JPH0613748B2 - Method for producing stress corrosion resistant aluminum-magnesium alloy soft material - Google Patents

Description

TECHNICAL FIELD The present invention relates to an Al—Mg alloy used for various large-scale welded structural materials such as LNG tanks, and in particular M for increasing strength.
The present invention relates to a method for improving the stress corrosion resistance of an Al-Mg alloy soft material having a high g content of 5.3% or more.

2. Description of the Related Art Conventional 5083 alloy, which is a typical Al-Mg alloy, is a high-strength non-heat treatment material. Therefore, LNG (liquefied natural gas) land storage tanks have advanced along with recent advances in Al welding technology. It has been widely used for large welded structures such as tanker tanks, but recently, for the purpose of cost reduction by reducing the amount of materials used, the strength of this type of alloy has been further improved to reduce the wall thickness. There is a strong desire for this.

By the way, according to JIS standard, 5083 alloy has Mg4.0-4.9
%, Mn 0.30 to 1.0%, Cr 0.05 to 0.25%, and the balance being Al and inevitable impurities,
Other impurity components: Cu 0.10% or less, Si 0.40%
Below, Fe 0.40% or less, Zn 0.25% or less, and Ti 0.15% or less are allowed.

The alloying elements that contribute to the strength of the 5083 alloy are mainly Mg, Mn, and Cr. Among these, the Mg content is particularly high, so that the contribution of Mg to the strength is highest. Therefore, the strength of Al-Mg alloy is
In order to raise the Mg content higher than that of the alloy, it is considered that the Mg addition amount is increased to 5.3% or more as compared with the case of the 5083 alloy, and the inventors of the present application have already disclosed such a Mg content in Japanese Patent Application No. 59-195516. Strengthened by increasing the amount of A
An l-Mg alloy is proposed.

Problems to be Solved by the Invention Al-Mg alloys generally have excellent corrosion resistance, but Mg
It has been clarified that stress corrosion cracking is likely to occur when the amount is increased, while it is known that the soft material (O material) is less likely to undergo stress corrosion cracking than the hard material (for example, H12 material). , Dix et al. (Reference name Corrosion 15 [2] (1959) P5
According to 5), it is clear that stress corrosion cracking may occur even in a soft material if Mg exceeds 5%. For structures such as LNG tanks that are used under various environments and under stress, it is necessary to avoid using materials that may cause stress corrosion cracking in order to ensure safety, and therefore high strength. Even in the case of Al-Mg synthetic soft material with more than 5.3% Mg added to make it
For use in NG tanks and the like, the possibility of stress corrosion cracking must be eliminated.

Generally, in a high-Mg alloy containing Mg in a supersaturated solid solution state, Mg tends to precipitate in the β phase (Al ₃ Mg ₂ ) at grain boundaries and on the trailing line due to long-term aging such as 20 years. It is known that, particularly when it precipitates at the grain boundary, the grain boundary is likely to be locally corroded, and the stress corrosion resistance decreases. According to the above-mentioned Dix et al., Such a long-term change with time is referred to as sensitization treatment at 100 to 120 ° C.
It is said that the heat treatment for one week can be used almost as a substitute, and therefore a stress corrosion cracking test can be performed in a state where such a sensitization treatment is performed to determine the stress corrosion resistance.

In order to reduce the amount of β-phase precipitation on the grain boundaries due to the long-term aging as described above, it is effective to limit the total amount of Mg and reduce the amount of Mg dissolved in supersaturation. ,
This method does not meet the object of the present invention to improve the strength of the Al-Mg alloy by increasing the amount of Mg. On the other hand, in order to promote the precipitation of the β phase in the grain and prevent the precipitation of the grain boundary of the β phase, a transition metal such as Zr or V is added to make the precipitate containing these elements uniform in the grain. However, in the case of this method, it causes an increase in the cost of alloying elements to be added, and depending on the amount added, coarse intermetallic compounds are generated in the ingot, which hinders the homogeneity of the structure. May lead to problems.

The present invention has been made in view of the above circumstances.
In a high Mg-based Al-Mg alloy soft material containing 5.3% or more, precipitation of β phase (Al ₃ Mg ₂ ) at grain boundaries is suppressed as much as possible without causing the above problems, and stress corrosion resistance. The basic purpose is to provide a method for sufficiently improving the sex.

