JPS59182940A - Aluminum alloy for welded structure - Google Patents

Abstract

PURPOSE:To develop an Al-Mg alloy for welded structure having a cryogenic characteristic by decreasing the content of Mn, Fe and Si in a specifically composed Al-Mg alloy and limiting the distributed quantity and size of an eutectic compd. CONSTITUTION:An alloy contains 4-5.5wt% Mg, 0.4-0.80wt% Mn, 0.05- 0.35wt% Cr and 0.005-0.2wt% Ti and has limited contents of Mg, Fe and Si which are the essential components of an eutectic alloy. The total of the contents of Mn and Fe therein is limited to <=0.80wt% and the content of Fe alone is limited to <=0.20wt% and the content of Si as an impurity to <=0.10wt%. The distributed quantity of the eutectic alloy of Al-Fe-Mn and Al-Mg-Si is decreased and the eutectic mixture is fined and dispersed, by which the Al alloy for welded structure having an excellent mechanical characteristic at an extra low temp. and suitable as a raw material for the member to be used for transportation and storage of LNG is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AQ-111 aluminum alloy used for welded structures such as tanks for transporting or storing liquefied natural gas (hereinafter referred to as LNG), and particularly relates to an aluminum alloy with cryogenic properties. The present invention provides an excellent aluminum alloy for AQ-Mll based welded structures.

As is well known, ^Q-■ based alloys, especially JIS5083 series alloys, have excellent weldability, corrosion resistance, low-temperature properties, etc., and have stable properties, so they are used for LNG transport ship tanks, LNG storage tanks, and various other vehicles and ships. Widely used in welded structures. Recently, such welded structures tend to become larger and larger, and as a result, thicker materials are required for the ^Q-Ut+ base alloy materials used in welded structures, such as moss. The equatorial part of the spherical tank loaded on a type LNG tanker has a thickness of 2001I1m.
Front and rear planks are now used.

By the way, as a thick-walled material for the equatorial portion of a spherical tank for LNG as described above, due to its structure, emphasis is placed on the mechanical properties of the material in the thickness direction, that is, ductility, toughness, and fatigue properties.
In particular, since it accommodates cryogenic LNG, it is required that these mechanical properties be excellent at cryogenic temperatures.

This invention has Ai! which has sufficient mechanical properties in the thickness direction at extremely low temperatures and is more stable than the current material! The object of the present invention is to provide an Mll-based aluminum alloy for welded structures.

In order to achieve the above-mentioned purpose, the present inventors have 8f! −)J
O-based alloys, especially JIS5083-based Al! -M(1-u
We conducted various experiments and studies to improve the mechanical properties of thick plates of n-based alloys in the thickness direction at extremely low temperatures. It was found that the second phase compound crystallized inside had a great effect. In other words, the production of thick plate materials as mentioned above,
In particular, thick and large ingots are required in the production of large and thick plate materials for large structures. However, in the DC casting method that is widely used for casting ingots of this type of alloy, as the wall thickness of the ingot increases, the cooling rate during solidification decreases, the internal structure of the ingot becomes coarser, and crystallized compounds It also tends to become coarser. In particular, in JIS 5083 series alloys, 8Q -Fe -kin series eutectic compounds and 80 IJil Si series eutectic compounds are used in the parts on both sides sandwiching the central part in the thickness direction, more precisely, when the plate thickness is t, the thickness is There is a tendency for coarse crystallization to become coarse near the position of t/8 on both sides of the center (i.e., near the position of 3t/8 from the skin), and such coarse crystallized substances tend to become coarse in the case of plates with a thickness of approximately 80 + + + m or more. Even during rolling, it is not sufficiently refined and dispersed. Further, such crystallized compounds are distributed near grain boundaries. Therefore, at extremely low temperatures where the grain boundaries become brittle, crack formation is promoted by the notch effect of the compound and propagates along the grain boundaries, which tends to cause a decrease in fracture energy. In particular, since grain boundaries are strongly involved in fracture in the thickness direction, strength, ductility, and toughness at extremely low temperatures are more easily affected by the distribution of crystallized compounds as described above than at room temperature. In fact, the location of the fracture in the thickness direction is also an area where the compound distribution is coarse, that is, 3t/3t from the skin.
It has been confirmed that they are concentrated near position 8. On the other hand, in the propagation of fatigue cracks, the amount of variation in stress intensity factor ΔK
In the region where the γ is high, crack propagation is promoted by the connection between the main crack and the fine cracks generated by the compound, so the influence of the compound distribution cannot be ignored.

Therefore, in order to improve and stabilize the mechanical properties in the thickness direction at extremely low temperatures, it is necessary to reduce the number of crystallized compounds distributed in the material, and to make the crystallized compounds -3= finer and more dispersed. However, in the case of thick plates, there is a restriction on the total rolling rate, so as mentioned above, refinement,
It is difficult to achieve dispersion in processes after rolling. Therefore, the present inventors considered suppressing the distribution amount and size of the crystallized compound by limiting the amount of 1"n, FeXSi, which is the main component of the eutectic compound, from the viewpoint of the composition of the material. As a result of repeated research, the total amount of Fe and Mn was less than 0.80% by weight, [e alone was less than 0.20% by weight,
It was discovered that mechanical properties at extremely low temperatures can be improved by suppressing Si to less than 0.10%, and this invention was achieved.

