HIGH-STRENGTH 6000-BASED ALLOY THICK PLATE HAVING UNIFORM STRENGTH IN PLATE THICKNESS DIRECTION AND METHOD FOR MANUFACTURING THE SAME

Abstract

The present invention relates to a high-strength aluminum alloy thick plate composed of an aluminum alloy including a prescribed quantity of Si, Mg, Ti, Fe, and the balance Al. The thick plate has a material structure in which an area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in a plate thickness central portion is 0.45% or less; and an area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in a region of 20 mm±1.5 mm from a plate surface in a plate thickness direction is 1.2 times or more and 3.0 times or less the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in the plate thickness central portion. The aluminum alloy thick plate has sufficient strength and good uniformity of strength in the plate thickness direction, and can be manufactured by cooling it after a solution treatment, so that suitable temperature difference occurs between a plate thickness central portion and a surface, and then performing a quenching treatment.

Claims

1. A high-strength aluminum alloy thick plate composed of an aluminum alloy comprising Si: 0.2 to 1.2 mass % (hereinafter, denoted by %), Mg: 0.2 to 1.5%, Ti: 0.005 to 0.15%, Fe: 1.0% or less, and the balance Al and inevitable impurities, wherein the high-strength aluminum alloy thick plate has a material structure in which: an area ratio of Mg2Si having circle equivalent diameters of 3 μm or more in a plate thickness central portion is 0.45% or less; and an area ratio of Mg2Si having circle equivalent diameters of 3 μm or more in a region of 20 mm±1.5 mm from a plate surface in a plate thickness direction is 1.2 times or more and 3.0 times or less the area ratio of Mg2Si having circle equivalent diameters of 3 μm or more in the plate thickness central portion.

2. The high-strength aluminum alloy thick plate according to claim 1, further comprising any one or more of Cu: 0.05 to 1.2%, Zn: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Cr: 0.05 to 0.5%, and Zr: 0.05 to 0.2%.

3. A method for manufacturing a high-strength aluminum alloy thick plate, the high-strength aluminum alloy thick plate being defined in claim 1, comprising the steps of: performing a solution treatment of heating an aluminum alloy at a temperature of 480° C. or higher for 1 hour or longer; then cooling the aluminum alloy so that a temperature of a plate thickness central portion of the aluminum alloy is 480° C. or higher and a temperature of a surface of the aluminum alloy is higher than the temperature of the plate thickness central portion by 10° C. or more and 30° C. or less; subsequently performing a quenching treatment of rapidly cooling the aluminum alloy so that a cooling rate of the plate thickness central portion of the aluminum alloy becomes 100° C./hr or larger; and furthermore, performing an artificial aging treatment.

4. A method for manufacturing a high-strength aluminum alloy thick plate, the high-strength aluminum alloy thick plate being defined in claim 2, comprising the steps of: performing a solution treatment of heating an aluminum alloy at a temperature of 480° C. or higher for 1 hour or longer; then cooling the aluminum alloy so that a temperature of a plate thickness central portion of the aluminum alloy is 480° C. or higher and a temperature of a surface of the aluminum alloy is higher than the temperature of the plate thickness central portion by 10° C. or more and 30° C. or less; subsequently performing a quenching treatment of rapidly cooling the aluminum alloy so that a cooling rate of the plate thickness central portion of the aluminum alloy becomes 100° C./hr or larger; and furthermore, performing an artificial aging treatment.

Description

BEST MODES FOR CARRYING OUT THE INVENTION

[0022] The present inventive high-strength aluminum alloy thick plate and the method for manufacturing the same will be described in more detail below. First, constituent elements and material structure of the aluminum alloy in the present invention will be described. As described above, the present inventive high-strength aluminum alloy thick plate contains Si, Mg, Ti, and Fe. Meanwhile, in the specification of the present application, a simple expression of “%” in the description of a component composition of an alloy means “mass %.”

[0023] Si: 0.2 to 1.2%

[0024] Si is solid-dissolved into a matrix by a solution treatment to contribute to strength improvement. Moreover, when Si coexists with Mg, minute Mg.sub.2Si deposits are formed by natural aging, which is deposited as Mg.sub.2Si by artificial aging to contribute to the improvement of strength. The effect is insufficient when the content is less than 0.2%, and is saturated when it exceeds 1.2%. Accordingly, the content of Si is desirably 0.2 to 1.2%, more preferably 0.4 to 0.8%.

