ALUMINUM ALLOY SHEET FOR FORMING AND METHOD FOR PRODUCING THE SAME

- UACJ CORPORATION

In an aluminum alloy sheet for forming, when hardness (Hv) is measured at intervals of 1/16 of a sheet thickness in a sheet thickness direction from a depth position at ½ the sheet thickness to a sheet surface, hardness distribution is plotted with the hardness (Hv) on a vertical axis and distance (mm) from the depth position at ½ the sheet thickness on a horizontal axis, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on the plotted hardness distribution, and a slope A of the linear function is determined by the least squares method, a product obtained by multiplying the slope A by the sheet thickness (mm) is 10 to 28. It is possible to provide an aluminum alloy sheet that can improve the effect of reducing the springback amount by bottoming.

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Description
TECHNICAL FIELD

The present invention relates to an aluminum alloy sheet for forming in which springback amount after press forming of a sheet material is reduced, and to a method for producing the same.

BACKGROUND ART

In press forming of a sheet material, shape and dimensional errors due to springback of returning the material to its original shape and dimensions by the amount of elastic deformation occur when the material has been bent and a tool is released. Such springback generally appears more and the amount thereof is greater in harder materials. Thus, it becomes necessary to lower the material strength and reduce the elastic deformation region of a stress-strain curve in order to reduce shape and dimensional errors.

In order to reduce the springback amount, bottoming is conventionally performed on a sheet material to apply pressure in the sheet thickness direction. This technique can be used to change the stress in the sheet thickness direction and change the in-plane stress of the sheet material so as to suppress the springback. However, with this technique, reduction in springback amount may be small for some materials, and further measures are needed.

In view of such circumstances, as a method for reducing the springback amount, for example, Patent Literature 1 discloses a method of improving shape fixability by applying a skin pass after solution treatment to provide a layer harder than the average hardness in a sheet from its surface layer to ¼ sheet thickness.

CITATION LIST Patent Literature

    • Patent Literature 1: Japanese Patent Publication 2006-283138-A

SUMMARY OF INVENTION Technical Problem

However, Patent Literature 1 involves a problem in that the number of regions harder than the average hardness increases. As mentioned above, since springback is plastic deformation related to the elastic deformation region, the regions harder than the average hardness are preferably minimized.

From this regard, to reduce the springback amount in press forming, the strength has to be reduced. Thus, it is not easy to fabricate an aluminum alloy sheet that has both high strength and reduced springback amount at the same time.

In view of this, it is an object of the present invention to provide an aluminum alloy sheet that can improve the effect of reducing springback amount by bottoming.

Solution to Problem

As a result of repeated diligent studies in order to solve the above-mentioned problems, the inventors of the present invention found that an aluminum alloy sheet having excellent shape fixability, which can reduce the springback amount while having high strength, can be obtained by setting the sheet to have a specific hardness distribution slope in the thickness direction and a specific sheet thickness, and have completed the present invention.

Specifically, the present invention (1) provides an aluminum alloy sheet for forming, in which, when hardness (Hv) is measured at intervals of 1/16 of a sheet thickness in a sheet thickness direction from a depth position at ½ the sheet thickness to a sheet surface, hardness distribution is plotted with the hardness (Hv) on a vertical axis and distance (mm) from the depth position at ½ the sheet thickness on a horizontal axis, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on the plotted hardness distribution, and a slope A of the linear function is determined by the least squares method, a product obtained by multiplying the slope A by the sheet thickness (mm) is 10 to 28.

The present invention (2) provides the aluminum alloy sheet for forming of (1), in which, when a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on, among the plotted hardness distribution, the plotted hardness distribution from a depth position at ¼ the sheet thickness to the sheet surface, a slope B1 of the linear function is determined by the least squares method, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on, among the plotted hardness distribution, the plotted hardness distribution from the depth position at ½ the sheet thickness to the depth position at ¼ the sheet thickness, and a slope B2 of the linear function is determined by the least squares method, an absolute value of the difference (B1−B2) between the slope B1 and the slope B2 is 10 or less.

The present invention (3) provides the aluminum alloy sheet for forming of (1) or (2), in which the aluminum alloy sheet has a tensile strength of 140.0 MPa or more.

The present invention (4) provides the aluminum alloy sheet for forming of any one of (1) to (3), in which the aluminum alloy sheet is made of a JIS 5000-series aluminum alloy.

The present invention (5) provides the aluminum alloy sheet for forming of any one of (1) to (3), in which the aluminum alloy sheet is made of a JIS 6000-series aluminum alloy.

The present invention (6) provides a method for producing an aluminum alloy sheet for forming made of a JIS 5000-series aluminum alloy, the method including performing at least one of: (1a) setting a cold working rate at 70.0% or more; (2a) performing a skin pass at a reduction of 1.0 to 10.0% after stabilizing treatment, (3a) performing three or more passes of cold working at a reduction of 25.0% or less per pass; and (4a) performing treatment with a leveler after the stabilizing treatment.