Therefore, the aluminum alloy for welded structures of this invention is
1114-5.5% by weight, Ijn O, 4-0.80% by weight, Qj 0.05-0.35% by weight, -0,005
In an aluminum alloy containing ~0.2% by weight and the remainder consisting of unavoidable impurities and 8Q, the total amount of fun and Fe as unavoidable impurities is less than 0.80% by weight, and the individual amount of Fe is 0.2% by weight. It is characterized by suppressing Si as an unavoidable impurity to less than 20% by weight and less than 0.10% by weight.

This invention will be explained in more detail below.

First, the reason for limiting the alloy components will be explained.

Un is an essential component of the 5083 series alloy targeted by this invention, and is useful for improving strength and preventing a decrease in corrosion resistance due to Fe, but 0.40! If it is less than I%, sufficient strength cannot be obtained. However, if Un exceeds 0.80% by weight, the coarsening of the compound will become noticeable, so 0.40 to 0.8
The range is 0-fold open air.

Si is normally included as an unavoidable impurity in this type of alloy, but if its content exceeds 0.10 mμ%, the amount of Ijl12Si crystallized compounds increases, causing problems in terms of material uniformity. It is not preferable, especially in terms of stabilizing and improving the mechanical properties in the thickness direction at extremely low temperatures, and therefore is limited to less than 0.10 ft%.

Fe forms a compound of the 8Q-re-kin type with kin. This compound is the second alloy of the system targeted by this invention.
Since it occupies most of the phase compounds, it is necessary to control its gate by considering the balance with Mnff1. As a result of detailed experiments, the present inventors found that while suppressing the individual amount of e to less than 0.20% by weight, the total amount of e and 4n was reduced to 0.80% by weight.
It has been found that by suppressing the amount of crystallization of the second phase compound to less than % by weight, the amount and size of the second phase compound can be controlled to the extent that they do not adversely affect the mechanical properties in the thickness direction at extremely low temperatures. Therefore, "e was limited to less than 0.20% by weight, and the total amount of Fe + un was limited to less than 0.80% by weight.

The other components are almost the same as those of conventional 5083 series alloys. In other words, VG is an essential component of 5083 series alloys and plays a role in improving strength through processing effects, but 4.
If it is less than 0% by weight, sufficient strength cannot be obtained;
If it exceeds ifi%, the workability will be significantly reduced, so 4.
0 to 4.8% by weight. Also, like Mn, Or is useful for improving strength and preventing deterioration of corrosion resistance, but 0.05% by weight
If it is less than 0.35% by weight, these effects cannot be expected, while if it exceeds 0.35% by weight, ductility will be impaired, so the content is set at 0.05 to 0.35% by weight. Ti contributes to the refinement of the ingot structure and the refinement and dispersion of crystallized compounds, but if it is less than o, oos weight %, no such effect can be expected, while if it exceeds 0.2 weight %, from a cost perspective, Also, since it is unfavorable from the viewpoint of castability, 0
．． 0.005 to 0.20% by weight. Furthermore, in the alloy of the present invention, as mentioned above, Fe,
Sia') and Cu may be included. It is desirable that the amount of Cu be as low as possible, but up to 0.15% by weight is permissible. If Cu exceeds 0.15% by weight, corrosion resistance will be impaired.

By suppressing Fe, Si, and IJn as described above, the amount and size of the second phase compound to be dispersed and crystallized can be controlled to such an extent that the mechanical properties in the thickness direction at low temperatures are not substantially affected. Specifically, it is desirable that the amount and size of crystallization of the second phase compound satisfy the following conditions. That is, regarding the amount of distribution, it is desirable that the amount of crystallization of the second phase compound in the cross section perpendicular to the rolling direction is 1.6% or less in terms of area ratio in any part of the cross section. Here, 1.6% or less in area ratio means 1.6% or less as a percentage of the area of the total crystallized compounds in the area when observed in a visual field where about 300 compounds (250 to 350 compounds) can be observed. .6 or less. In addition, as mentioned above, the amount of crystallization is highest near the position t/8 on both sides from the center of the plate thickness, so it is sufficient if the result observed at that position is 1.6% or less in terms of area ratio. . On the other hand, regarding the size of the second phase compounds, it is desirable that the number of compounds with a maximum diameter of 100JJll1 or more is 0.3% or less of the total number of compounds with a maximum diameter of 1 note or more. In this case as well, the compound near the position t/8 on both sides from the center of the plate thickness is the coarsest, so when observed at that position, 100ul+
It is sufficient if the number of the above compounds is 0.3% or less.

