JP2017082304A - Aluminum alloy structural member having excellent impact resistance in cryogenic range - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy structural member having excellent impact resistance in a cryogenic range.SOLUTION: The present invention provides an aluminum alloy structural member having excellent impact resistance in a cryogenic range, the aluminum alloy structural member containing Mg: 0.3-5.5 mass%, Mn: 0.01-1.0 mass%, and Ti: 0.001-0.1 mass%, satisfying the relational expression [Mg]≤-7.7[Mn]+8.0 with the Mg content and Mn content defined as [Mg] and [Mn], with the balance being Al and inevitable impurities, and having a conductivity: 27%IACS or more.SELECTED DRAWING: None

Description

  The present invention relates to an aluminum alloy structural member having excellent impact resistance in a cryogenic temperature range.

  As a low temperature aluminum (Al) alloy, JIS standard 5083 alloy has been developed and used as an alloy for liquefied natural gas (LNG) (liquefaction temperature: -162 ° C.) tanks. However, even the 5083 alloy has a problem that the impact resistance in the extremely low temperature region of liquid hydrogen temperature (hydrogen liquefaction temperature), that is, −253 ° C. or lower, is extremely low. This problem is that insoluble compounds containing impurities such as Fe and Si remaining during Al refining do not form solid solutions in the alloy even when the treatment temperature in the homogenization treatment or solution treatment is increased, and crystallize into particles. Caused by adversely affecting Note that the cryogenic temperature range in this specification refers to a temperature range of −253 ° C. or lower (that is, a temperature range from −253 ° C. to −273.15 ° C., which is absolute zero).

  As a means for improving the impact resistance of such an Al—Mg-based alloy in a cryogenic temperature region, it is conceivable to limit the content of impurity elements such as Fe and Si that cause insoluble compounds as much as possible. However, limiting the contents of Fe and Si, which are inevitable impurities, as much as possible, that is, requiring Al metal with extremely high purity, is costly and lacks practicality. Under such circumstances, for example, Patent Document 1 proposes an invention for improving the impact resistance of an Al—Mg-based alloy in a cryogenic temperature region.

  In Patent Document 1, Mg: 3.5 to 5.5% by mass, Fe: 0.5% by mass or less, Si: 0.5% by mass or less are satisfied, and Cr, Mn, and Zr are included. There is described an Al—Mg-based alloy containing at least one selected type within a specified range, with the balance being Al and inevitable impurities. This Al-Mg alloy is subjected to hot rolling or hot rolling and cold rolling and heat treatment after continuous casting, and the maximum length of insoluble compound grains containing Fe and Si is 1.0 μm or less, and Volume fraction is controlled to 1.0% or less.

JP-A-8-20833

  The invention described in Patent Document 1 improves impact resistance in a cryogenic temperature region by reducing the amount of crystallized material. However, in the future, the amount of liquid hydrogen transported and stored is expected to increase, and the development of an aluminum alloy structural member that further improves impact resistance in the cryogenic temperature region is desired.

  This invention is made | formed in view of the said condition, and makes it a subject to provide the aluminum alloy structural member excellent in the impact resistance in a cryogenic region.

The aluminum alloy structural member excellent in impact resistance in the cryogenic temperature range according to the present invention is Mg: 0.3 to 5.5% by mass, Mn: 0.01 to 1.0% by mass, Ti: 0.001 to 0.1% by mass, and when Mg content and Mn content are [Mg] and [Mn] respectively, satisfy the relational expression [Mg] ≦ −7.7 [Mn] +8.0, and the balance Is made of Al and inevitable impurities, and has an electrical conductivity of 27% IACS or higher.
As described above, according to the present invention, since the chemical component and the electrical conductivity are set as specific conditions, an aluminum alloy structural member excellent in impact resistance in a cryogenic temperature region can be provided.

In the aluminum alloy structural member excellent in impact resistance in the cryogenic temperature region according to the present invention, the cryogenic structural member is preferably a rolled material, a forged material or an extruded material.
As described above, the aluminum alloy structural member having excellent impact resistance in the cryogenic temperature range according to the present invention is excellent in impact resistance in the cryogenic temperature range. It can be suitably used for a forging material or an extruded material.