The present invention (7) provides a method for producing an aluminum alloy sheet for forming made of a JIS 6000-series aluminum alloy, the method including performing at least one of: (1b) setting a cold working rate at 70.0% or more; (2b) performing a skin pass at a reduction of 1.0 to 10.0% after artificial aging treatment; (3b) performing three or more passes of cold working at a reduction of 25.0% or less per pass; and (4b) performing treatment with a leveler after solution treatment or after the artificial aging treatment.

Advantageous Effect of Invention

According to the present invention, it is possible to provide an aluminum alloy sheet that can improve the effect of reducing the springback amount by bottoming.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an aluminum alloy sheet for forming according to the present invention.

FIG. 2 is a graph of the hardness of Example A.

FIG. 3 is a graph of the hardness of Example B.

FIG. 4 is a graph of the hardness of Example C.

FIG. 5 is a graph of the hardness of Example D.

FIG. 6 is a graph of the hardness of Example E.

FIG. 7 is a graph of the hardness of Example F.

FIG. 8 is a graph of the hardness of Example G.

FIG. 9 is a graph of the hardness of Comparative Example H.

FIG. 10 is a graph of the hardness of Comparative Example I.

FIG. 11 is a graph of the hardness of a 6000-series reference material.

FIG. 12 is a graph of the hardness of Example J.

FIG. 13 is a graph of the hardness of Example K.

FIG. 14 is a graph of the hardness of Example L.

FIG. 15 is a graph of the hardness of a 5000-series reference material.

 DESCRIPTION OF EMBODIMENTS

An aluminum alloy sheet for forming according to the present invention is an aluminum alloy sheet for forming, in which, when hardness (Hv) is measured at intervals of 1/16 of a sheet thickness in a sheet thickness direction from a depth position at ½ the sheet thickness to a sheet surface, hardness distribution is plotted with the hardness (Hv) on a vertical axis and distance (mm) from the depth position at ½ the sheet thickness on a horizontal axis, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on the plotted hardness distribution, and a slope A of the linear function is determined by the least squares method, a product obtained by multiplying the slope A by the sheet thickness (mm) is 10 to 28.

The aluminum alloy sheet for forming according to the present invention will be described with reference to FIGS. 1, 3, and 9. FIG. 1 is a schematic sectional view of the aluminum alloy sheet for forming according to the present invention. FIG. 3 is a graph on which hardness distribution of Example B is plotted with the hardness (Hv) on the vertical axis and the distance (mm) from a depth position at ½ the sheet thickness on the horizontal axis. FIG. 9 is a graph on which hardness distribution of Comparative Example His plotted with the hardness (Hv) on the vertical axis and the distance (mm) from a depth position at ½ the sheet thickness on the horizontal axis.

With reference to FIG. 1, positions where the hardness (Hv) is to be measured will be described. FIG. 1 is a sectional view of an aluminum alloy sheet 1 for forming, cut in a plane perpendicular to the sheet surface. The position indicated by reference sign 3 in FIG. 1 is a depth position at ½ the sheet thickness. In other words, the depth position 3 at ½ the sheet thickness is a position separated from a sheet surface 7 in a sheet thickness direction 6 by a length q that is ½ of a sheet thickness p. The position indicated by reference sign 5 is a depth position at ¼ the sheet thickness. In other words, the depth position 5 at ¼ the sheet thickness is a position separated from the sheet surface 7 in the sheet thickness direction 6 by a length r that is ¼ of the sheet thickness p.

The following describes a method of determining the slope A. When the hardness (Hv) is measured at intervals of 1/16 of the sheet thickness in the sheet thickness direction from the depth position at ½ the sheet thickness to the sheet surface, hardness distribution is plotted with the hardness (Hv) on a vertical axis and the distance (mm) from the depth position at ½ the sheet thickness on a horizontal axis, and a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on the plotted hardness distribution, the slope A is a slope of the linear function determined by the least squares method. To begin with, on a cross section of the aluminum alloy sheet for forming, the hardness (Hv) is measured at intervals of 1/16 of the sheet thickness p in the sheet thickness direction 6 from the depth position 3 at ½ the sheet thickness to a sheet surface position 4. Subsequently, as illustrated in FIG. 3, results of the hardness (Hv) at the respective measurement positions are plotted with the hardness (Hv) on the vertical axis and the distance (mm) from the depth position at ½ the sheet thickness on the horizontal axis Then, based on the plotted hardness distribution thus obtained, the relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function, and the slope A of the linear function is determined by the least squares method. In Example B illustrated in FIG. 3, the slope A is determined to be 16 by the least squares method. In Comparative Example H, the same procedure is used and, as illustrated in FIG. 9, the slope A is determined to be 4.9 by the least squares method.