[0025] Mg: 0.2 to 1.5%

[0026] Mg is solid-deposited into a matrix and contributes to strength improvement in the same way as Si, and, furthermore, when Mg coexists with Si, minute Mg.sub.2Si deposits are formed by natural aging and Mg.sub.2Si is deposited by artificial aging to contribute to the improvement of strength. The effect is insufficient when the content is less than 0.2%, and is saturated when it exceeds 1.5%. Accordingly, the content of Mg is desirably 0.2 to 1.5%, more preferably 0.8 to 1.2%.

[0027] Ti: 0.005 to 0.15%

[0028] Ti acts on fining of crystal grains in casting. The effect is insufficient when the content is less than 0.005%, and, when it exceeds 0.15%, the effect is saturated and coarse compounds are formed easily. Accordingly, the content of Ti is desirably 0.15% or less.

[0029] Fe: 1.0% or Less

[0030] Fe is an element contained as an impurity. Fe forms an Al—Fe-based compound to deteriorate the elongation and toughness of the alloy. Therefore, the content of Fe is desirably as little as possible. Industrially, it may be 1.0% or less.

[0031] Further, the present inventive high-strength aluminum alloy thick plate can further contain any one or more of Cu, Zn, Mn, Cr and Zr, in addition to Si, Mg and Ti.

[0032] Cu: 0.05 to 1.2%

[0033] Cu is solid-dissolved into a matrix and has a role to enhance strength. The effect is insufficient when the content is less than 0.05%, and corrosion resistance deteriorates when it exceeds 1.2%. Accordingly, the content of Cu is desirably 0.05 to 1.2%. When particularly high strength is needed, it is particularly desirable to be 0.2% to 1.2%.

[0034] Zn: 0.05 to 0.5%

[0035] Zn is solid-dissolved into a matrix and has a function of enhancing strength. The effect is insufficient when the content is less than 0.05%, and is saturated when it exceeds 0.5% and the corrosion resistance deteriorates. Accordingly, the content of Zn is desirably 0.05 to 0.5%.

[0036] Mn: 0.05 to 1.0%

[0037] Mn is solid-dissolved into a matrix or disperses fine deposits and has a role to enhance strength. The effect is insufficient when the content is less than 0.05%, and, when it exceeds 1.0%, the effect is saturated and coarse compounds are formed easily. Accordingly, the content of Mn is desirably 0.05 to 1.0%.

[0038] Cr: 0.05 to 0.5%

[0039] Cr has a role to disperse fine deposits in a matrix to enhance strength. The effect is insufficient when the content is less than 0.05%, and, when it exceeds 0.5%, the effect is saturated and huge crystallized products are formed easily. Accordingly, the content of Cr is desirably 0.05 to 0.5%.

[0040] Zr: 0.05 to 0.2%

[0041] Zr has a role to disperse fine deposits in a matrix to enhance strength. The effect is saturated and huge crystallized products are formed easily. Accordingly, the content of Zr is desirably 0.05 to 0.2%.

[0042] Constituent elements other than the above-described component elements that constitute the alloy in the present invention are Al and inevitable impurities. The inevitable impurities are allowed in a range that does not affect the present invention. Desirably, contents of respective elements contained as inevitable impurities are 0.05% or less and are 0.15% or less in total.

[0043] Next, the material structure of the present inventive aluminum alloy will be described.

[0044] The aluminum alloy of the present invention is so constituted that the alloy has uniform strength in a plate thickness direction by controlling the size of Mg.sub.2Si being a deposit and distribution of the deposits in a plate thickness direction. The size of Mg.sub.2Si in the structure of a plate material is various, and the present inventors paid attention, particularly, to Mg.sub.2Si having circle equivalent diameters of 3 μm or more, and found that it is possible to reduce variation of strength in the thickness direction of a plate material by controlling an area ratio thereof.

[0045] As conditions for the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more, it is first and foremost necessary that the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in a plate thickness central portion is 0.45% or less. This is a condition for securing strength of the plate thickness central portion. That is, when an area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in the plate thickness central portion exceeds 0.45%, strength of the plate thickness central portion decreases, and a plate material with sufficient strength cannot be obtained. Meanwhile, it is important to reduce Mg.sub.2Si having circle equivalent diameters of 3 μm or more as much as possible. Accordingly, the present invention has no problem even if the lower limit of the Mg.sub.2Si area ratio is 0%. Further, the plate thickness central portion means, as described, the central portion of a thick plate material in the plate thickness direction.

[0046] Further, the present invention requires that a deposition quantity of coarse deposits in the plate thickness surface layer portion is larger than a deposition quantity thereof in the plate central portion. Concretely, in a region of 20 mm±1.5 mm from the plate surface in the plate thickness direction, an area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more is set to be 1.2 times or more and 3.0 times or less that in the plate thickness central portion.