  ADVANTAGE OF THE INVENTION According to this invention, the aluminum alloy structural member excellent in the impact resistance in a cryogenic region can be provided.

It is explanatory drawing explaining the test piece used for the Charpy impact test. It is a graph which shows the relationship between electrical conductivity and the impact absorption energy in about -173 degreeC. In the figure, the horizontal axis represents conductivity (% IACS), and the vertical axis represents impact absorption energy (J) at about −173 ° C. It is a graph which shows the relationship between the amount of Mg and the amount of Mn about the Example whose impact absorption energy in about -173 degreeC becomes 55 J or more, and the comparative example which is not so. In the figure, the horizontal axis represents the amount of Mn (mass%), and the vertical axis represents the amount of Mg (mass%).

  Hereinafter, an embodiment of an aluminum alloy structural member excellent in impact resistance in a cryogenic temperature range according to the present invention will be described in detail. In the following description, “aluminum alloy” may be described as “Al alloy”, and “aluminum alloy structural member excellent in impact resistance in a cryogenic temperature region” is described as “Al alloy material”. There is also.

The Al alloy material according to the present invention contains Mg: 0.3 to 5.5% by mass, Mn: 0.01 to 1.0% by mass, Ti: 0.001 to 0.1% by mass, and Mg contained When the amount and the amount of Mn are [Mg] and [Mn], respectively, the relational expression [Mg] ≦ −7.7 [Mn] +8.0 is satisfied, and the balance is made of Al and inevitable impurities.
Further, the Al alloy material according to the present invention has a conductivity of 27% IACS (International Annealed Copper Standard) or more.
Hereinafter, these components in the Al alloy material according to the present invention will be described in detail.

(Mg: 0.3-5.5% by mass)
Mg is an element that greatly contributes to improving the strength of the Al alloy material. If the amount of Mg is less than 0.3% by mass, sufficient strength cannot be obtained. Here, sufficient strength means that the proof stress value at room temperature is 65 MPa or more. That is, if the amount of Mg is less than 0.3% by mass, the effect of improving the yield strength at room temperature cannot be obtained sufficiently. On the other hand, when the amount of Mg exceeds 5.0% by mass, impact resistance in a cryogenic temperature range is lowered, and an impact absorption energy of 55 J or more can be obtained at about −173 ° C. (−173 ± 2 ° C.). Not only can it be done, but workability will also be reduced. Therefore, the Mg amount is set to 0.3 to 5.5% by mass. The Mg content is preferably 1% by mass or more, more preferably 2% by mass or more from the viewpoint of improving the yield strength at room temperature. In addition, the Mg amount is preferably 5% by mass or less, more preferably 4.5% by mass or less, from the viewpoint of preventing a decrease in impact resistance in a cryogenic region.

(Mn: 0.01 to 1.0% by mass)
Mn is an element that refines the crystal grain of the Al alloy and improves the impact resistance in the cryogenic temperature range. Mn is an element that contributes to the refinement of crystal grains by forming fine dispersed particles of about 0.1 to 0.5 μm by combining with Al. When the amount of Mn is less than 0.01% by mass, it is difficult to make crystal grains fine, and the effect of improving the impact resistance in a cryogenic temperature region and the yield strength at room temperature cannot be sufficiently obtained. On the other hand, when the amount of Mn exceeds 1.0 mass%, a coarse crystallized product is formed, workability is lowered, and impact resistance in a cryogenic region is lowered. Therefore, the amount of Mn is set to 0.01 to 1.0% by mass. The amount of Mn is preferably 0.05% by mass or more and more preferably 0.1% by mass or more from the viewpoint of improving impact resistance in a cryogenic temperature region and yield strength at room temperature. Further, the amount of Mn is preferably 0.7% by mass or less, and more preferably 0.6% by mass or less from the viewpoint of preventing a decrease in impact resistance in a cryogenic region.