In the present invention, the hardness (Hv) is a value measured by a method in accordance with JIS Z 2244, in which, for example, an aluminum alloy sheet is resin embedded and mirror polished, and then the hardness is measured on a rolled perpendicular cross-section thereof (a plane perpendicular to the sheet surface) with a micro-Vickers hardness tester (FM-110, manufactured by FUTURE-TECH CORP.) on measurement conditions that the test load is 10 gf (0.098 N) and the holding time is 10 seconds.

In order to measure the hardness accurately for a portion near the sheet surface at a position of 1/16 from the sheet surface, the hardness (Hv) may be measured from a position separated from the surface by approximately 1/16 of the thickness or more if the sheet thickness is thin, in the present invention.

The following describes a method of determining the slopes B1 and B2. Among the plotted hardness distribution obtained in determining the slope A, based on the plotted hardness distribution from the depth position 5 at ¼ the sheet thickness to the sheet surface position 4, the relation between the hardness (Hv) from the depth position 5 at ¼ the sheet thickness to the sheet surface position 4 and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function. The slope B1 of the linear function is determined by the least squares method. Among the plotted hardness distribution obtained in determining the slope A, based on the plotted hardness distribution from the depth position 3 at ½ the sheet thickness to the depth position 5 at ¼ the sheet thickness, the relation between the hardness (Hv) from the depth position 3 at ½ the sheet thickness to the depth position 5 at ¼ the sheet thickness and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function. The slope B2 of the linear function is determined by the least squares method.

In the aluminum alloy sheet for forming according to the present invention, based on the plotted hardness distribution from the depth position at ½ the sheet thickness to the sheet surface position obtained as described above, “a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function, and a slope A of the linear function is determined by the least squares method” from the depth position at ½ the sheet thickness to the sheet surface position. A product obtained by multiplying the value of the slope A by the sheet thickness (mm) (slope A×sheet thickness (mm)) is 10 to 28, preferably 10 to 20, and particularly preferably 12 to 17. If the product of slope A×sheet thickness (mm) is within the above range, reduction in springback amount by bottoming increases, resulting in excellent shape fixability. In contrast, if the product of slope A×sheet thickness (mm) is less than the above range, reduction in springback amount by bottoming decreases, resulting in poor shape fixability. In order to increase the slope A, it is necessary to apply more work hardening. However, in order to set the product of slope A×sheet thickness (mm) to a value exceeding the above range, either excessively great work hardening is required to increase the slope A, or the sheet thickness needs to be set excessively large. And if the sheet thickness is too large, it is difficult to create, in the sheet thickness direction, a hardness distribution for the aluminum alloy sheet for forming according to the present invention.

In the present invention, based on the plotted hardness distribution from the depth position at ½ the sheet thickness to the sheet surface position, “a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function, and a slope A of the linear function is determined by the least squares method” from the depth position at ½ the sheet thickness to the sheet surface position. The coefficient (R2) of correlation between the linear function having the slope A and the plotted hardness distribution from the depth position at ½ the sheet thickness to the sheet surface position is 0.50 or more, preferably 0.70 or more, and particularly preferably 0.80 or more.

In the aluminum alloy sheet for forming according to the present invention, based on the plotted hardness distribution from the depth position at ½ the sheet thickness to the sheet surface position obtained as described above, “a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function, and a slope B1 of the linear function is determined by the least squares method” from the depth position at ¼ the sheet thickness to the sheet surface based on the plotted hardness distribution from the depth position at ¼ the sheet thickness to the sheet surface, and “a relation between the hardness (Hv) and the distance (mm) from the depth position at the ½ the sheet thickness is approximated by a linear function, and a slope B2 is determined by the least squares method” from the depth position at ½ the sheet thickness to the depth position at ¼ the sheet thickness based on the plotted hardness distribution from the depth position at ½ the sheet thickness to the depth position at ¼ the sheet thickness. The absolute value of the difference (B1−B2) between the slope B1 and the slope B2 is preferably 10 or less, and particularly preferably 8 or less. The value of difference between the slope B1 and the slope B2 based on the plotted hardness distribution from the depth position at ½ the sheet thickness of the aluminum alloy sheet for forming to the sheet surface position is set within the above range, whereby the effect of reducing of the springback amount by bottoming can be increased.

In the present invention, the reason that the hardness is measured at intervals of 1/16 of the sheet thickness in the sheet thickness direction is as follows. When a portion of the sheet to be measured is divided into two regions: one from the depth position at ¼ the sheet thickness to the sheet surface position; and the other from the depth position at ½ the sheet thickness to the depth position at ¼ the sheet thickness, measuring the hardness at intervals of 1/16 of the thickness allows an approximate linear function to be drawn at four measurement points in each region, which increases reliability of the value of the slope of the linear function. In contrast, for example, if the hardness is measured at intervals of 1/10 of the sheet thickness in the sheet thickness direction, an approximate linear function has to be drawn at two measurement points in consideration of the difficulty of measuring the hardness near the uppermost surface, which reduces the reliability of the value of the slope of the linear function.

