JPH0820833A - Aluminum-magnesium alloy for liquid hydrogen storage, excellent in toughness, and its production - Google Patents

Abstract

PURPOSE:To develop an Al alloy for liquid hydrogen storage, excellent in tough ness at cryogenic temp., by subjecting a continuously cast slab of an Al-Mg alloy containing specific small amounts of Fe and Si to hot rolling and to cold rolling and controlling the grain size and volume fraction of contained Fe- and Si-type insoluble compounds, respectively. CONSTITUTION:A molten Al-Mg alloy, containing, by weight, 3.5-5.5% Mg, <0.5% Fe, <0.5% Si, and one or more kinds among 0.1-0.3% Cr, 0.1-0.8% Mn, and 0.1-0.3% Zr, is cast and then cooled and solidified at R ( deg.C/sec) cooling rate. At this time, Fe and Si are forcedly allowed to enter into solid solution by regulating R to >=5 when the total content of Fe and Si in the Al alloy is <=0.4% and also regulating R to a value defined by R<=7.5 ([Fe]+[Si])+2.5 when the total content of Fe and Si is >=0.4%, and the maximum length and volume fraction of insoluble compound grains containing Fe and Si are controlled to <=1.0mum and <=1.0%, respectively. The resulting ingot is hot-rolled and cold-rolled, by which the Al alloy material excellent in toughness at low temp. and used for a vessel material for storing liquid hydrogen of <=-253 deg.C can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an Al-Mg alloy for liquid hydrogen storage having excellent toughness and a method for producing the same, and in particular, it is suitable as a storage container for liquid hydrogen which is liquefied at an extremely low temperature of -253 ° C or lower. To an Al-Mg based alloy excellent in toughness and a method for producing the same.

[0002]

2. Description of the Related Art JIS is a low temperature aluminum alloy
A standard 5083 alloy has been developed and used as an alloy for LNG tanks. However, even the above 5083 alloy has a problem that the toughness is extremely low in the liquid hydrogen temperature, that is, in the extremely low temperature range of −253 ° C. or lower. This is due to an insoluble compound containing impurities such as Fe and Si remaining during Al refining, and the insoluble compound is solidified in the alloy even if the treatment temperature in the homogenization treatment or the solution treatment is increased. It does not melt and crystallizes in a granular form, which adversely affects the physical properties.

As a means for improving the low temperature toughness of such an Al--Mg type alloy, F, which causes the insoluble compound, is used.
A possible method is to limit the content of impurity elements such as e and Si. However, limiting the contents of Fe and Si, which are inevitable impurities, as much as possible requires Al ingot with extremely high purity, resulting in high cost and poor practicability.

[0004]

The present invention has been made in view of the above circumstances, and is an unavoidable impurity Fe.
Although the content of Al and Si is about the same as the conventional one, by improving the manufacturing method, Al- for liquid hydrogen storage excellent in toughness
It is intended to provide an Mg-based alloy and a method for producing the same.

[0005]

The Al-Mg alloy according to the present invention, which has been able to solve the above-mentioned problems, includes: Mg: 3.5 to 5.5% by weight Fe: 0.5% by weight or less (0 Si: 0.5% by weight or less (not including 0% by weight) and at least one selected from the group consisting of Cr, Mn and Zr, respectively: Cr: 0.1 ~ 0.3 wt% Mn: 0.1 to 0.8 wt% Zr: 0.1 to 0.3 wt% Al alloy with the balance of Al and inevitable impurities, after continuous casting , Hot rolling or hot rolling and cold rolling and heat treatment,
The maximum length of insoluble compound particles containing Fe and Si is 1.0
The gist is that the volume fraction is controlled to be not more than μm and the volume fraction is not more than 1.0%.

As one of the preferred means for producing the above Al-Mg alloy, the present invention is a cooling rate R during solidification.
Is R ≧ 5 (1) R ≧ 7.5 ([Fe] + [Si]) + 2.5 (2) where R: cooling rate during solidification (° C./sec) [Fe], [ [Si]: after continuous casting under conditions satisfying the Fe (Si) content (% by weight) in the Al alloy, immediately after the cast slab is kept at the hot rolling temperature or higher, or at the hot rolling temperature. A method of hot rolling after adjusting is provided. The recommended range represented by the conditional expressions (1) and (2) is such that the value of [Fe] + [Si] is 0.4 as shown in FIG.
When the amount is less than or equal to wt%, the value is calculated according to the formula (1), and
When the value of [Si] is more than 0.4% by weight, it means that it is defined by the equation (2).