[0047] That the area ratio of coarse deposits in the plate thickness surface layer portion is large as described above is caused by a deposition treatment of deposits in the manufacturing process of the plate material, and, hereby, the uniformity of strength in the plate thickness direction is secured. That is, in the present invention, coarse deposits are made to be deposited prior to quenching in the plate thickness surface layer portion on which the effect of rapid cooling becomes largest in quenching, to make the area ratio thereof high. Hereby, it becomes possible to decrease a number density of deposits (fine Mg.sub.2Si) in the region that will be deposited in a subsequent aging treatment. On the other hand, the plate thickness central portion has been cooled rapidly from a high temperature that is equal to or higher than the temperature at which coarse deposits are deposited, and, therefore, deposition of coarse deposits are suppressed. In the plate thickness central portion, although the effect of rapid cooling in quenching is small, the deposition density of coarse deposits is low (Mg.sub.2Si area ratio of 0.45% or less) and, therefore the strength increases by deposits in an aging treatment, and difference in the strength from that of the plate thickness surface layer portion can be reduced.

[0048] Furthermore, the present inventive aluminum alloy requires that the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more is 1.2 times or more and 3.0 times or less that in the plate thickness central portion, in a region of 20 mm±1.5 mm from the plate surface in the plate thickness direction. The reason is that, when the area ratio in the plate thickness surface layer portion is less than 1.2 times the area ratio in the plate thickness central portion, deposits are deposited finely with high density in the plate thickness surface layer portion in an aging treatment, and strength in the plate thickness surface layer portion becomes high to increase a difference in the strength from that of the plate thickness central portion. On the other hand, as for 3.0 times being the upper limit, the efficiency in manufacturing thick plates is considered. As will be described later, a deposition treatment in the plate thickness surface layer portion performed prior to quenching is a treatment for forming a temperature difference between the plate surface portion and the plate thickness central portion, but there is a limit in a formable temperature difference in an aluminum alloy having high thermal conductivity, and it is difficult to manufacture one having an area ratio of the plate thickness surface layer portion exceeding three times an area ratio in the plate thickness central portion.

[0049] Next, the present inventive method for manufacturing a high-strength aluminum alloy thick plate will be described. As described above, the present inventive method for manufacturing a high-strength aluminum alloy thick plate includes performing a solution treatment on an ingot of an aluminum alloy, then performing a treatment of cooling the aluminum alloy while controlling the temperature of a plate thickness surface to deposit coarse deposits in the plate thickness surface layer portion, subsequently performing a quenching treatment, and further performing an artificial aging treatment. Hereinafter, detailed description will be given.

[0050] First, an aluminum alloy having the above-described component composition is smelted according to an ordinary method. An aluminum alloy is cast by suitably selecting a usual casting method such as a continuous casting method or a semi-continuous casting method (DC casting method).

[0051] Then, a homogenizing treatment can be performed as necessary on an obtained aluminum alloy. When a homogenizing treatment is to be performed, the treatment conditions are not particularly limited, and preferably, heating is performed at a temperature of 480 to 590° C. for 0.5 to 24 hours, more preferably at a temperature of 500 to 560° C. for 1 to 20 hours. When a homogenizing treatment temperature is lower than 480° C. or a treating time is shorter than 0.5 hours, the effect of homogenization may not be obtained sufficiently. On the other hand, when a homogenizing treatment temperature exceeds 590° C., there is the risk that the material melts. Further, when a treating time exceeds 24 hours, the productivity lowers.

[0052] Hot rolling can be performed on an aluminum alloy having been subjected to a homogenizing treatment as necessary. When hot rolling is to be performed, in a process from completion of the homogenizing treatment to start of hot rolling, any of following treatment methods can be applied as necessary. That is, subsequent to cooling to ordinary temperature or near to ordinary temperature in a cooling process after the homogenizing treatment, it is possible to perform anew heating to start temperature of hot rolling and start hot rolling. Further, it is also possible to perform cooling to start temperature of hot rolling in a cooling process after a homogenizing treatment, and to start directly hot rolling. Then, hot rolling can be performed under conventional general conditions, and the temperature may be controlled to a temperature allowing hot rolling, for example, with hot rolling starting temperature set to be 250° C. or higher and lower than 580° C. and hot rolling end temperature set to be 150° C. or higher.