As mentioned above, in order to control the content of Fe and Si, it is sufficient to select and use a material with high purity as a melting raw material such as Δg metal.

A more specific explanation will be given below based on Examples.

1 to a thickness of 500 m containing the components shown in sample numbers 1 to 9 in Table 1. The above large ingot was obtained by DC casting, heated at 510 to 540°C for 10 to 15 hours, and then hot rolled to obtain a super 8-thick plate with a thickness of 200 a+m. Tables 2 and 3 show the results of examining the dispersion state of the second phase compound of the obtained ultra-thick plate and the mechanical properties (1) in the thickness direction at room temperature and at an extremely low temperature (-196°C). Note that the dispersion status of the second phase compound in Table 2 is from the epidermis to 3 (
The amount of crystallization at the /8 position M (position t/8 from the center of the plate thickness to both sides) is the largest and coarsest, and the fracture position in the ultra-low temperature tensile test is also concentrated around this area, so it is considered that rolling Five visual fields were observed in which approximately 300 compounds were observed at a position 3'8 from the epidermis in a cross section perpendicular to the direction, and the average value was expressed.

As is clear from these tables, Fe is 0.20% by weight
In the above materials (sample numbers 7 to 9) and the material containing 0.10% by weight or more of Si (sample number 8), the crystallization of second phase compounds is large and the proportion of coarse compounds is high. The ductility (elongation) at cryogenic temperatures is lower than at room temperature. This is contrary to the inherent characteristics of the aluminum material, and is thought to be affected by the coarse second phase compound. In addition, Fe is less than 0.20% and Si
is less than 0.10% by weight, but Fe +1. In the case of a material with a total In content of 0.80% by weight or more (sample number 6), the proportion of coarse compounds is still high, and the properties at extremely low temperatures are comparable to those at room temperature, but they are still not sufficient.

On the other hand, Fe O, less than 20% by weight, less than 9i 0,10% by weight, and the total amount of Fe and Lln is 0.80% by weight.
In the materials (sample numbers 1 to 5), the crystallization of the second phase compound is small, and the proportion of coarse compounds is also small (in this case, extremely low), and the ductility of the product is much higher than that at room temperature. It is clear that the original properties of aluminum material are maintained. In particular, when Fed is less than 10% by weight and SiO is less than 10% by weight (sample numbers 1 to 2), the crystallization depressions of the second phase compound are extremely small and the proportion of coarse compounds is also extremely small. It is clear that the mechanical properties, especially the ductility, at extremely low temperatures are significantly superior.

FIG. 1 shows the influence of the Fe and Ijn contents on the ductility in the thickness direction in each of the above samples 1 to 9. In FIG. 1, the O mark indicates that the elongation at extremely low temperatures is superior to the elongation at air temperature, and the * mark indicates that the elongation at extremely low temperatures is less than the elongation at room temperature.

From FIG. 1, it is clear that in order to improve the ductility at extremely low temperatures, both conditions of less than 2.0% Fe and less than 80% Fe + un O are required.

Furthermore, among the above samples, sample number 2.4.6.9 was subjected to a fatigue crack propagation test at an extremely low temperature of -196°C, and the variation Δ in the stress intensity factor was 40ko・1I.
When the fatigue crack propagation length per cycle in llIl-tongue was investigated, the results shown in Table 4 were obtained.

However, the fatigue crack propagation test piece is based on ASTM standard (E39
9 & 647), that is, a CT test piece in which the notch was placed in a plane perpendicular to the plate thickness direction in a direction perpendicular to the rolling direction, and the notch position was 3t/8 from the skin. The stress ratio R was 0.5.

From Table 4, sample 2.4 within the composition range of this invention is “
It is clear that the resistance to the propagation of fatigue cracks is higher than that of sample 6 in which the total amount of e+sn is excessive and sample 9 in which both Fe and Si are excessive.

As is clear from the above description, the aluminum alloy for welded structures of the present invention has high mechanical properties at extremely low temperatures, especially ductility and resistance to fatigue crack propagation, in thick plates with a thickness of about 8Q++u++ or more, and therefore is suitable for LNG use. This material is safer than current materials when used in various large structures such as tanks.

[Brief explanation of drawings]

FIG. 1 is a correlation diagram showing the influence of the Fe and νn contents in the material on the ductility in the thickness direction. Applicant Sky Aluminum Co., Ltd. Representative Patent Attorney Yutaka 1) Hisashi Take (and 1 other person) 15- Figure 1 Fe-containing At (additional 1%)

Claims (1)

[Claims] 1g4-5.5 weight 1-4%, MnO, 4-0
．． 80% by weight, Oro, 05-0.35% by weight,
In an aluminum alloy for welded structures containing 0.005 to 0.2% by weight of 7i and the remainder consisting of unavoidable impurities and 8Q, the total content of un and e as an impurity component is 0.80% by weight. less than 0.
Less than 20% by weight, the content of 81 as an impurity component is 0
．． An aluminum alloy for welded structures characterized by being regulated to less than 10% by weight and less than 10% by weight.