[0007]

With the aim of developing an alloy for liquid hydrogen storage, the inventors of the present invention have conducted extensive studies on the problem of improving the toughness of Al-Mg alloys, which have no problem in strength, at low temperatures. Even with an Al-Mg-based alloy containing Fe and Si to some extent, Fe and Si are forcibly solid-solved in the alloy by continuous casting, and the maximum length of insoluble compound particles containing Fe and Si is 1.0 μm or less. Moreover, it has been found that when the volume fraction is controlled to 1.0% or less, the toughness in the cryogenic range can be improved without impairing the strength. Furthermore, in order to control the maximum length and volume fraction of the insoluble compound particles, it is best to control the cooling rate during solidification in the continuous casting process within a certain condition to force Fe and Si to form a solid solution in the alloy. The present invention has been completed by identifying the above method.

First, the reasons for limiting the component composition according to the present invention will be described below. Mg: 3.5 to 5.5 wt% Mg is an element that greatly contributes to the strength improvement of the Al alloy. If the amount of Mg is less than 3.5% by weight, sufficient strength cannot be obtained. On the other hand, if the amount of Mg exceeds 5.5% by weight, the toughness is lowered and continuous casting cannot be performed. Therefore, the amount of Mg is 3.5
It is necessary to be ˜5.5 wt%. The lower limit of Mg is more preferably 3.8% by weight or more, further preferably 4.0% by weight or more, while the upper limit is preferably 5.0.
It is less than or equal to wt%, more preferably less than or equal to 4.5 wt%.

Cr: 0.1 to 0.3% by weight Mn: 0.1 to 0.8% by weight Zr: 0.1 to 0.3% by weight In the Al alloy of the present invention, the above-mentioned three metal elements are used. By incorporating at least one of the above within the above range, A
It is possible to refine the crystal grains of the 1-alloy and improve the toughness. Moreover, the above three kinds of metal elements are combined with Al to form 0.1
The formation of fine precipitates of about 0.5 μm also contributes to the improvement of strength. However, if the content is too small, the effect of improving toughness and strength is not sufficient, and if the content is too large, coarse crystallized substances are formed and the toughness is reduced. Therefore, the amount of Cr is 0.1 to 0.3% by weight, the amount of Mn is 0.1 to 0.8% by weight, and the amount of Zr is 0.1 to 0.3% by weight.
It is necessary to be within the range.

The lower limit of Cr is more preferably 0.15% by weight or more, further preferably 0.2% by weight or more, while the upper limit is preferably 0.25% by weight or less, more preferably 0.23% by weight. It is the following. The lower limit of Mn is more preferably 0.15% by weight or more, further preferably 0.2% by weight or more, while the upper limit is preferably 0.6% by weight or less, more preferably 0.5% by weight or less. Zr
As a lower limit of, more preferably 0.15 wt% or more,
More preferably 0.2 wt% or more, while the upper limit is preferably 0.25 wt% or less, more preferably 0.23 wt%
It is the following.

Fe: 0.5% by weight or less (not including 0% by weight) Si: 0.5% by weight or less (not including 0% by weight) Fe and Si are inevitable impurities remaining during Al refining and are insoluble. Since it forms a compound, it is generally an undesirable element for Al alloys and is usually limited as much as possible. In the present invention, both the amount of Fe and the amount of Si are 0.5
Although up to 0.5% by weight can be tolerated, if it exceeds 0.5% by weight, the insoluble compound is likely to grow, coarse and large amounts are formed, and the toughness is significantly reduced.

In the Al-Mg alloy according to the present invention, it is very important to control the maximum length of insoluble compound grains containing Fe and Si and the volume fraction thereof in order to obtain high toughness. The maximum length of the insoluble compound particles is 1.0
When it is not more than μm and its volume fraction is not more than 1.0%, it exhibits excellent toughness in an extremely low temperature range. The maximum length of the insoluble compound particles is 0.8μ to obtain high toughness.
m or less is preferable, and 0.6 μm or less is more preferable. On the other hand, the volume fraction of the insoluble compound is preferably 0.8% or less,
It is more desirable if it is 0.6% or less.

