JP2602374B2 - Method for producing extruded aluminum alloy for welded structure with improved weld cracking - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an aluminum alloy extruded material for a welded structure used by welding as a material for a tank structure such as LNG, a hull structure for an LNG tanker, a fishing boat or the like. More particularly, the present invention relates to a method for manufacturing an extruded aluminum alloy material for welded structures with improved weld cracking.

[0002]

2. Description of the Related Art Aluminum alloy materials used for the above applications, particularly extruded Al-Mg alloys, are:
In addition to being lightweight, it has excellent corrosion resistance and mechanical properties at low temperatures, and also has excellent weldability, and it is generally recognized that welding cracks do not occur except during special welding. .

[0003] However, even with such an extruded Al-Mg alloy, if the thickness of the surface recrystallized microstructure layer generated in the manufacturing process reaches about 0.3 mm or more and is too thick, general welding can be performed. It has been found by the inventors of the present invention that micro cracks are generated in the heat-affected zone (HAZ zone) particularly when the welding strain is large when the welding is performed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an aluminum alloy extruded material for a welded structure having improved welding cracks by suppressing surface recrystallization.

[0005]

In order to achieve the above object, the present invention provides an Al-Mg based alloy billet containing 3.0 to 6.0 wt% of Mg at 420 to 450 ° C x 5 to 1%.
After the homogenization treatment was performed for 5 hours, the extruded material was extruded to reduce the thickness of the recrystallized structure layer on the surface of the extruded material to 0.15 mm.
It is characterized by the following , thereby suppressing the surface recrystallization of the extruded material and improving the weld crack.

[0006] First, in the extruded Al-Mg alloy, which is the subject of the present invention, the content of Mg is specified to be 3.0 to 6.0 wt%.
This is because the superiority of the corrosion resistance, mechanical properties at low temperatures, and weldability of the l-Mg-based alloy is sufficiently exhibited. When the Mg content is less than 3.0 wt%, the effects are poor.
Conversely, if it exceeds 6.0% by weight, the extrusion processability is deteriorated.

The above Al—Mg alloy is manufactured into a billet by DC casting or the like, and then the billet is subjected to a homogenizing treatment.
It is necessary to perform under the condition of 0 to 450 ° C. × 5 to 15 hours.
This is to suppress recrystallization of the surface of the extruded material after extrusion as much as possible. That is, typical homogenization treatment condition Al-Mg-based alloy is the order of four hundred eighty to five hundred and twenty ° C. × 4 to 10 hours, the heating temperature exceeding 450 ° C., Mn crystallized matter or the like in the heating time exceeding 15 hours Is coarsened and the surface recrystallized structure layer is thickened, and as a result, it is not possible to prevent the occurrence of welding cracks in the heat-affected zone. On the other hand, when the heating temperature is less than 420 ° C. and the heating time is less than 5 hours, the purpose of the homogenization treatment itself cannot be sufficiently achieved due to insufficient heating, and a speed crack is easily generated at the time of extrusion. Therefore,
By performing the homogenization treatment at a relatively low heating temperature of 420 to 450 ° C. and setting the heating time to 5 to 15 hours, the effect of the homogenization treatment itself is sufficiently provided to suppress the surface recrystallization. Things. Particularly preferred homogenization treatment conditions are 440 to 450 ° C. × 8 to 12 hours.

The aluminum alloy billet that has been subjected to the homogenization treatment is then subjected to extrusion. Extrusion conditions are not particularly limited, and general extrusion conditions may be appropriately selected.

By the way, in such extrusion, 0.5 m
It is also possible to suppress recrystallization of the surface of the extruded material by extruding at a product speed of not more than / min or setting a low billet heating temperature during extrusion. However, the method of lowering the extrusion speed has a disadvantage that the yield productivity is reduced. In addition, in the method of lowering the billet heating temperature, the total length of the billet must be shortened, which also has the disadvantage that the production efficiency is not good. On the other hand, when the homogenization processing conditions are suppressed as in the present invention,
Without reducing the extrusion speed and shortening the billet,
The thickness of the recrystallized structure layer on the surface of the extruded material can be suppressed to 0.05 to 0.15 mm or less while adopting the same conditions as in the normal extrusion.

[0010]

[Effect] 5-1 at relatively low heating temperature of 420-450 ° C.
Since the homogenization treatment is performed for 5 hours, surface recrystallization of the extruded material after extrusion is suppressed, and welding cracks in the heat-affected zone during welding are suppressed.

[0011]

EXAMPLE Three billets were manufactured by DC casting using an A5083 alloy containing 4.5 wt% of Mg. Each of these alloy billets is subjected to homogenization treatment under the conditions of (a) 450 ° C. × 10 hours, (b) 500 ° C. × 8 hours, and (c) 560 ° C. × 5 hours, and then the billet heating temperature is 480. C., Extruded product speed: Extruded into a 12 mm thick flat bar under extrusion conditions of 1.5 m / min.

When the thickness of the surface recrystallized structure layer of the extruded material obtained as described above was measured, the thickness of the sample (a) was 0.05 to 0.15 mm, whereas the thickness of the sample (b) was
(C) was 0.8 to 1.2 mm.

Next, for each extruded material, MIG was performed under the same conditions.
When the butt welding was performed, the sample (a)
No weld cracking was observed for
For samples (b) and (c), small cracks occurred in the heat-affected zone.

Therefore, it has been confirmed that according to the present invention, surface recrystallization can be suppressed and weld cracking can be improved.

[0015]

According to the present invention, as described above, Mg: 3.
A recrystallized structure of the surface of the extruded material is obtained by performing homogenization treatment at a relatively low temperature of 420 to 450 ° C. for 5 to 15 hours on an Al—Mg based alloy billet containing 0 to 6.0 wt% and extruding. Since the thickness of the layer is set to 0.15 mm or less, the surface recrystallization can be suppressed and the thickness of the surface recrystallized structure layer of the extruded material can be reduced. Therefore, it is possible to eliminate welding cracks in the heat-affected zone, which has conventionally occurred due to the large thickness of the surface recrystallization structure layer.
The advantages of the l-Mg based alloy extruded material as a welded structural material can be fully exhibited. Moreover, by setting the homogenization treatment conditions, the surface recrystallization of the extruded material is suppressed, so that the extrusion can be performed without requiring a reduction in the extrusion speed and a reduction in the billet length, A decrease in yield productivity can be avoided.

Claims (1)

(57) [Claims] 1. A containing Mg: 3.0 to 6.0 wt%.
420-450 ° C x 5-15 for l-Mg based alloy billet
After performing the homogenization treatment under the conditions of time, by extruding, the thickness of the recrystallized structure layer on the surface of the extruded material is set to 0.15 mm or less.
A method for producing an aluminum alloy extruded material for a welded structure, wherein the material has improved weld cracking.