JP2010144186A - High-strength aluminum alloy for lng spherical tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy for an LNG (Liquefied Natural Gas) spherical tank having excellent strength without remarkably reducing its intergranular corrosion resistance. <P>SOLUTION: An ingot having a composition comprising 5.0 to 6.0% Mg, 0.5 to 1.0% Mn, 0.05 to 0.25% Cr and 0.05 to 0.5% Cu, and in which, as impurities, the content of Si is regulated to <0.25% and the content of Fe is regulated to <0.25%, respectively, and the balance Al with the other inevitable impurities is heated at 460 to 540°C for 1 to 24 hr as homogenizing treatment, thereafter, hot rolling is started at ≥400°C, so as to be a prescribed sheet thickness by the hot rolling, and annealing is performed at 340 to 420°C for ≥1 hr, so as to have a TS (tensile strength) of ≥320 MPa. Further, 0.05 to 0.35% Zn may be incorporated therein, and, it is also possible that, by controlling hot rolling finishing temperature to ≥340°C, annealing treatment may be obviated. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

    The present invention relates to a method for producing an Al—Mg—Mn—Cr—Cu non-heat-treatable aluminum alloy. An excellent aluminum alloy for a LNG spherical tank is obtained.

  As an alloy for an LNG spherical tank, an Al—Mg—Mn—Cr alloy is used as represented by 5083. 5083 alloy has high strength and high corrosion resistance as a non-heat-treatable alloy in aluminum alloy, and is used for welding structures such as ships, vehicles, low temperature tanks, pressure vessels, etc. in addition to LNG spherical tanks.

  The LNG spherical tank is required to be enlarged with an increase in capacity, but the current standard value (TS: 275 MPa or more, YS: 127 MPa or more) requires an increase in the plate thickness and the tank weight also increases. For this reason, there is a need to reduce the tank weight by increasing the strength. As a policy for increasing the strength, it has been found that it is effective to improve the tensile strength of the current 5083 material from calculation of allowable stress with respect to film stress.

  However, in order to further increase the strength and reduce the weight with this Al-Mg alloy, it is known that the addition of Mg is effective, but at the same time, it also includes SCC (stress corrosion cracking) resistance characteristics. It is known that corrosion resistance decreases. In particular, this alloy is often used for ships or LNG tanks, and the occurrence of SCC is a major safety issue.

Therefore, the addition of Zn has been proposed for the purpose of reducing the β phase (Al ₃ Mg ₂ ) generated at the grain boundaries of the high Mg content alloy. For example, Patent Document 1 adds 0.4 to 0.9% Zn. Thus, the SCC resistance and the layered corrosion resistance are improved by lowering the volume fraction of the β phase at the grain boundary.

However, Patent Document 1 mentions that Zr is added in an amount of 0.05 to 0.25% to increase the strength and improve the crack resistance during welding. However, Al-Mg alloy is recrystallized during hot rolling. It is known that the interface between the Al—Zr-based precipitate (Al ₃ Zr) and the matrix becomes inconsistent with recrystallization. Precipitates such as MgZn ₂ that are formed when the β-phase that precipitates later and a large amount of Zn are contained are likely to be generated at the non-matching interface. In addition, the addition of Zn has the effect of lowering the volume fraction of the β phase at the grain boundary, but when the added amount of Zn increases, it changes to the β phase and changes between Al-Zn-Mg compounds, MgZn ₂ and other metals. It has been found that since the compound appears at the grain boundary, it is difficult to obtain a great effect of intergranular corrosion.

For this reason, the present invention further examined the influence of components on mechanical properties and corrosion resistance.
Japanese Patent No. 3262278

  The present invention provides an Al-Mg-Mn-Cr-Cu-based aluminum-based alloy that provides a high-strength aluminum alloy for LNG spherical tanks that does not significantly reduce intergranular corrosion.

  In order to solve the above problems, in claim 1, Mg: 5.0 to 6.0%, Mn: 0.5 to 1.0%, Cr: 0.05 to 0.25%, Cu: 0 0.05% to 0.5%, Si as an impurity is controlled to less than 0.25%, Fe is controlled to less than 0.25%, and the balance is Al and other inevitable impurities, and TS (tensile strength) is It is set as the high intensity | strength aluminum alloy for LNG spherical tanks characterized by being 320 Mpa or more.

  The alloy according to claim 2 is the alloy according to claim 1 further containing Zn: 0.05 to 0.35%.

  In Claim 3, as a manufacturing method of the high intensity | strength aluminum alloy for LNG spherical tanks of Claims 1 and 2, after heating for 1 to 24 hours at 460-540 degreeC as a homogenization process to an ingot, it is 400 It prescribes that hot rolling is started at a temperature of not lower than 0 ° C., a predetermined thickness is obtained by hot rolling, and annealing is performed at a temperature of 340 to 420 ° C. for 1 hour or longer.

  According to a fourth aspect of the present invention, the annealing treatment is omitted by setting the hot rolling end temperature to 340 ° C. or higher.

  It is possible to obtain a high-strength high-strength aluminum alloy for LNG spherical tanks without greatly reducing the intergranular corrosion properties.

  The reasons for limitation of the present invention will be described from the above examination results.

  First, the reasons for limiting the alloy composition will be described.

  Mg is an element that improves the strength by dissolving in aluminum in this alloy. If the addition amount is less than 5.0%, the target strength of the present invention cannot be obtained. If the addition amount exceeds 6.0%, hot rolling cracks are likely to occur during industrial production, making it difficult.