Means for Solving the Problems This invention is basically for producing a high Mg-based Al—Mg alloy soft material (O material) containing Mg in an amount of 5.3% or more and containing Fe and Mn. After rolling and finishing annealing as necessary to finish the soft material with the required plate thickness, cold working strain is further applied under specific conditions to make β-phase intragranular precipitation more aggressive than the conventional method. To promote
This is intended to prevent the deterioration of stress corrosion resistance due to long-term aging.

That is, the inventors of the present application have found that, with respect to an Al-Mg alloy soft material containing 5.3 to 9% of Mg and containing a predetermined amount of Mn and Fe, a cold set of 0.5% to 2.0% in permanent set by tensile straightening. When a processing strain is applied, a change corresponding to long-term aging is promoted, for example, even after sensitizing treatment at 120 ° C for 1 week, uniform precipitation of β phase in the grains is promoted, and stress corrosion resistance is significantly improved. Therefore, they have come to make the present invention.

Specifically, the manufacturing method of the aluminum-magnesium alloy soft material of the first invention of the present application is Mg5.3-9%, Mn0.0.
5 to 1.0%, Cr 0.05 to 0.3%, Ti 0.005 to 0.2%, Fe
An alloy containing 0.25 to 1.00% with the balance Al and unavoidable impurities is used as a raw material, rolled and optionally subjected to finish annealing to finish a soft material having a required plate thickness, and then by tensile straightening. It is characterized by imparting a cold work strain of more than 0.5% and not more than 2.0% in terms of permanent strain to improve stress corrosion resistance.

The method for producing a soft aluminum-magnesium alloy material according to the second aspect of the present invention is to apply the same process as described above, using an alloy containing 0.05 to 0.3% of Cu in addition to the components specified in the first aspect of the invention. is there.

Action Mn and Fe added to an Al-Mg alloy having a Mg content of 5.3 to 9% are uniform as an Al-Fe-Mn-based second phase compound through a casting solidification process and a soaking / hot rolling process. , And finely dispersed. Thus, if a tensile load is applied to the metal structure in which the second phase compound is uniformly dispersed,
The cold-working strain does not concentrate as grain deformation at grain boundaries but is uniformly distributed corresponding to the distribution of the second phase compound.
As a result, the β phase (A
The precipitation of l ₃ Mg ₂ ) will be carried out uniformly not only at the grain boundaries but also within the grains. Therefore, the possibility of occurrence of stress corrosion cracking due to preferential precipitation of β phase at grain boundaries is reduced, and stress corrosion resistance is improved.

Here, Al-M having a required plate thickness and a soft material
The cold-working strain applied to the g-alloy plate by the straightening must be more than 0.5% and not more than 2% in terms of permanent set. If it is 0.5% or less, the effect of promoting uniform precipitation of β phase in the grains cannot be obtained, and if it exceeds 0.5%, uniform precipitation in grains can be promoted only. On the other hand, if a permanent strain of more than 2% is given, not only the characteristics as a soft material are impaired, but also a shear band is generated and the precipitation of β phase becomes uneven, which is not preferable for improving the stress corrosion resistance. Therefore, a permanent set of more than 0.5% and 2.0% or less is set.

Here, the step before applying the cold working strain by the straightening of the tension as described above, that is, the method for producing the soft material having the required thickness may be a general method, for example, the DC casting method or the semi-continuous casting method. , After casting the ingot according to a conventional method such as continuous casting method, if necessary, homogenization treatment of the ingot at a temperature of 400 ~ 500 ℃, for example 400 ~ 500
Performing hot rolling by heating to ℃, a method of finishing the final plate thickness by only the hot rolling, or after performing hot rolling in the same manner as described above, perform intermediate annealing at 300 ~ 500 ℃, if necessary, Further, a method may be applied in which cold rolling is performed to finish the final plate thickness, and then finish annealing is performed at 300 to 500 ° C. to obtain a soft material. However, even when finishing to the final plate thickness only by hot rolling, the final temperature of hot rolling is 35
If the temperature is lower than 0 ° C, it is necessary to perform finish annealing at a temperature of 300 to 500 ° C to obtain a soft material.

Next, the reasons for limiting the component composition of the alloy used in the present invention will be described.