(Ti: 0.001 to 0.1% by mass)
Ti is an element that refines crystal grains and improves strength (tensile strength and yield strength at room temperature) and impact resistance in a cryogenic temperature range. When the amount of Ti is less than 0.001% by mass, the crystal grains of the ingot are coarsened, and the effect of improving the strength and impact resistance in a cryogenic region cannot be sufficiently obtained. Moreover, a crack generate | occur | produces at the time of hot processing, and subsequent processing cannot be continued. On the other hand, if the amount of Ti exceeds 0.1% by mass, a coarse crystallized product will be formed. On the other hand, the impact resistance in the cryogenic temperature range is lowered, and the workability of the Al alloy material is reduced. There is a risk of adversely affecting the above. Therefore, the Ti amount is set to 0.001 to 0.1% by mass. The Ti content is preferably 0.002% by mass or more, and more preferably 0.005% by mass or more from the viewpoint of improving strength and Charpy impact characteristics in a cryogenic region. Further, the amount of Ti is preferably 0.08% by mass or less from the viewpoint of preventing a decrease in impact resistance in a cryogenic region and the workability of the Al alloy material, and is 0.05% by mass or less. More preferably.

([Mg] ≦ −7.7 [Mn] +8.0)
Even if Mg and Mn are contained in the respective ranges described above, if the amount of Mg and the amount of Mn are large, the effect of excellent impact resistance in the cryogenic temperature range exhibited by the present invention may not be achieved. As a result of researches by the present inventors, it has been found that the effect of excellent impact resistance in a cryogenic temperature region can be surely exhibited when Mg and Mn satisfy the following relational expression (1). In the following relational expression, [Mg] is the amount of Mg contained in the Al alloy, and [Mn] is the amount of Mn contained in the Al alloy. The derivation of the following relational expression will be described in [Example] described later.

[Mg] ≦ −7.7 [Mn] +8.0 (1)

  The relationship between the amount of Mg and the amount of Mn is that the amount of Mg and the amount of Mn contained in the Al alloy are in the ranges described above, and the amount of Mg contained in the Al alloy ([Mg]) is −7.7 [Mn]. If it is +8.0 or less, the effect of excellent impact resistance in a cryogenic temperature region can be obtained, and the amount of Mg contained in the Al alloy ([Mg]) is −7.7 [Mn] +8. If it is greater than 0, the effect of excellent impact resistance in a cryogenic temperature region cannot be achieved.

(Remainder)
The balance is Al and inevitable impurities. Inevitable impurities are impurities that are inevitably mixed when the material is dissolved. Basically, it is preferable not to contain them, but it is contained within a range that does not impair the effect of excellent impact resistance in the cryogenic temperature region according to the present invention. It is permissible to do. Examples of the inevitable impurities in the present invention include Fe, Cr, Si, Zr, Zn, and Cu. In the present invention, it is acceptable if Fe is 0.35% by mass or less, Cr is 0.25% by mass or less, Si is 0.35% by mass or less, and Zr is 0.15% by mass or less. When Fe, Cr, Si, and Zr each exceed the above upper limit value, coarse crystallized substances are formed, which lowers the formability and lowers the impact resistance in the cryogenic temperature range. In addition, in order to suppress the coarsening of the crystal grain of an ingot, it is desirable to make Fe, Cr, Si, and Zr each 0.10 mass% or less. In the present invention, Zn is acceptable as long as it is 0.1% by mass or less. If Zn exceeds the upper limit, the strength may be too high, which may adversely affect the workability of the Al alloy material according to the present invention. Moreover, when Zn exceeds the said upper limit, there exists a possibility that corrosion resistance and weldability may become too low. Furthermore, in the present invention, Cu is acceptable if it is 0.2 mass% or less. When Cu exceeds the upper limit, corrosion resistance and weldability may be deteriorated. Inevitable impurities may include V, Hf, Sc, etc. in addition to the above.