[0053] A solution treatment is performed on an aluminum alloy cast in this way, or an aluminum alloy material having been subjected to a homogenizing treatment or hot rolling as necessary. The present inventive aluminum alloy is a heat treatment-based alloy, and an intended strength is obtained by causing a crystallized product such as Mg.sub.2Si generated in casting to be solid-dissolved into a matrix. This treatment is called a solution treatment. Temperature in the solution treatment shall be 480° C. or higher. When the temperature is lower than 480° C., above-described effects cannot be obtained sufficiently. The upper limit temperature in the solution treatment is not particularly prescribed, but, when it exceeds a melting point, there is the risk that internal defects such as porosity occur, and therefore, it shall be lower than a melting point, particularly preferably 560° C. or lower.

[0054] Treatment time in the solution treatment is preferably set to be 1 hour or longer. When it is shorter than 1 hour, diffusion of elements is insufficient and a uniform solid-solution state cannot be obtained. Further, the upper limit of the treating time is not particularly defined, but, industrially, an economical and sufficient effect can be obtained by setting it to be 48 hours or shorter, more preferably 24 hours or shorter.

[0055] In a general method for manufacturing an aluminum alloy plate material, a quenching treatment is performed immediately after the solution treatment. However, in the present invention, performed is a treatment of cooling an aluminum alloy held at high temperatures in the solution treatment prior to quenching to deposit deposits of coarse Mg.sub.2Si in the plate thickness surface layer portion. In the deposition treatment, the cooling is performed so that a temperature of the plate thickness central portion of an ingot becomes 480° C. or higher and a temperature at the surface of the ingot becomes lower than the temperature of the plate thickness central portion in a range of 10° C. or more and 30° C. or less.

[0056] In the deposition treatment, when the surface temperature of an aluminum alloy plate is higher than “the temperature of the plate thickness central portion—10° C.,” it is in a state where Mg and Si are solid-dissolved in a matrix in large quantities, and coarse deposits have not been deposited sufficiently. If an artificial aging treatment is performed while maintaining this state, solid-dissolved Mg and Si are deposited as fine Mg.sub.2Si, and therefore, an increase in the strength of the plate thickness surface layer portion becomes large and difference in strength between the plate thickness central portion and the plate thickness surface layer portion becomes large. Consequently, it is necessary to set surface temperature of an aluminum alloy to be lower than the temperature of the plate thickness central portion by 10° C. or more. However, aluminum has high thermal conductivity and, therefore, it is difficult to hold a surface temperature of a plate to be lower than the temperature of the plate thickness central portion by 30° C. or more.

[0057] Further, in the deposition treatment, the temperature of the plate thickness central portion is set to be 480° C. or higher. When it becomes 480° C. or lower, coarse Mg.sub.2Si deposits are deposited sparsely in the plate thickness central portion, and, even after a subsequent artificial aging treatment, sufficient strength cannot be obtained in the plate thickness central portion. As the result, difference in strength between the plate thickness central portion and the plate thickness surface layer portion becomes large.

[0058] The above cooling method for a deposition treatment of an aluminum alloy is not particularly restricted, and treatments, in which temperature difference between the surface temperature of an aluminum alloy and the temperature of the plate thickness central portion becomes 10° C. or more and 30° C. or less, are acceptable. If it is a suitable temperature difference, for example, a method of contacting a cooling medium to the vicinity of the surface of an aluminum alloy is acceptable. However, as a suitable and simple method from an industrial viewpoint, there is mentioned a method of exposing an aluminum alloy having been subjected to a solution treatment to an atmosphere for performing a quenching treatment to cool it, and performing a quenching treatment when the temperature difference between a surface temperature and a temperature of the plate thickness central portion becomes 10° C. or more and 30° C. or less.

[0059] A quenching treatment is performed on the aluminum alloy having been subjected to the above deposition treatment. Quenching is a treatment of leaving an aluminum alloy in the state where elements solid-dissolved into a matrix in the solution treatment remain being solid-dissolved, without depositing elements solid-dissolved in a matrix by rapid cooling the aluminum alloy. In the quenching treatment, cooling is performed at a cooling rate of 100° C./hr or larger. When the cooling rate is smaller than 100° C./hr, quenching becomes insufficient, and sufficient strength cannot be obtained in an artificial aging treatment. Accordingly, the cooling rate in a solution treatment is desirably 100° C./hr or larger. As the cooling rate, preferably a cooling rate in the central portion in a plate thickness direction of an aluminum alloy is applied.

[0060] Meanwhile, when quenching is performed directly from a solution treatment temperature without performing a deposition treatment, a deposition quantity of coarse deposits decreases in the plate thickness surface layer portion on which the effect of rapid cooling is high. Further, in the plate thickness surface layer portion, fine deposits are deposited densely in a subsequent artificial aging treatment. In the situation, the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more in the region of 20 mm±1.5 mm in a plate thickness direction, which is required in the present invention, becomes small, and the area ratio becomes less than 1.2 times an area ratio of Mg.sub.2Si in the plate thickness central portion having a small cooling rate. Such a plate material has a large difference in strength between the plate thickness surface layer portion and the plate thickness central portion, and is not applicable to the aluminum alloy thick plate of the present invention capable of solving the problem.