  Mn, which was forcibly dissolved during casting, formed a fine Al-Mn compound in the homogenization process and hot rolling process and contributed to strength improvement as dispersion strengthening. At the same time, recrystallized grains were refined in this alloy. Work as. If the added amount is less than 0.5%, it is not sufficient, and if it exceeds 1.0%, the effect is saturated and a large intermetallic compound is easily formed during solidification.

  Cr forms fine intermetallic compounds in the matrix and acts as a refinement of the crystal grain size. The effect is insufficient if it is less than 0.05%, and if it exceeds 0.25%, a huge intermetallic compound is likely to be formed during solidification as in the case of Mn.

Cu, like Mg, has a function of increasing the strength by dissolving in aluminum. Furthermore, a part of the grain boundary β phase (Al ₃ Mg ₂ ), which deteriorates the corrosion resistance of the Al—Mg alloy, changes to an AlMgCu compound, thereby dividing the grain boundary β phase and improving the grain boundary corrosion resistance. There is. If the effect is less than 0.05%, it is not sufficient, and if it exceeds 0.5%, the effect is saturated and the hot rolling property is deteriorated.

Zn, like Cu, has a function of substituting a part of the grain boundary β phase (Al ₃ Mg ₂ ) of the Al—Mg alloy with an AlZnMg compound to improve grain boundary corrosion. The effect is not sufficient if it is less than 0.05%, and if it exceeds 0.35%, the effect is saturated and at the same time, preferentially corrodes easily.

  Fe and Si are impurity elements inevitably contained in an industrial aluminum alloy, but if both are less than 0.25%, the characteristics are not impaired.

Although not particularly specified, a finer containing TiB ₂ is generally added in an amount of about 0.01 to 0.15% in terms of Ti for the purpose of crystal grain refinement during casting.

  Other than the above, it consists of Al and other inevitable impurities.

  Next, manufacturing conditions will be described.

  The homogenization treatment has the purpose of improving the microsegregation of the components generated during solidification and at the same time finely and uniformly dispersing Mn, Cr, etc. in the aluminum. The temperature is preferably 460 to 540 ° C., and the holding time is industrially sufficiently achieved if it is industrially 1 hr or more and 24 hr or less. In addition, although not particularly specified, a two-stage process in which the temperature is kept at a temperature lower than 450 ° C. which is the melting temperature of the β phase at the time of homogenizing the Al—Mg alloy, and the temperature is increased after sufficiently dissolving and diffusing. Alternatively, a multistage heating pattern is often used.

  Regarding the hot rolling temperature, it is necessary to select a temperature at which the rolling process can be performed uniformly industrially. If it is less than 400 ° C., the deformation resistance increases and the rolling load becomes too high, so that it is difficult to roll a material having a wide plate width. In particular, the upper limit temperature is not stipulated, but at high temperatures close to the melting point, the grain boundary becomes weaker than that of the matrix, and cracking may occur during hot rolling. For this reason, it is preferable to start hot rolling at 440-520 degreeC.

  The LNG spherical tank material is manufactured in a completely annealed state (so-called O material tempering) because it is bent to assemble it into a spherical tank. As final annealing conditions, desired characteristics can be obtained by holding at 340 to 420 ° C. for 1 hour or more.

  The spherical tank material is a hot-rolled sheet with a thickness of 20 mm or more. When the temperature at the end of hot-rolling is 340 ° C or more, complete annealing is performed as in the case of final annealing by self-heating. Since mechanical characteristics equivalent to those of the material can be obtained, desired characteristics can be obtained without performing final annealing again.

  An ingot of t70xw200xL180mm is produced by DC casting of 11 types of Al-Mg alloys shown in Table 1, and then homogenized and hot-rolled under the production conditions shown in Table 2 to obtain a plate with a final thickness of 20 mm Was made. These plate materials were subjected to an annealing treatment of 360 ° C. × 2 hr as a complete annealing treatment (O material refining).

  As a characteristic evaluation of the obtained plate material, a round bar tensile test piece having a diameter of 6 mm was taken from the direction perpendicular to the rolling direction (LT), and the mechanical characteristics were investigated. Further, as a corrosion resistance evaluation, a grain boundary corrosion test was performed in accordance with ASTM G67, and the weight loss was measured.

  Table 3 shows the results of characteristic evaluation. From this, it can be seen that in the present invention, high tensile strength of 320 MPa or more is obtained and sufficient intergranular corrosion properties are obtained. No. No. 11 has a high strength, but the intergranular corrosive deterioration is recognized.

Claims (4)

Mg: 5.0 to 6.0% (mass%, the same applies hereinafter), Mn: 0.5 to 1.0%, Cr: 0.05 to 0.25%, Cu: 0.05 to 0.5% Characterized in that Si as an impurity is controlled to less than 0.25%, Fe is controlled to less than 0.25%, the balance is Al and other inevitable impurities, and TS (tensile strength) is 320 MPa or more. High strength aluminum alloy for LNG spherical tank. The high strength aluminum alloy for LNG spherical tank according to claim 1, further comprising Zn: 0.05 to 0.35%. The ingot is heated at 460 to 540 ° C. for 1 to 24 hours as a homogenization treatment, and then hot rolling is started at 400 ° C. or more, and a predetermined plate thickness is obtained by hot rolling, and a temperature of 340 to 420 ° C. The method for producing a high-strength aluminum alloy for an LNG spherical tank according to claim 1, wherein annealing is performed for 1 hour or more. After the ingot is heated at 460 to 540 ° C. for 1 to 24 hours as a homogenization treatment, hot rolling is started at 400 ° C. or higher, and the hot rolling is completed to obtain a predetermined plate thickness. The method for producing a high-strength aluminum alloy for an LNG spherical tank according to claim 1, wherein the annealing treatment is omitted by setting the temperature to 340 ° C or higher.