The sheet thickness of the aluminum alloy sheet for forming according to the present invention is 0.4 to 5.0 mm, preferably 0.8 to 2.7 mm. In aluminum alloy sheets having sheet thicknesses less than or exceeding the above ranges, it is difficult to create a hardness distribution in which the product of “slope A×sheet thickness (mm)” is 10 to 28, preferably 10 to 20, and particularly preferably 12 to 17.

The basic composition of the aluminum alloy sheet for forming according to the present invention is not limited to a particular one, and the aluminum alloy of the aluminum alloy sheet for forming according to the present invention may be various aluminum alloys of 1000, 2000, 3000, 4000, 5000, 6000, and 7000 series, for example.

The aluminum alloy sheet for forming according to the present invention is preferably made of a JIS 5000-series aluminum alloy or a JIS 6000-series aluminum alloy.

The chemical composition of the JIS 5000-series aluminum alloy more preferably comprises Si: 0.25 mass % or less, Fe: 0.40 mass % or less, Cu: 0.10 mass % or less, Mn: 0.10 mass % or less, Mg: 2.20 to 2.80 mass %, Cr: 0.15 to 0.35 mass %, and Zn: 0.10 mass % or less, with the balance being aluminum and inevitable impurities.

The chemical composition of the JIS 6000-series aluminum alloy more preferably comprises Si: 0.20 to 0.60 mass %, Fe: 0.35 mass % or less, Cu: 0.10 mass % or less, Mn: 0.10 mass % or less, Mg: 0.45 to 0.90 mass %, Cr: 0.10 mass % or less, Zn: 0.10 mass % or less, and Ti: 0.10 mass or less, with the balance being aluminum and inevitable impurities.

The tensile strength of the aluminum alloy sheet for forming according to the present invention is preferably 140.0 MPa or more, more preferably 150.0 to 300.0 MPa, and particularly preferably 160.0 to 290.0 MPa. In the aluminum alloy sheet for forming according to the present invention, the product of “slope A×sheet thickness (mm)” is set to 10 to 28, preferably 10 to 20, particularly preferably 12 to 17, and more preferably the absolute value of “slope B1−slope B2” is set to preferably 10 or less, particularly preferably 8 or less, whereby reduction in springback amount by bottoming is increased and the shape fixability is excellent while the tensile strength thereof is high, which is preferably 140.0 MPa or more, more preferably 150.0 to 300.0 MPa, particularly preferably 160.0 to 290.0 MPa.

In Example B in which the hardness distribution illustrated in FIG. 3 was plotted, the sheet thickness was 0.81 mm. A JIS A6063 aluminum alloy was cast by a conventional method, and hot rolled and cold rolled by common procedures to obtain a sheet thickness of 0.81 mm. Subsequently, solution treatment and artificial aging treatment were performed, and a skin pass of 3.0% was performed after the artificial aging treatment. As illustrated in FIG. 3, in the plotted hardness distribution in Example B, the hardness is lowest in the center of the sheet thickness (depth position at ½ the sheet thickness), and the hardness varies linearly to the sheet surface. Then, based on the plotted hardness distribution in FIG. 3, the slope A determined by the least squares method is 16. Thus, the product of “slope A×sheet thickness (mm)” is 13 (16×0.81). The aluminum alloy sheet of Example B was then subjected to a 90-degree bending test at 20 kgf (196) N, 100 kgf (980) N, and 200 kgf (1961 N). The springback amounts were 5.7°, 3.7°, and 3.7°, respectively. Each springback amount was determined as the difference from 90°. In the aluminum alloy sheet of Example B, a phenomenon in which the springback amount was reduced was observed under the high load condition of the 90° bending test, which is a condition corresponding to what is called “bottoming”.

In Comparative Example H in which the hardness distribution illustrated in FIG. 9 was plotted, the sheet thickness was 0.82 mm. A JIS A6063 aluminum alloy was cast by the common procedure, and hot rolled and cold rolled by the common procedures to obtain a sheet thickness of 0.82 mm. Subsequently, a skin pass of 3.0% was performed, and then solution treatment, followed by artificial aging treatment, was performed. As illustrated in FIG. 9, in the plotted hardness distribution in Comparative Example H, the slope A determined by the least squares method is 4.9. Thus, the product of “slope A×sheet thickness (mm)” is 4.0 (4.9×0.82). The aluminum alloy sheet of Comparative Example H was then subjected to the 90-degree bending test at 20 kgf, 100 kgf, and 200 kgf. The springback amounts were 8.7°, 10.3°, and 8.0°, respectively. In the aluminum alloy sheet of Comparative Example H, no change was observed in the springback amount even under the high load condition of the 90° bending test, which is a condition corresponding to what is called “bottoming”.