[0061] In the present inventive aluminum alloy, the strength can be enhanced by performing a solution treatment, quenching and, furthermore, an artificial aging treatment to deposit fine Mg.sub.2Si. Temperature in the artificial aging treatment is preferably 150 to 250° C. When it is lower than 150° C., a long time aging treatment is necessary until sufficient strength is obtained, which is uneconomical. On the other hand, when it exceeds 250° C., coarse Mg.sub.2Si is deposited easily, which may lower the strength.

[0062] Further, as for the time of an artificial aging treatment, holding time is preferably 1 to 24 hours. Setting of aging time has a strong relationship with an aging temperature, and, when aging treatment time is shorter than 1 hour, sufficient strength cannot be obtained, or variation of strength becomes large. The upper limit is not particularly defined, but is preferably 24 hours or shorter from the economical viewpoint. As for conditions of an artificial aging treatment, a treatment is more preferably performed at 170 to 190° C. for 6 to 12 hours. Under the conditions, the manufacturing is industrially and stably possible.

[0063] Meanwhile, in the present inventive method for manufacturing an aluminum alloy thick plate, it is possible to perform suitably a flattening treatment of an ingot surface of an aluminum alloy. Examples of performable flattening treatments include, for example, mechanical processing such as facing and polishing, chemical polishing, etc. A flattening treatment can be performed prior to a solution treatment and a quenching treatment.

[0064] The present inventive aluminum alloy material manufactured via the above processes may exert strength of 200 MPa or more, and yield strength of 140 MPa or more in the plate thickness surface layer portion and plate thickness central portion. Moreover, difference in strength between the plate thickness surface layer portion and the plate thickness central portion is reduced to be 5.0 MPa or less. Meanwhile, the present inventive aluminum alloy material can give strength of 200 MPa or more and yield strength of 140 MPa or more that exceed largely those of H112 material of the JIS 5052 alloy, which is not a heat treatment-based alloy, and the application of the same to more broad fields is expected.

[0065] Thickness of the present inventive aluminum alloy thick plate is not particularly limited. Aluminum alloy thick plates having arbitrary thicknesses can be obtained as long as the material structure or the manufacturing condition having been described heretofore is satisfied. However, in instances of thick plates having thicknesses exceeding 650 mm, an aluminum thick plate itself works as a heat source to make it difficult to obtain a sufficient cooling rate. Further, the present invention is particularly effective for an application to a plate having thickness of 200 mm or more, manufacturing of which is considered to be difficult from circumstances such as restriction on flattening correction described above. Accordingly, as for an application range of the present invention, aluminum alloy thick plates of 200 mm or more and 650 mm or less are preferable.

Examples

First Embodiment

[0066] Hereinafter, concrete embodiments of the present invention will be described with Comparative Examples. In the present embodiment, aluminum alloy thick plates of various compositions were manufactured and measurement of strength and observation of a material structure were performed.

[0067] [Manufacturing of Aluminum Alloy Thick Plate]

[0068] Aluminum alloy ingots of compositions shown in Table 1 (T 320 mm×W 1500 mm×L 3500 mm) were produced on an industrial scale, which were cut to give an aluminum alloy material (T 320 mm×W 1400 mm×L 3000 mm). Meanwhile, T shows plate thickness, W shows plate width, and L shows plate length.

TABLE-US-00001 TABLE 1 Unit: mass % Alloy No. Si Mg Fe Ti Cu Zn Mn Cr Zr Al Example A 0.61 0.91 0.4 0.06 0.2 0.5 0.05 — — Balance B 0.48 1.18 0.44 0.15 — 0.2 — — — Balance C 0.55 1.49 0.15 0.006  0.81 — 0.5  0.5 — Balance D 0.75 1.02 0.35 0.1  0.06 — — — — Balance E 0.22 0.85 0.81 0.12 1.2 — — — 0.2  Balance F 0.88 0.21 0.25 0.13 — — 0.9   0.04 — Balance G 0.45 0.65 0.7 0.05 —  0.05 — 0.1 0.05 Balance H 1.1 1.25 0.35 0.1  0.04 — — — 0.12 Balance Comparative I 0.66 1.8 1.1 0.06 — — 0.74 — — Balance example J 0.11 0.63 0.35 0.001 0.2 0.5 0.03 — — Balance K 1.6 1.3 0.6 0.16 0.3 0.1 0.81 — — Balance L 0.34 0.17 0.4 0.06 0.2 0.5 0.05 — — Balance

[0069] Facing of 10 mm for a side was performed on the obtained aluminum alloy material as a surface flattening treatment, and then a solution treatment was performed. As for conditions of the solution treatment, high temperature retention at 530° C.×10 hours was performed.

