereto, and the scanning range is 0% of the focused spot diameter of the laser beam. By laser-welding the aluminum-plated steel sheet while scanning the beam to be laser-welded in the welding progress direction so as to be 0.5 to 3 times, the tensile strength and the fatigue strength of the welded portion can be further improved.

This is achieved by using an assist gas containing 50% by volume or more of nitrogen gas and scanning a laser beam during welding to stir the weld pool of the weld metal, thereby locally forming molten aluminum in the molten metal. The purpose is to suppress the concentration, promote the formation of aluminum nitride of dissolved aluminum and dissolved nitrogen in the molten metal, promote the dispersal of the aluminum nitride, and promote the homogenization of the weld metal structure.

In order to sufficiently obtain such an effect, it is necessary to set the scanning range of the laser beam during welding to 0.5 to 3 times the focused spot diameter of the laser beam. When the diameter is less than 0.5 times, the local concentration of aluminum cannot be sufficiently prevented due to insufficient stirring of the molten metal, and when the diameter exceeds 3 times the focused spot diameter of the laser beam, spatters are scattered during welding. In addition, the required melting depth cannot be obtained because the laser beam penetration ability is reduced.

The scanning direction of the laser beam is perpendicular or parallel to the welding direction, and the scanning method can be a constant reciprocating motion or a simple vibration. In the case of one-dimensional scanning in a reciprocating motion at a constant speed perpendicular to the welding traveling direction, the center of the beam follows a locus as shown in FIG. In the case of two-dimensional scanning in a circular motion around the reference axis of the laser beam, the center of the beam follows a locus as shown in FIG.

In the present invention, the above-mentioned nitrogen gas is supplied at 50 vol.
% Of an assist gas containing at least 0.1%
By providing a 0.5 times gap and performing laser welding, the tensile strength and fatigue strength of the welded portion can be further improved.

This is accomplished by using an assist gas containing 50 vol% or more of nitrogen gas and providing a gap of 0.1 to 0.5 times the thickness of the aluminum-plated steel sheet around the welded portion by laser welding. In addition to suppressing the coarsening of ferrite crystal grains due to the dissolution of nitrogen in the weld metal as described above, the local concentration of aluminum in the weld metal is effectively suppressed by allowing low melting point aluminum to escape from the gap. is there.

In the present invention, the gap between the aluminum-plated steel sheet around the welded portion is set to be 0.1 to 0.5 times the sheet thickness because:
If it is less than 0.1 times, the low melting point aluminum cannot escape from the gap to obtain a sufficient effect of suppressing the local concentration of aluminum in the weld metal, and if it exceeds 0.5 times, the molten metal into the gap cannot be obtained. This causes the problem of under-beads due to the flow of water.

[0035]

EXAMPLES The effects of the present invention will be described below with reference to the following examples.

FIG. 6 shows a plan view and a side view of a laser-welded joint of an aluminum-plated steel sheet as a material to be joined used in the actual machine test. The overlapping portions of the aluminum-plated steel sheets 1 and 2 having a thickness of 0.8 mm and having a basis weight of 30 gr / m ² on one side shown in FIG. 6 and having a thickness of 0.8 mm are shown in Table 1 or Table 2. Was welded with a CO ₂ laser or a YAG laser at a gap 8 of a laser beam scanning condition and a welding speed, and the tensile strength and fatigue strength of the welded joint were evaluated.

As for the laser welding conditions, in the case of a CO ₂ laser, the laser oscillation output was 5 kW and the welding speed was 3
m / min, and in the case of a YAG laser, the laser oscillation output was 3 kW and the welding speed was 2 m / min. The focal length of the laser beam is 250 mm when the laser beam is scanned, and 20 mm when the laser beam is not scanned.
0 mm.

Both the CO ₂ laser and the YAG laser focused the laser beam directly on the steel plate, and the focused spot diameter was about 0.5 mm. The welding was performed so that the weld bead 5 was formed at a position 5 mm from the tip of the overlapped portion having a length of 10 mm.