The maximum length of the insoluble compound grains is, for example, the maximum diameter when a cut surface is formed so as to give the maximum diameter in the case of spherical or disc-shaped crystals, and is a substantially cubic or substantially rectangular parallelepiped crystal. If there is a crystal with an unspecified shape, it means the length of the longest diagonal, if any, and the length between two points on the most distant surface. When measuring the maximum length of the insoluble compound particles, it may be calculated from the microscope field using an electron microscope.

In order to control the maximum length and volume fraction of the insoluble compound particles, Al which satisfies the above-mentioned compositional requirements.
An alloy is used, and the cooling rate R during solidification is the following equations (1) and (2) R ≧ 5 (1) and R ≧ 7.5 ([Fe] + [Si]) + 2.5. (2) However, R: cooling rate during solidification (° C./sec) [Fe], [Si]: continuous casting may be performed under conditions satisfying Fe and Si contents (wt%) in the Al alloy. is important.

That is, in the Al alloy of the present invention, it is cooled rapidly during solidification by the continuous casting method to forcibly dissolve inevitable impurity elements such as Fe and Si to suppress the growth of insoluble compounds which adversely affect toughness. The purpose is to limit the size and amount. If the above conditions are satisfied, Fe and S
The maximum length of insoluble compound particles containing i can be controlled to 1.0 μm or less and the volume fraction can be controlled to 1.0% or less. On the other hand, if the cooling rate during solidification does not satisfy the above conditions, Fe and S
Since the impurity element i cannot be sufficiently forced to form a solid solution, the maximum length of the insoluble compound particles containing Fe and Si and the volume fraction thereof cannot be controlled within the range of the present invention.
Sufficient toughness cannot be obtained.

As described above, the recommended range represented by the conditional expressions (1) and (2) is based on the expression (1) when the value of [Fe] + [Si] is 0.4% by weight or less, and Fe] +
When the value of [Si] is more than 0.4% by weight, it means that it is defined by the equation (2). Thus, A according to the present invention
When the content of unavoidable impurity elements such as Fe and Si is large in the production of the 1-alloy, the cooling rate during solidification in the continuous casting process is increased according to the content, and the maximum length of the insoluble compound particles and It is desirable to control the volume fraction.

The Al alloy of the present invention has the above composition A
It is possible to obtain excellent toughness by controlling the maximum length and volume fraction of the insoluble compound particles containing Fe and Si by continuously casting the alloy with a cooling rate under specific conditions. The production conditions are not particularly limited, but the following production methods can be exemplified.

First, an Al alloy having the alloy composition according to the present invention is used as a melt, and this melt is continuously cast. As the continuous casting method, a water-cooled continuous casting method, a twin roll type continuous casting method, a belt type continuous casting method, a block type continuous casting method, etc. can be adopted, but the transition time from the continuous casting to the hot rolling step It is preferable to adjust the timing after the inside of the slab has fallen below the solidus temperature and has completely solidified.

In the present invention, the temperature of the moving strip obtained by continuous casting is maintained at a hot rolling temperature or higher and hot rolling is immediately performed, and subsequently or once wound, it is sent to a cold rolling process. It is advantageously applied to continuous casting and direct-feed rolling, but in addition to this method, after continuous casting, it is temporarily held and sent to hot rolling before the slab temperature drops substantially, and further cold rolling is performed. Can also be applied.

When hot rolling is performed, the starting temperature is 450.
It is preferable to finish at a finishing temperature of 300 to 350 ° C. In the continuous casting method, a moving strip having a wall thickness of about 4 to 30 mm is usually continuously produced, and hot rolling is performed, and if necessary, cold rolling is performed to obtain a thickness of about 0.1 to 10 mm. It is rolled into a thick Al alloy plate. The rolling reduction is preferably 50% or more.

The hot-rolled or hot-rolled and cold-rolled Al alloy sheet is subjected to solution heat treatment, water quenching, and then an aging treatment. As the conditions for the solution treatment, it is desirable that the treatment temperature is 500 to 550 ° C. and the treatment time is 0.1 to 1 hour.

[0022]

EXAMPLE An Al alloy having the composition shown in Table 1 was used as a melt, a moving strip having a thickness of 20 mm was prepared by a continuous casting method, and immediately hot-rolled to obtain a plate having a thickness of 3 mm. The cooling rate during continuous casting was 10 ° C / sec, the rolling start temperature of the hot rolling was 500 ° C, and the finishing temperature was 330 ° C. The plate material was subjected to solution treatment at 500 ° C. for 1 hour and water-quenched.