[0070] Then, the aluminum alloy material after the solution treatment was cooled to a predetermined temperature in the air whose atmospheric temperature was controlled prior to quenching to perform a deposition treatment for depositing coarse deposits in a plate thickness surface layer portion. In the deposition treatment, a thermocouple was attached to the plate thickness central portion of the aluminum alloy surface to survey the temperature, and it was confirmed that the temperature of the plate thickness central portion became 480° C. or higher and the temperature of the aluminum alloy surface was lower than the temperature of the plate thickness central portion by 10° C. or more. Temperatures of the surface and the central portion of an aluminum alloy before quenching treatment are listed in Table 2.

[0071] A quenching treatment was performed by cooling the aluminum alloy material with water. At this time, a thermocouple was attached to the plate thickness central portion to survey a cooling rate, and an average cooling rate between material temperatures of 450° C. to 250° C. was measured. Measured cooling rates are listed in Table 2.

[0072] Then, an artificial aging treatment was performed on the aluminum alloy material after the quenching treatment. The artificial aging treatment was performed under conditions of 180° C.×10 hours.

[0073] [Structure Observation of Aluminum Alloy Thick Plate]

[0074] Material structures of the aluminum alloy thick plates manufactured in the present embodiment were observed to measure the area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more. For the observation of a material structure, a scanning electron microscope (SEM) was used. In the structure observation, for a region located at 300 mm from the edge portion in a length direction of the alloy plate material and at the central portion in a width direction, cross-section structures of the surface layer portion (a region of 20 mm±1.5 mm from the plate surface in the plate thickness direction) and the plate thickness central portion were observed and imaged. At this time, an image of 3.7×10.sup.5 μm.sup.2 was photographed at a magnification of 250 times to measure an area ratio of Mg.sub.2Si in the range. In the measurement of the area ratio, the area ratio was obtained by use of a particulate analysis capability of a commercially available image analysis software (trade name “A zo-kun” manufactured by Asahi Kasei Engineering Corporation) for the obtained image (one visual field).

[0075] [Strength Measurement of Aluminum Alloy Thick Plate]

[0076] Next, strength measurements of the surface layer portion and plate thickness central portion were performed on the aluminum alloy thick plate manufactured in the present embodiment. Here, JIS No. 4 test pieces (φ 14 mm) were gathered at a position of 20 mm from the surface of the obtained aluminum alloy thick plate in the plate thickness direction, and from the plate thickness central portion, and a tensile test (in plate width direction) was performed. The tensile test was performed on two pieces respectively based on JIS Z 2241 standard, and the average value was used as an evaluation object. In the present embodiment, as a criterion for deciding a passing status regarding the strength of the manufactured aluminum alloy thick plate, the minimum values of tensile strength (TS) and yield strength (YS) of the plate thickness central portion were evaluated. Furthermore, difference between tensile strengths (TS) of the plate thickness surface layer portion and the plate thickness central portion was calculated to evaluate a passing status of presence or absence of the strength difference in the plate thickness direction. Meanwhile, as a criterion of deciding the passing status of strength of a thick plate, there were adopted tensile strength of 200 MPa or more and yield strength of 140 MPa or more, which were prescribed in JIS standard for H112 material of JIS 5052 alloy, which was not a heat treatment-based alloy having been actually used for a vacuum chamber material etc., and those higher than this was decided to be “success” and those lower than this was decided to be “failure.” On the other hand, as for the presence or absence of the strength difference in the plate thickness direction, those in which the difference in strengths between the plate thickness surface layer portion and the plate thickness central portion was 50 MPa or less were decided as “success.”

[0077] [Measurement of Warpage Quantity of Aluminum Alloy Thick Plate]

[0078] For the obtained aluminum alloy thick plate (T 300 mm×W 1400 mm×L 3000 mm), there was measured magnitude of warpage generated when the plate was cut from the surface to the plate thickness central portion in the plate thickness direction. In the measurement of a warpage quantity, a cut plate was placed on a surface plate and magnitude of a gap generated by a curve of the plate was measured. A larger warpage quantity generated at this time means that a warpage quantity is larger when cutting processing is performed, and it was decided as “success” when the warpage quantity per 1000 mm of width was 3 mm or less and as “failure” when it exceeded 3 mm.