The aluminum alloy sheet for forming according to the present invention has excellent formability in the 90° bending test at a high load corresponding to the bottoming condition by setting the product of “slope A×sheet thickness (mm)” to 10 to 28, preferably 10 to 20, and particularly preferably 12 to 17. In general, hard materials are strongly affected by springback, and thus it is difficult to fabricate materials having high strength and excellent formability. Against such a technical background, in the aluminum alloy sheet for forming according to the present invention, a region in which the hardness is low is created in the sheet thickness direction, and the change in hardness in the sheet thickness direction is controlled such that the product of “slope A×sheet thickness (mm)” satisfies 10 to 28, preferably 10 to 20, and particularly preferably 12 to 17, whereby the springback amount can be reduced by changing the in-plane stress of the sheet material by bottoming, resulting in high strength and excellent shape fixability.

The aluminum alloy sheet for forming according to the present invention is suitably produced by a method for producing an aluminum alloy sheet for forming according to a first embodiment of the present invention or a method for producing an aluminum alloy sheet for forming according to a second embodiment of the present invention described below.

The method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention is a method for producing an aluminum alloy sheet for forming made of a JIS 5000-series aluminum alloy, and the method includes performing at least one of: (1a) setting a cold working rate at 70.0% or more; (2a) performing a skin pass at a reduction of 1.0 to 10.0% after stabilizing treatment; (3a) performing three or more passes of cold working at a reduction of 20.0% or less per pass; and (4a) performing treatment with a leveler after the stabilizing treatment.

In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, a casting step of casting an aluminum alloy ingot having a composition of the JIS 5000-series aluminum alloy, homogenization treatment, hot rolling, cold rolling, and the stabilizing treatment are performed in order. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, the casting method, the homogenization treatment method, and the stabilizing treatment method are not limited to particular ones, and are selected as appropriate.

In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, by performing at least one of: (1a) setting the cold working rate at 70.0% or more; (2a) performing a skin pass at a reduction of 1.0 to 10.0% after stabilizing treatment; (3a) performing three or more passes of cold working at a reduction of 20.0% or less per pass; and (4a) performing treatment with a leveler after the stabilizing treatment, the degree of work is increased to promote work hardening in the sheet thickness direction, whereby the hardness distribution in the aluminum alloy sheet for forming according to the present invention described above can be obtained. Any one of the above (1a) to (4a) may be performed, or two or more of the above (1a) to (4a) may be performed in combination. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, the value of the slope A can be easily increased by combining two or more of the above (1a) to (4a).

(1a) is to set the cold working rate in cold working at 70.0% or more, preferably 70.0 to 80.0%. The cold working rate is a total working rate in cold working, and is a value calculated as “Cold working rate (%)=((Sheet thickness before the first pass of cold working−Sheet thickness after the last pass of cold working)/Sheet thickness before the first pass of cold working)×100”.

(2a) is to perform a skin pass at a reduction of 1.0 to 10.0%, preferably 3.0 to 10.0%, after stabilizing treatment. The skin pass is a process of reducing the thickness of a sheet in the cold working such that a reduction calculated as “Reduction (%)=((Sheet thickness before skin pass−Sheet thickness after skin pass)/Sheet thickness before skin pass)×100” is within a range of 1.0 to 10.0%, preferably 3.0 to 10.0%. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, it is preferable to perform the skin pass specified in (2a) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

(3a) is to perform three or more passes of cold working at a reduction of 25.0% or less per pass. The reduction per pass of cold working is a reduction calculated as “Reduction (%)=((Sheet thickness before pass−Sheet thickness after pass)/Sheet thickness before pass)×100”. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, it is preferable to perform three or more passes of cold working at a reduction of 10.0 to 25.0% per pass, it is more preferable to perform four or more passes of cold working at a reduction of 10.0 to 20.0% per pass, and it is particularly preferable to perform six or more passes of cold working at a reduction of 10.0 to 15.0% per pass. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, it is preferable to perform the passes of cold working specified in (3a) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

(4a) is to perform treatment with a leveler after the stabilizing treatment. The leveler is a device commonly used for the purpose of correcting warping of a thin sheet. The treatment with the leveler is a process of passing a sheet between at least two sets of rolls provided such that the points of action of the rolls are displaced slightly with respect to the direction in which the sheet travels, thereby bending the sheet at least twice in opposite directions. In the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention, it is preferable to perform the treatment with the leveler specified in (4a) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

The method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention is a method for producing an aluminum alloy sheet for forming made of a JIS 6000-series aluminum alloy, the method includes performing at least one of: (1b) setting the cold working rate at 70.0% or more; (2b) performing a skin pass at a reduction of 1.0 to 10.0% after artificial aging treatment; (3b) performing three or more passes of cold working at a reduction of 20.0% or less per pass; and (4b) performing treatment with a leveler after solution treatment or after the artificial aging treatment.