The Al alloy plate thus obtained was subjected to a tensile test at a liquid hydrogen temperature to determine the area surrounded by the load-displacement curve and both coordinate axes, that is, the work required for breaking, and the gauge of the test piece was measured. The work volume per unit volume was divided by the volume of the length portion, and the toughness was evaluated by the magnitude of this work volume. The size and volume fraction of the insoluble compound were determined by observation with a scanning electron microscope and image analysis, and the solidification rate during casting was determined by measuring the dendrite arm space. Further, the strength was evaluated by measuring the tensile strength. The results are also shown in Table 1.

[0024]

[Table 1]

No. 1 to 14 are Al-Mg according to the present invention
Since they are all alloys that satisfy the conditions of the present invention, they have sufficient strength and excellent toughness in the cryogenic range.

On the other hand, No. Since Nos. 15 to 24 do not satisfy any one or more of the conditions according to the present invention, the strength or toughness is insufficient. No. Reference numeral 15 is a comparative example in which the alloy composition is within the range of the present invention and the cooling conditions during casting do not satisfy the range of the present invention. Since the cooling rate during solidification is low, the size and volume fraction of the insoluble compound are also present in the present invention. Not satisfying the range and poor toughness. No. 16 is M
This is a comparative example when the amount of g is too small, and the strength is insufficient. No. No. 17 is a comparative example when the amount of Mg is too large, and continuous casting could not be performed. No. 18 is Cr, M
This is a comparative example when both n and Zr are too small, and the strength is insufficient. No. 19 has too much Cr, N
o. No. 20 had too much Mn, and No. No. 21 is a comparative example in the case where Zr is too much, and the size and volume fraction of the insoluble compound do not satisfy the range of the present invention, and the toughness is low. N
o. No. 22 has too much Fe, and No. No. 23 is a comparative example when the amount of Si is too large, and has poor toughness. No. 24
Is manufactured by a conventional continuous casting method using JIS5083 alloy, and the cooling rate during solidification is too low, resulting in poor toughness.

[0027]

The present invention is constituted as described above, and the maximum length of the insoluble compound grains in the Al-Mg alloy is 1.0 μm or less by the continuous casting method, and the volume fraction thereof is 1. Since the content is controlled to be 0% or less, in addition to the high strength property of the Al-Mg alloy, the toughness in the cryogenic range can be improved, and the Al-Mg alloy suitable as a liquid hydrogen storage container is suitable. And a manufacturing method thereof can be provided.

[Brief description of drawings]

FIG. 1 is a graph showing a preferable cooling rate condition during continuous casting used in the manufacturing method of the present invention.

Claims (4)

[Claims] 1. Requirements for Mg: 3.5 to 5.5% by weight Fe: 0.5% by weight or less (not including 0% by weight) Si: 0.5% by weight or less (not including 0% by weight) And at least one selected from the group consisting of Cr, Mn, and Zr, respectively: Cr: 0.1 to 0.3 wt% Mn: 0.1 to 0.8 wt% Zr: 0.1 to 0 In an Al alloy containing 3% by weight and the balance being Al and inevitable impurities, after continuous casting, hot rolling or hot rolling and cold rolling and heat treatment are performed,
The maximum length of insoluble compound particles containing Fe and Si is 1.0
Al-M for liquid hydrogen storage excellent in toughness characterized by controlling the volume fraction to be 1.0 μm or less and the volume fraction to be 1.0% or less
g-based alloy. 2. In producing the Al—Mg alloy according to claim 1, the cooling rate R during solidification is R ≧ 5 and R ≧ 7.5 ([Fe] + [Si]) + 2.5. , R: cooling rate during solidification (° C./sec) [Fe], [Si]: after continuous casting under conditions satisfying the Fe and Si contents (% by weight) in the Al alloy, A method for producing an Al-Mg-based alloy for liquid hydrogen storage excellent in toughness, which is characterized by holding at a hot rolling temperature or higher and performing hot rolling. 3. The method according to claim 2, wherein the continuously cast moving strip is immediately sent to the hot rolling step. 4. The manufacturing method according to claim 2, wherein the continuously cast slab is adjusted to a hot rolling temperature and sent to a hot rolling step.