Mg: Mg is an element effective for increasing the strength in the non-heat treatment type Al alloy, but if the amount is less than 5.3%, the high strength as intended in the present invention cannot be achieved, and the M content is less than 5.3%.
In the case of a soft material, a decrease in stress corrosion resistance due to long-term aging does not pose a serious problem in terms of g. On the other hand, over 9% M
Even if the amount of g is increased, the contribution to the improvement of strength is reduced and it becomes uneconomical. Therefore, the range is limited to 5.3 to 9%. In order to obtain a particularly high strength within this range, it is desirable that the Mg content be 6% or more, and further 7% or more.

Mn: Mn is effective for securing strength, especially proof stress, and as described above, coexistence with Fe produces a stable second phase compound of Al-Fe-Mn system at high temperature in various thermal history processes after casting. In addition, it contributes to the improvement of stress corrosion resistance by being uniformly broken and dispersed in the hot rolling or cold rolling process. That is, in the state in which the second phase compound is finely and uniformly dispersed, the cold working strain is imparted with a permanent strain of more than 0.5% and 2% or less by the tensile straightening as described above, and the strain is only the grain boundary. Not evenly distributed in the grains,
As a result, β-phase precipitation due to long-term aging is performed uniformly not only at grain boundaries but also within grains. Where M
If the amount of n is less than 0.05%, such an effect cannot be obtained. On the other hand, if the amount of Mn exceeds 1.0%, the amount of compound crystallization increases, and sometimes the giant intermetallic compound is crystallized to obtain a sound ingot. The quality of the rolled material may be impaired,
Mn was limited to the range of 0.05 to 1.0%.

Fe: Fe is also an element effective for crystallizing an Al-Fe-Mn compound by coexistence with Mn and improving the stress corrosion resistance similarly to the above. If Fe is less than 0.25%, the effect cannot be sufficiently obtained, while if it exceeds 1.0%, the distribution number of the compound becomes too large and ductility and toughness may be deteriorated. Therefore, Fe is set within the range of 0.25 to 1.0%.

Cr: Cr is also an element effective for securing strength, particularly for securing proof stress and improving stress corrosion resistance, but if it is less than 0.05%, its effect is not sufficiently obtained, while if it exceeds 0.3%, a large metal in the ingot is formed. Since the intermetallic compound may be crystallized and impair the homogeneity of the quality of the rolled material, it is limited to the range of 0.05 to 0.3%.

Ti: Ti is an element effective in refining the crystal grains of the ingot,
If it is less than 0.005%, the effect is not recognized, while if it exceeds 0.2%, the toughness is deteriorated. Therefore, Ti is set within the range of 0.005 to 0.2%. If B is added in addition to Ti, the effect of refining the ingot crystal grains becomes more remarkable. However, B is 0.00
If it is less than 1%, its effect is small, and if B exceeds 0.1%, the toughness is lowered. Therefore, when B is added in combination with Ti, the amount of B is preferably in the range of 0.001 to 0.1%.

Cu: Cu is an element effective for improving the stress corrosion resistance, and is therefore added in the second invention of the present application. However, C
If u is less than 0.05%, the effect is not observed, while 0.30%
If it exceeds the range, the hot workability of the Al-Mg alloy containing Mg in a high concentration may be impaired.
The amount added was in the range of 0.05 to 0.30%.

In addition, Zn, Si, etc. are usually contained as impurities in Al alloys, but if Zn exceeds 0.50%, a problem occurs in the corrosion resistance of the weld heat affected zone, and Si is less than 0.
If it exceeds 40%, the amount of Al-Mg-Si eutectic compound increases, and this tends to melt in high heat input welding and cause microcracks in the heat affected zone. Therefore, Zn is 0.50% or less,
It is preferable to control Si to 0.40% or less.

Since the alloy of the present invention is an alloy containing easily oxidizable Mg in a high concentration, it is preferable to add Be in an amount of about 0.0001 to 0.002% in order to prevent molten metal oxidation during melting and casting of the alloy. .