[0079] There are listed, in Table 2, results of area ratio measurement of deposits and evaluation of mechanical properties performed for respective aluminum alloy thick plates manufactured in the present embodiment.

TABLE-US-00002 TABLE 2 Ingot temperature before Area ratio of Mg.sub.2Si (%) Tensile properties of quenching Surface plate thickness Difference Central Cooling Surface layer/ central portion in Evaluation Manufacturing Alloy Central portion − rate layer Central Central TS YS EL strength of No. No. Surface portion Surface (° C./hr)*.sup.1 portion portion portion (Mpa) (Mpa) (%) (Mpa) warpage Example 1 A 478° C. 503° C. 25° C. 368 0.71 0.29 2.45 226 172 8.9 40 ◯ 2 B 476° C. 500° C. 24° C. 370 0.65 0.23 2.83 219 168 9.1 37 ◯ 3 C 476° C. 500° C. 24° C. 372 0.79 0.33 2.39 240 187 8.2 36 ◯ 4 D 480° C. 502° C. 22° C. 371 0.74 0.31 2.39 221 170 8.6 41 ◯ 5 E 481° C. 506° C. 25° C. 365 0.81 0.44 1.84 222 169 9.4 34 ◯ 6 F 485° C. 503° C. 18° C. 375 0.74 0.43 1.72 209 146 11 29 ◯ 7 G 476° C. 500° C. 24° C. 371 0.67 0.33 2.03 213 157 10.1 32 ◯ 8 H 472° C. 500° C. 28° C. 368 0.62 0.30 2.07 232 178 7.9 47 ◯ Comparative 9 I 471° C. 499° C. 28° C. 367 0.83 0.54 1.54 234 179 7.6 51 X example 10 J 483° C. 504° C. 21° C. 369 0.49 0.17 2.88 194 138 12.3 34 ◯ 11 K 481° C. 502° C. 21° C. 373 0.89 0.62 1.44 237 182 6.9 54 X 12 L 478° C. 503° C. 25° C. 372 0.45 0.16 2.81 167 109 16.2 31 ◯ *.sup.1The cooling rate is an average cooling rate between material temperatures of 450 to 250° C. in the plate thickness central portion

[0080] From Table 2, it can be confirmed that each of manufacturing Nos. 1 to 8 corresponding to Examples of the present invention obtained tensile strength of 200 MPa or more and yield strength of 140 MPa or more and ultra thick plates that have strengths exceeding largely the strength of JIS 5052 alloy-H112 thick plate material. Further, it was also confirmed that, in these thick plate materials, the difference in strength between the plate thickness surface layer portion and the plate thickness central portion was 50 MPa or less, and that the difference in strength in the plate thickness direction was reduced.

[0081] In contrast, alloy thick plates of manufacturing Nos. 9 to 12 being Comparative Examples were decided “failure” in either the strength of the thick plate or the strength difference in the plate thickness direction. That is, the manufacturing Nos. 9 and 11 showed strength difference exceeding 50 MPa between the plate thickness surface layer portion and the plate thickness central portion. These aluminum alloy thick plates were alloys containing Mg exceeding the standard quantity (manufacturing No. 9) or containing Si exceeding the standard quantity (manufacturing No. 11) in the alloy composition. Si and Mg are additive elements that form fine Mg.sub.2Si deposits to contribute to the improvement of the material strength. It is considered that, when these elements become excessive, the strength of a thick plate rises, but the difference in strength between the plate thickness surface layer portion and the plate thickness central portion tends to become large proportionately.

[0082] Further, manufacturing Nos. 10 and 12 could not clear standards that tensile strength of the plate thickness central portion was 200 MPa or more and the yield strength was 140 MPa or more. It is considered that the manufacturing No. 10 is an aluminum alloy containing Si of less than the standard quantity and strength rise caused by deposits was small. Moreover, Manufacturing No. 12 is an aluminum alloy containing Mg exceeding the standard quantity, and it is considered that, in the instance of the alloy, the concentration of Si that is bonded with Mg to generate deposits is near the lower limit, and therefore, the strength rise by deposits was small.

Second Embodiment

[0083] In the present embodiment, mainly, plural kinds of thick plates composed of the aluminum alloy having the composition of an alloy No. A were manufactured under varied manufacturing conditions, and their strength and observation of material structure were measured. In the present embodiment, while conditions of a solution treatment, a subsequent deposition treatment by cooling and a cooling rate of quenching were adjusted, aluminum alloy thick plates were manufactured. Meanwhile, also in the present embodiment, prior to a solution treatment and a quenching treatment, facing of 10 mm on a side was performed as a surface flattening treatment.