The static tensile strength shown in Tables 1 and 2 is good (○) when the strength is 90% or more of the base material strength, slightly poor (△) when the strength is less than 90% and 80% or more, and 80% or less. Was determined as poor (x). In addition, the fatigue test of the laser welded joint was performed with a complete pulsation at a load ratio of 0 (= minimum load / maximum load) and a frequency of 1 Hz, and the fatigue strength when vibrated 20,000 times was obtained.

The fatigue strengths shown in Tables 1 and 2 indicate the 20,000 times fatigue of laser welded joints when pure Ar was used as a shielding gas and steel plates were welded together without any gaps. When the strength is set to 1.0, the relative fatigue strength to the fatigue strength is shown.

Table 1 shows the test results of laser welded joints when using a CO ₂ laser. Table 2 shows that YAG
The test result of the laser welding joint when using a laser is shown.

From Tables 1 and 2, it can be seen that the shielding gas, the gap between the overlapping portions, and the scanning range of the laser beam are within the scope of the present invention (Examples 4 to 8 in Table 1 and No. 7 in Table 2).
To 11) are comparative examples (N in Table 1) out of the scope of the present invention.
o. Nos. 1 to 3 and Table 2 Laser welded joints in which both the static tensile strength and the fatigue strength of the weld metal were superior to those of 1-6) were obtained.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

According to the present invention, it is possible to improve both the static tensile strength and the fatigue strength of the weld metal in lap laser welding of an aluminum-plated steel sheet, which is extremely advantageous for the automobile industry.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of an embodiment of a lap laser welding method for an aluminum-plated steel sheet according to the present invention.

FIG. 2 is a schematic view showing an example of an embodiment of the laser welding method for lapping aluminum-plated steel sheets of the present invention.

FIG. 3 is a schematic diagram showing an example of an embodiment of the present invention when scanning with a laser beam is not performed.

FIG. 4 is a schematic view showing an example of a scanning pattern according to an embodiment of the present invention when scanning with a laser beam.

FIG. 5 is a schematic diagram showing an example of a scanning pattern according to an embodiment of the present invention when scanning with a laser beam.

FIG. 6 is a view showing a welded joint of the present invention, wherein (a) is a plan view and (b) is a side view.

[Explanation of symbols]

 1, 2 aluminized steel sheet 3, 3a, 3b restraint 4, 4a, 4b restraint 5 welding bead (welded portion) 6 laser irradiation torch 7 assist gas 8 gap of overlapping portion 9 laser beam 10 laser beam locus

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Yasunobu Miyazaki 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division (72) Inventor Takashi Tanaka 20-1 Shintomi, Futtsu City Nippon Steel Corporation Inside the Technology Development Division (72) Inventor Masahiro Ohara 1 Oshino-shi, Oita, Nippon Steel Corporation Inside Oita Works (72) Inventor Masanori Kondo 1-Toyota-cho, Toyota-shi, Aichi Prefecture Toyota Motor Corporation F-term (reference) 4E068 CE02 DA14 DB01 DB15

Claims (3)

[Claims] In the lap laser welding method for aluminized steel sheet, nitrogen gas is used as an assist gas.
A method for superposing laser welding of aluminum-plated steel sheets, wherein a mixed gas containing at least vol.% and at least one of argon gas, helium gas and carbon dioxide gas is used as the other gas. 2. The method according to claim 1, further comprising the step of performing one-dimensional scanning of the laser beam in a welding direction or a direction perpendicular thereto, or two-dimensional scanning in a welding direction or a direction perpendicular thereto. 2. The lamination of the aluminized steel sheet according to claim 1, wherein laser welding is performed in a welding direction so that the scanning range is 0.5 to 3 times the focused spot diameter of the laser beam. Laser welding method. 3. The method of lap laser welding of an aluminum-plated steel sheet, further comprising providing a gap of 0.1 to 0.5 times the thickness of the aluminum-plated steel sheet around the welded portion. 3. The method for laser welding of superposed aluminum-plated steel sheets according to claim 1.