In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, a casting step of casting an aluminum alloy ingot having a composition of the JIS 6000-series aluminum alloy, homogenization treatment, hot rolling, cold rolling, the solution treatment, and the artificial aging treatment are performed in order. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, the casting method, the homogenization treatment method, the solution treatment method, and the artificial aging treatment method are not limited to particular ones, and are selected as appropriate.

In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, by performing at least one of: (1b) setting the cold working rate at 70.0% or more; (2b) performing a skin pass at a reduction of 1.0 to 10.0% after artificial aging treatment; (3b) performing three or more passes of cold working at a reduction of 20.0% or less per pass; and (4b) performing treatment with a leveler after the artificial aging treatment, the degree of work is increased to promote work hardening in the sheet thickness direction, whereby the hardness distribution in the aluminum alloy sheet for forming according to the present invention described above can be obtained. Any one of the above (1b) to (4b) may be performed, or two or more of the above (1b) to (4b) may be performed in combination. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, the value of the slope A can be easily increased by combining two or more of the above (1b) to (4b).

(1b) is to set a cold working rate in cold working at 70.0% or more, preferably 70.0 to 80.0%. The cold working rate is a total working rate in cold working, and is a value calculated as “Cold working rate (%)=((Sheet thickness before the first pass of cold working−Sheet thickness after the last pass of cold working)/Sheet thickness before the first pass of cold working)×100”.

(2b) is to perform a skin pass at a reduction of 1.0 to 10.0%, preferably 3.0 to 10.0%, after artificial aging treatment. The skin pass is a process of reducing the thickness of a sheet in the cold working such that a reduction calculated as “Reduction (%)=((Sheet thickness before skin pass−Sheet thickness after skin pass)/Sheet thickness before skin pass)×100” is within a range of 1.0 to 10.0%, preferably 3.0 to 10.0%. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, it is preferable to perform the skin pass specified in (2b) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

(3b) is to perform three or more passes of cold working at reduction of 25.0% or less per pass. The reduction per pass of cold working is a reduction calculated as “Reduction (%)=((Sheet thickness before pass−Sheet thickness after pass)/Sheet thickness before pass)×100”. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, it is preferable to perform three or more passes of cold working at a reduction of 10.0 to 25.0% per pass, it is more preferable to perform four or more passes of cold working at a reduction of 10.0 to 20.0% per pass, and it is particularly preferable to perform five or more passes of cold working at a reduction of 10.0 to 15.0% per pass. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, it is preferable to perform the passes of cold working specified in (3b) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

(4a) is to perform treatment with a leveler after solution treatment or after the artificial aging treatment. The leveler is a device commonly used for the purpose of correcting warping of a thin sheet. The treatment with the leveler is a process of passing a sheet between at least two sets of rolls provided such that the points of action of the rolls are displaced slightly with respect to the direction in which the sheet travels, thereby bending the sheet at least twice in opposite directions. In the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention, it is preferable to perform the treatment with the leveler specified in (4b) when the cold working rate in cold working (total working rate in cold working) is 30.0 or more and less than 70.0%, particularly when it is 30.0 to 60.0%.

The reasons that the above-described hardness distribution in the aluminum alloy sheet for forming according to the present invention is obtained in the method for producing an aluminum alloy sheet for forming according to the first embodiment of the present invention or the method for producing an aluminum alloy sheet for forming according to the second embodiment of the present invention are as follows.

The hardness distribution in the aluminum alloy sheet for forming according to the present invention is considered to be derived from work hardening. Dislocations multiply and internal stresses accumulate in metals due to work stress. Work hardening is believed to occur when the number of these dislocations increases too much, causing the material itself to harden as the dislocations become entangled or break off from each other. Large-scale deformation as in hot rolling does not produce hardness distribution in the sheet thickness direction because stress is transmitted uniformly throughout the entire sheet thickness. In contrast, in the cold rolling or the skin pass of (1a) to (3a) and (1b) to (3b), work hardening can easily occur in the surface layer and its vicinity due to smaller-scale deformation than in hot rolling, and thus hardness distribution satisfying the specifications for the aluminum alloy sheet for forming according to the present invention can be created in the sheet thickness direction. This is evident also in Examples A to F. For the reasons described above, even in a method of bending a sheet for the purpose of forcing the sheet to warp with the leveler of (4a) and (4b), for example, the surface layer of the sheet can be work hardened, whereby the hardness distribution satisfying the specifications for the aluminum alloy sheet for forming according to the present invention can be created in the sheet thickness direction. This is evident also in Example G.

The effect of reducing the springback amount in the aluminum alloy sheet for forming according to the present invention is derived from the hardness distribution in the sheet thickness direction derived from work hardening, and thus the same tendency can be obtained not only with work-hardened alloys such as JIS 5000-series alloys but also with heat-treated alloys such as JIS 6000-series alloys. In heat-treated alloys, many precipitates are formed before and after artificial aging, and thus the hardness changes under the influence of these precipitates. However, since the hardness distribution in the sheet thickness direction for the aluminum alloy sheet for forming according to the present invention is derived from work hardening by the cold rolling, the skin pass, and the like, it is considered that the slope of the hardness distribution itself is not affected before and after aging. This is evident also in Examples A to G.