[0084] The manufacturing process of an aluminum alloy thick plate in the present embodiment is basically the same as that in the first embodiment, and manufacturing conditions other than the solution treatment (temperature and time), temperature in the deposition treatment and the cooling rate of quenching were set to be the same as those in the first embodiment. Further, the structure observation and the method and condition of strength measurement after manufacturing aluminum alloy thick plates were also set to be the same as those in the first embodiment. The evaluation results are shown in Table 3.

TABLE-US-00003 TABLE 3 Ingot temperature before Area ratio of Mg.sub.2Si (%) Tensile properties of quenching Surface plate thickness Differ- Evalu- Condition Central Cooling Surface layer/ central portion ence in ation Manufacturing Alloy of solution Central Portion rate layer Central Central TS YS EL strength of No. No. treatment Surface portion surface (° C./hr)*.sup.1 portion portion portion (Mpa) (Mpa) (%) (Mpa) warpage Example 13 A 530° C. × 478° C. 503° C. 25° C. 370 0.69 0.31 2.23 226 172 8.9 35 ◯ 11 hr 14 A 540° C. × 475° C. 502° C. 27° C. 310 0.7 0.35 2.00 225 162 7.9 36 ◯ 2 hr 15 A 490° C. × 471° C. 481° C. 10° C. 180 0.59 0.44 1.34 209 161 7.8 45 ◯ 20 hr 16 A 520° C. × 452° C. 482° C. 30° C. 230 0.79 0.33 2.39 218 156 7.7 29 ◯ 17 hr 17 A 530° C. × 478° C. 503° C. 25° C. 120 0.74 0.34 2.18 204 154 7.9 33 ◯ 11 hr Compar- 18 A 500° C. × 481° C. 496° C.  7° C. 370 0.44 0.39 1.13 222 171 8.8 53 X ative 11 hr example 19 C 540° C. × 480° C. 504° C. 24° C. 80 0.78 0.49 1.59 187 121 9.5 21 ◯ 8 hr *.sup.1The cooling rate is an average cooling rate between material temperatures of 450 to 250° C. in the plate thickness central portion

[0085] From Table 3, it was confirmed that each of manufacturing Nos. 13 to 17 corresponding to Examples was good aluminum alloy thick plates having high strength and a small difference in strength in the plate thickness direction. For these aluminum alloy thick plates, good warpage evaluations were obtained. When concrete examinations are performed, manufacturing Nos. 15 and 16 are Examples lying near the upper and lower limits of conditions of the difference in temperatures at the aluminum alloy surface and the plate thickness central portion (10° C. or more and 30° C. or less) in the deposition treatment prior to quenching. Each of these alloys shows good properties. Further, manufacturing No. 17 is a thick plate manufactured near the lower limit of the condition of a cooling rate (100° C./h or more) in a quenching treatment, and the tensile strength exceeds 200 MPa to give a good result.

[0086] In contrast, manufacturing No. 18 is a thick plate treated at a temperature falling below the lower limit (10° C.) of the difference in temperatures between the aluminum alloy surface and the plate thickness central portion in a deposition treatment prior to the quenching. In the aluminum alloy thick plate, as for an area ratio of Mg.sub.2Si having circle equivalent diameters of 3 μm or more, the plate thickness surface layer portion falls below 1.2 times the plate thickness central portion. Consequently, it was confirmed that the difference in strength between the plate thickness surface layer portion and the plate thickness central portion exceeded 50 MPa and the difference in strength in the plate thickness direction became large. Further, in the manufacturing No. 19, it was confirmed that the cooling rate in the quenching treatment was too low and, therefore, the area ratio of coarse deposits in the plate thickness central portion exceeded 0.45%, and that it could not clear the standard of tensile strength of 200 MPa or more and yield strength of 140 MPa or more to show strength poverty.

INDUSTRIAL APPLICABILITY

[0087] As described hereinbefore, the present inventive high-strength 6000-based alloy thick plate is a high-strength thick plate material having uniform strength in the plate thickness direction. In the manufacturing method of the high-strength aluminum alloy thick plate, it is possible to manufacture a thick plate of 200 mm or more without considering restriction on facilities for flat correction for reducing internal stress that has been necessary for conventional methods, because the flat correction is not indispensable. The present inventive high-strength 6000-based alloy thick plate may be applied as a constituent material of manufacturing apparatuses of electronic components such as a liquid crystal panel, and machine components of semiconductor manufacturing apparatuses or vacuum chambers, etc., and may also respond to a requirement for increase in size of these apparatuses.