EXAMPLES Alloys having the compositional compositions shown in Alloy Nos. 1 to 6 in Table 1 were
1 to 2 ppm of Be is added and melted, and the thickness is 40 mm depending on the mold.
It was cast into an ingot with a width of 110 mm and a height of 150 mm. Then, after subjecting the ingot to homogenization heat treatment at 460 ° C for 30 hours or more, both sides in the thickness and width directions are 1 mm on each side.
Each was chamfered to a thickness of 38 mm, a width of 108 mm, and a height of 150 mm.
After the ingot after the chamfering is heated at 460 ° C for 1 hour, it is rolled at a pass of 2 mm in one pass by an experimental rolling mill and 460 ° C.
Reheating in the furnace was repeated, hot rolling was performed from an initial thickness of 38 mm to 5 mm, and cold rolling was further performed from 5 mm to 1 mm. After cold-rolling, each plate was subjected to finish annealing at 350 ° C for 5 hours to make O material (soft material), and then various tensile tests of 0.5%, 1% and 2% of permanent strain were made by a tensile tester. Added distortion. A sensitizing treatment of 120 ° C. for 1 week was performed on each of the plates to which the tensile strain was applied and the plate to which the tensile strain was not applied (as it was the O material) to examine the β-phase precipitation distribution state.

As a result, in alloys 1 and 2 of the composition of the present invention, when a permanent strain of 1% or 2% exceeding 0.5% is applied, transition metals such as Zr and V of alloy numbers 4, 5 and 6 are given. In the same manner as in the conventional method in which the β phase is precipitated in the grain by the addition of β, the precipitation of the β phase after the sensitization treatment is uniformly generated in the grain, while the pure Al-Mg binary system is used. In alloy 3 of
It was found that the phases were constantly deposited at the grain boundaries.

FIG. 1 shows a typical example of the β-phase precipitation state of each alloy sheet after the tensile set and the sensitization treatment.

Next, among the alloys 1 to 6 described above, the O material plates of alloys 1 to 3 (without permanent set) and 1
%, 2%, 3%, the plate was stretched, and then subjected to the same sensitization treatment as described above, and then subjected to a stress corrosion cracking test by alternate immersion in salt water using a U-shaped bending test piece. The results are shown in Table 2. The U-shaped bending test piece used in this stress corrosion cracking test had a bending direction of 9 mm and a sampling direction of LT, and a salt water of 30 ° C.
Alternate immersion was repeated for 10 minutes and 50 minutes for drying for 6 months, and the state of cracking after 6 months was examined. [N / of the test results in Table 2
m] was represented by the number of test samples m and the number of crack generation samples n.

As shown in Table 2, in the alloys 1 and 2 having the component composition of the present invention, the sensitizing treatment corresponding to the long-term aging was performed in the state where the O material was subjected to permanent strain of 1% and 2%. Although no stress corrosion cracking was observed at all when applied, A
It is clear that the 1-Mg pure binary alloy 3 is in a state where stress corrosion cracking is likely to occur remarkably.

EFFECTS OF THE INVENTION As is clear from the above examples, according to the method of the present invention, an Al-Mg alloy soft material containing Mg in a high concentration of 5.3 to 9% for the purpose of increasing strength is manufactured. In this case, the β phase was allowed to precipitate uniformly in the grains even with a long-term aging, so that the stress corrosion resistance was sufficiently improved.
An l-Mg alloy soft material can be obtained. Therefore, the Al-Mg alloy soft material obtained by the method of the present invention is
It is suitable for structural materials that require high strength and excellent stress corrosion resistance, such as large structures represented by LNG tanks.

[Brief description of drawings]

FIG. 1 is a photograph of a metallographic cross section showing the state of precipitation of β phase after cold permanent strain imparting and sensitizing treatment of each alloy sheet according to an embodiment of the present invention.

Claims (2)

[Claims] 1. Mg 5.3-9% (weight%, the same applies hereinafter), M
n0.05-1.0%, Cr0.05-0.3%, Ti0.005-0.2%,
An alloy containing Fe 0.25 to 1.00% and the balance Al and unavoidable impurities is used as a raw material, and after rolling and finishing annealing as necessary, a soft material having a required plate thickness is finished, and then stretched. A method for producing an aluminum-magnesium alloy soft material, which comprises imparting a cold working strain of more than 0.5% and 2.0% or less by straightening to improve stress corrosion resistance. 2. Mg 5.3-9% (weight%, the same applies hereinafter), M
n0.05-1.0%, Cr0.05-0.3%, Ti0.005-0.2%,
An alloy containing Fe 0.25 to 1.00% and Cu 0.05 to 0.3% with the balance Al and unavoidable impurities as the raw material. Rolled and finish annealed as necessary to obtain a soft plate with the required thickness. After the material is finished, it is set to 0 with permanent set by tension straightening.
A method for producing an aluminum-magnesium alloy soft material, which comprises applying a cold work strain of more than 5% and not more than 2.0% to improve stress corrosion resistance.