Springback is primarily due to elastic deformation during bending of a material. The elastic deformation region is narrower for a material having lower strength, and thus the springback amount is smaller for a material having lower strength. In the aluminum alloy sheet for forming according to the present invention, a soft layer exists in the center of the sheet thickness, and the elastic deformation region in that portion is narrow, and consequently the effect of reducing the springback amount can be increased. In other words, the more widely the soft layer exists in the sheet thickness direction, the more the layer is plastically deformed, and thus it is considered that the effect of reducing the springback amount can be increased.

Examples will be given below to specifically describe the present invention, but the present invention is not limited to the Examples given below.

EXAMPLES Example 1

A JIS A6063 aluminum alloy and a JIS 5052 aluminum alloy were ingot casted by DC casting. Subsequently, the JIS A6063 aluminum alloy was subjected to homogenization treatment, hot rolling, cold rolling, solution treatment, and artificial aging treatment in order, and the JIS 5052 aluminum alloy was subjected to homogenization treatment, hot rolling, cold rolling, and stabilizing treatment in order. At these steps, operations under producing conditions given in Table 1 or Table 2 were performed. From the JIS A6063 aluminum alloy, aluminum alloy sheets having sheet thicknesses given in Table 1 were produced, in which the target thickness was 0.80 mm. From the JIS 5052 aluminum alloy, aluminum alloy sheets having sheet thicknesses given in Table 2 were produced, in which the target thickness was 2.70 mm. As the conditions of the homogenization treatment, the solution treatment, the artificial aging treatment, and the stabilizing treatment, common conditions therefor were used.

The difference in the conditions from those for the reference sheet is as follows. Example A: The cold working rate was increased. Example B: A skin pass at a reduction of 3.0% was performed after artificial aging treatment. Example C: The number of passes of cold working was increased. Example D: The number of passes of cold working was increased, and a skin pass at a reduction of 3.0% was performed after artificial aging treatment. Example E: A skin pass at a reduction of 5.0% was performed after artificial aging treatment. Example F: A skin pass at a reduction of 10.0% was performed after artificial aging treatment. Example G: Treatment with the leveler was performed after solution treatment. Comparative Example H: A skin pass at a reduction of 3.0% was performed after cold rolling and before solution treatment. Comparison Example I: The number of passes of cold working was reduced. Example J: The cold-rolling working rate was increased. Example K: The number of passes of cold working was increased. Example L: A skin pass at a reduction of 3.0% was performed after stabilizing treatment.

TABLE 1 Hot rolling condition Sheet Cold rolling condition Skin pass Leveler Hot rolling thickness Sheet Skin pass after treatment starting after hot The number thickness Cold Skin pass after artificial after Producing temperature rolling of passes after cold rolling Reduction after cold solution aging solution condition [° C.] [mm] [times] rolling [mm] rate [%] [%] per pass rolling treatment treatment treatment A 410 4.00 2 0.81 79.8 39.90 — — — — B 410 2.00 2 0.81 59.5 29.75 — — 3.0% — C 410 2.00 5 0.81 59.5 11.90 — — — — D 410 2.00 5 0.81 59.5 11.90 — — 3.0% — E 410 2.00 2 0.81 59.5 29.75 — — 5.0% — F 410 2.00 2 0.78 61.0 30.50 — — 10.0%  — G 410 2.00 2 0.81 59.5 29.75 — — — Performed H 410 2.00 2 0.82 59.0 29.50 3.0% — — — I 410 2.00 1 0.78 61.0 61.00 — — — — 6000-series 410 2.00 2 0.82 59.0 29.50 — — — — reference material

TABLE 2 Hot rolling condition Sheet Cold rolling condition Hot rolling thickness Sheet Skin pass starting after hot The number thickness Cold Skin pass after Producing temperature rolling of passes after cold rolling Reduction after cold stabilizing condition [° C.] [mm] [times] rolling [mm] rate [%] [%] per pass rolling treatment J 410 9.00 2 2.70 70.0 35.00 — — K 410 7.00 6 2.70 61.4 10.23 — — L 410 7.00 2 2.70 61.4 30.70 — 3.0% 5000-series 410 7.00 2 2.70 61.4 30.70 — — reference material

A method and a criterion for each evaluation are as follows.

<Hardness (Hv) Measurement>

The hardness (Hv) was measured by a method in accordance with JIS Z 2244.

Each aluminum alloy sheet was resin embedded and mirror polished, and then the hardness (Hv) was measured on a rolled perpendicular cross-section thereof (a plane perpendicular to the sheet surface) with a micro-Vickers hardness tester (FM-110, manufactured by FUTURE-TECH CORP.) on measurement conditions that the test load was 10 gf (0.098 N) and the holding time was 10 seconds. This measurement was performed in the thickness direction at predetermined intervals, measurements were taken at three points in each measurement position, and the average was used as the hardness (Hv) at the position.