(Conductivity: 27% IACS or higher)
The higher the Al ratio in the matrix, the higher the impact resistance in the cryogenic range. For example, in an Al-Mg alloy, the electrical conductivity changes (increases / decreases) according to the amount of Mg added and the precipitation state of the Mg-based precipitate (β phase). That is, when the amount of Mg added is large, the solid solubility of Mg in the matrix phase increases and the conductivity of the Al alloy material decreases.
Moreover, when there is much precipitation amount of Mg type | system | group deposit, since the solid solubility of Mg in a mother phase will become small and the ratio of Al in a mother phase will become high, the electrical conductivity of Al alloy material will become high. That is, the higher the electrical conductivity of the Al alloy material, the higher the impact resistance in the cryogenic temperature region.
If the amount of Mg-based precipitates is small, the solid solubility of Mg in the matrix phase increases, and the Al ratio in the matrix phase decreases, so the conductivity of the Al alloy material decreases. In this case, the impact resistance of the Al alloy material in the cryogenic temperature region is lowered.

In the present invention, by setting the electrical conductivity of the Al alloy material to 27% IACS or more, the impact resistance in the cryogenic temperature region can be made excellent. If the electrical conductivity of the Al alloy material is less than 27% IACS, the impact resistance in the cryogenic temperature region cannot be made excellent.
Note that, from the viewpoint of further improving the impact resistance in the cryogenic temperature region, the electrical conductivity of the Al alloy material is preferably 28% IACS or more, and more preferably 30% IACS or more.

  As a method for setting the electrical conductivity of the Al alloy material to 27% IACS or more, for example, annealing is performed after hot rolling or forging, and cooling after annealing is performed more slowly than usual. For example, after annealing, the cooling rate to 120 ° C. (average cooling rate) may be 40 ° C./hr or less. If the cooling rate to 120 ° C. is 40 ° C./hr or less after annealing, for example, the relational expression [Mg] ≦ −7.7 [Mn] +8.0 can be satisfied. In the present invention, it is only necessary to increase the amount of Mg-based precipitates, and the present invention is not limited to this.

  As described above, since the Al alloy material according to the present invention is excellent in impact resistance in a cryogenic temperature region, it is preferably used for a cryogenic structural member. Examples of the cryogenic structural member include a tank, a pipe, or a joining member for accommodating a cryogenic substance such as liquid hydrogen, liquid nitrogen, LNG, and liquid helium. In addition, a rolling material, a forging material, or an extrusion material can be used suitably for the structural member excellent in the impact resistance in a cryogenic region.

  The Al alloy material according to the present invention can be manufactured with general equipment. For example, the raw material is melted in a melting furnace and adjusted to the above-described chemical composition Al alloy, and then the ingot is cast with a casting apparatus. Thereafter, the ingot is subjected to surface grinding and homogenization heat treatment, and heating is performed as necessary. Subsequently, it can obtain by performing hot rolling, forging, or extrusion using the ingot which performed these processes, and also performing annealing. Hot rolling, forging or extrusion can be repeated, and these steps may be combined. Moreover, you may anneal between these processes. In the present invention, the cooling rate after annealing is set to 40 ° C./hr or less as described above. The product thus obtained is, for example, a structural member (rolled material, forged material, or extruded material) that is excellent in impact resistance in the cryogenic region described above.

  In the production of the Al alloy material according to the present invention, it can be performed under general conditions except for the cooling rate after annealing. For example, the raw material may be melted as long as the raw material can be melted, for example, a molten metal temperature of 750 to 800 ° C. The temperature of the homogenization heat treatment can be performed at 450 to 550 ° C. Moreover, the temperature of a hot rolling can be performed at 300-520 degreeC, the forging can be performed at 300-520 degreeC, and the temperature of an extrusion process can be performed at 300-520 degreeC. Note that the hot rolling, the forging, and the extrusion can all be 50% or more. Furthermore, the holding temperature of annealing can be 300-450 degreeC.

The Al alloy material according to the present invention described above is excellent in impact resistance in a cryogenic temperature range because the chemical composition and the electrical conductivity are set as specific conditions.
Moreover, since the Al alloy material according to the present invention is excellent in impact resistance in a cryogenic region, it is suitable as a structural member for cryogenic use (rolled material, forged material or extruded material). Examples of the cryogenic structural member include a tank, a pipe, or a joining member for transporting and storing liquid hydrogen. In addition, since the Al alloy material according to the present invention is excellent in impact resistance in a cryogenic temperature range (−253 ° C. or lower), a tank for carrying and storing liquid nitrogen, LNG, etc. having a higher temperature than this. It can also be suitably used for pipes or joining members.

  Next, this invention is demonstrated based on an Example. In addition, this invention is not limited to the Example shown below.

  No. in Table 1 Ingots were produced by casting Al alloys according to 1-30. Thereafter, homogenization heat treatment was performed at 450 to 540 ° C., forging was performed at a workability of 360 to 420 ° C. and 50 to 75%, and annealing was performed at a holding temperature of 345 ° C. The cooling rate after annealing (average cooling rate) was performed under the conditions shown in Table 1. Forged materials of Al alloys according to 1 to 29 were produced. In addition, No. Since the Al alloy according to 30 was cracked during forging, it was not possible to produce a forged material.

Since the metal properties of the Al alloy change with time, in this study, the manufactured No. The forging materials according to 1 to 29 were subjected to accelerated aging treatment. The accelerated aging treatment was carried out at 120 ° C. for 7 days.
No. for which accelerated aging treatment was performed. For the forged materials according to 1 to 29, the conductivity was measured and the impact resistance at about -173 ° C was evaluated as follows. No. The yield strength at room temperature of the forgings according to 1 to 29 was measured.

(Measurement of conductivity)
The conductivity was measured using an eddy current type measuring instrument (manufactured by Forster Co., Ltd., trade name: Sigma Tester). No. A sample of a predetermined size was cut out from the forging material according to 1 to 29, and the measurement surface was cut. Then, the measurement surface was made into a mirror surface state with emery paper, the probe attached to the measuring instrument was brought into contact with the measurement surface, and the conductivity was measured. The sample temperature was measured at 5 points at 20 ° C., and the average value thereof was defined as the conductivity of the sample.

(Evaluation of impact resistance at about -173 ° C)
According to the Charpy impact test (JIS Z 2242: 2005), evaluation of Charpy impact characteristics at about -173 ° C. (measurement of impact absorption energy) was performed. In the Charpy impact test, a V-notch test piece (see FIG. 1) was used. As shown in FIG. 1, the V-notch test piece 1 was collected from the forging material so that the direction of the V-notch 2 was parallel to the direction of the forging line. In the case of an extruded material, the direction of extrusion is made parallel to the direction of the V notch, and in the case of a rolled material, the direction of rolling is taken so as to be parallel to the direction of the V notch. After immersing the V-notch test piece 1 in liquid nitrogen for 10 minutes or more, the test piece 1 was taken out, the test piece 1 was placed on a platform of a testing machine, and a hammer was shaken down to perform the test. The temperature of the test piece 1 when the hammer collides with the test piece 1 is about −190 ° C. 5 seconds after the test piece 1 is taken out from liquid nitrogen, and the temperature when the hammer is made to collide after 15 seconds is about − It was 173 ° C.

As a result of prior studies using a low Mg low Mn alloy and a JIS standard 5083 alloy, as a characteristic of a forging material of an Al alloy, if the impact absorption energy at about −173 ° C. is 55 J or more, liquid hydrogen in a very low temperature region It was speculated that sufficient shock absorption energy (for example, 10 J or more) can be secured even at a temperature of −253 ° C.
Therefore, as described above, after immersing the test piece in liquid nitrogen (temperature: -196 ° C) and cooling it sufficiently, the cooled test piece is taken out from the liquid nitrogen, and the test piece is placed in a Charpy impact tester. When the temperature of the test piece reached about −173 ° C., the hammer was shaken down and the impact absorption energy was measured.
The impact absorption energy at about −173 ° C. (average value of 3 test pieces) was 55 J or more, and it was evaluated that the impact resistance in the cryogenic temperature range was excellent. On the other hand, the impact absorption energy at about −173 ° C. (average value of three test pieces) was less than 55 J, and it was evaluated that the impact resistance in an extremely low temperature range was inferior.

(Measurement of tensile properties)
A No. 4 test piece was prepared according to JIS Z 2241: 2011, and the tensile properties were measured at room temperature. An average value of the proof stress of three test pieces was 65 MPa or more, and less than 65 MPa was rejected. The No. 4 test piece was sampled so that the longitudinal direction of the tensile test piece was perpendicular to the direction of the forging line (in the case of a rolled material, sampled to be perpendicular to the rolling direction and extruded. If it is a material, it is collected so as to be perpendicular to the extrusion direction).

  No. The chemical composition (mass%) of the Al alloy according to 1 to 29, whether or not the relational expression [Mg] ≦ −7.7 [Mn] +8.0 is satisfied, the average cooling rate from the annealing temperature to 120 ° C. (° C. / hr) (simply referred to as “cooling rate” in Table 1), conductivity (% IACS) and impact absorption energy (J) at about −173 ° C. are shown in Table 1. Table 1 also shows the yield strength (MPa) at room temperature. In addition, the underline in Table 1 indicates that the requirement of the present invention is not satisfied, and “−” indicates that cracking occurred during forging, and the forging material could not be manufactured, so measurement and evaluation could not be performed. It shows that.

As shown in Table 1, no. Since the forging materials according to 1 to 13 satisfied the requirements of the present invention, the impact absorption energy at about −173 ° C. was 55 J or more, and the impact resistance in the cryogenic temperature range was excellent (all examples). .
In contrast, no. Since the forged materials according to 14 to 27 did not satisfy the requirements of the present invention, the impact absorption energy at about −173 ° C. was less than 55 J, and the impact resistance in the cryogenic temperature range was inferior (all are comparative examples) ). No. In the forging material according to No. 28, the amount of Mg does not satisfy the requirements of the present invention. Since the forging material according to No. 29 did not satisfy the requirements of the present invention for Mn, the yield strength at room temperature was low (all were comparative examples).

Here, from the result of this study, a correlation was found between the conductivity and the impact absorption energy at about −173 ° C., and the graph is shown in FIG.
As shown in FIG. 2, it was confirmed that the higher the conductivity, the higher the impact absorption energy at about −173 ° C.

In addition, when Mg is contained in an amount of 0.3 to 5.5% by mass and Mn is contained in an amount of 0.01 to 1.0% by mass, if the amount of Mg and the amount of Mn are large, an impact at about −173 ° C. The absorbed energy may be less than 55J. That is, it has been found that the effect of excellent impact resistance in the cryogenic temperature range of the present invention may not be achieved. The graph is shown in FIG. FIG. 3 shows an example in which the impact absorption energy at about −173 ° C. is 55 J or more (“◯” in FIG. 3) and a comparative example (“ X ") is plotted, and these are distinguished.
As shown in FIG. 3, it is confirmed that whether or not the desired effect of the present invention is achieved can be distinguished by satisfying the relational expression [Mg] ≦ −7.0 [Mn] +8.0. It was done. [Mg] is the amount of Mg (mass%) contained in the Al alloy, and [Mn] is the amount of Mn (mass%) contained in the Al alloy.

  As mentioned above, although the aluminum alloy structural member excellent in impact resistance in the cryogenic temperature range according to the present invention has been specifically described by the embodiments and examples, the gist of the present invention is not limited to these.

Claims (2)

Mg: 0.3 to 5.5% by mass,
Mn: 0.01 to 1.0% by mass,
Ti: 0.001 to 0.1% by mass,
When the amount of Mg and the amount of Mn contained are [Mg] and [Mn], respectively, the relational expression [Mg] ≦ −7.7 [Mn] +8.0 is satisfied,
The balance consists of Al and inevitable impurities,
Electrical conductivity: 27% IACS or more An aluminum alloy structural member excellent in impact resistance in a cryogenic temperature region.   The aluminum alloy structural member having excellent impact resistance in a cryogenic temperature range according to claim 1, which is a rolled material, a forged material or an extruded material.

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