For the JIS 6000-series aluminum alloy sheets, measurement was performed at intervals of 0.05 mm in the thickness direction. For the JIS 5000-series aluminum alloy sheets, measurement was performed at intervals of 0.168 mm in the thickness direction.

Note that the hardness was not measured at the uppermost surface layer location because the Vickers hardness indentation would extend even into the resin.

The results are given in Tables 3 and 4. The respective hardness distributions are also illustrated in FIGS. 2 to 15. The slopes A determined by the least squares method are given in Table 5.

<Springback Amount (Formability)>

To evaluate formability, five sheets of 60 mm in the rolling direction×30 mm in the width direction were prepared and subjected to a 90° bending test with a bend radius of R=5.0 mm. The test loads were 20 kgf (196 N), 100 kgf (980 N), and 200 kgf (1961 N). After the loads were released, the angle of the sheet was measured with a protractor and the difference from 90° was taken as the springback amount. The springback amount was the average of those of the five sheets.

The evaluation results are given in Table 5. Evaluations in which the reduction in springback amount at 200 kgf with respect to at 20 kgf was 1° or more were rated as “Good” indicating excellent formability, and evaluations in which the reduction was less than 1° were rated as “Poor” indicating inferior formability.

From Table 5, it was found that the springback amount at 200 kgf decreased by 1° or more with respect to at 20 kgf when the product obtained by multiplying the slope A of each approximate linear function, based on the hardness distributions illustrated in FIGS. 2 to 15, by the sheet thickness (mm) (slope A×sheet thickness (mm)) was 10 or more.

The “slope A×sheet thickness (mm)” was 10 or more in the producing conditions A, B, C, D, E, F, G, J, K, and L. In contrast, in the producing conditions H and I, the “slope A×sheet thickness (mm)” became less than 10 because the slope of the hardness distribution in the sheet thickness direction was too small, and it is thought that, as a result of this, the springback amount did not decrease.

TABLE 3 Distance from depth position at ½ the sheet thickness [mm] 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 A 79.50 77.96 76.24 77.16 75.51 74.57 74.56 73.94 B 78.83 77.83 77.23 75.83 75.27 75.52 73.62 73.10 C 79.24 78.23 77.45 76.52 76.73 75.03 73.74 73.42 D 81.62 80.86 77.23 77.73 77.97 77.46 75.05 73.64 E 81.67 80.62 81.59 78.75 78.21 76.32 76.23 75.83 F 79.63 80.10 79.50 76.89 75.90 74.40 74.52 73.20 G 79.23 79.09 78.49 76.51 75.89 75.50 75.20 75.00 H 77.48 78.75 76.37 75.53 76.97 75.77 76.77 76.00 I 78.00 76.70 76.96 77.99 78.20 77.90 76.09 76.00 6000-series 76.74 75.27 75.03 75.99 75.06 75.04 75.77 76.25 reference material

TABLE 4 Distance from depth position at ½ the sheet thickness [mm] 1.344 1.176 1.008 0.840 0.672 0.504 0.336 0.168 J 86.53 85.30 85.96 83.38 82.90 82.47 80.22 79.50 K 87.88 85.11 83.92 83.03 83.58 82.52 81.36 80.41 L 90.01 89.09 87.43 86.16 85.98 84.50 83.73 84.18 5000-series 81.93 80.85 82.54 82.33 81.51 81.70 83.20 80.69 reference material

TABLE 5 Reduction in springback amount [°] Absolute Sheet Slope Correlation Springback amount [°] at 200 kgf Tensile Slope Slope Slope value of thickness A × Sheet coefficient 20 100 200 with respect strength Formability A B1 B2 B1-B2 [mm] thickness R{circumflex over ( )}2 kgf kgf kgf to at 20 kgf [MPa] evaluation A 15 9.4 17 8.0 0.81 12 0.90 11.3 9.7 7.0 4.3 249 Good B 16 17 19 2.3 0.81 13 0.96 5.7 3.7 3.7 2.0 244 Good C 17 22 18 4.6 0.81 14 0.97 10.3 9.3 9.0 1.3 248 Good D 20 31 31 0.2 0.81 16 0.85 9.3 7.7 6.7 2.7 235 Good E 19 14 16 1.1 0.81 15 0.91 9.5 8.5 6.2 3.3 252 Good F 21 16 18 1.7 0.78 16 0.93 9.4 7.9 5.3 4.1 251 Good G 14 5.9 18 12 0.81 11 0.91 8.2 7.8 6.0 2.2 246 Good H 4.9 3.8 17 13 0.82 4.0 0.33 8.7 10.3 8.0 0.7 239 Poor I 3.4 17 −0.5 17 0.78 2.6 0.21 7.3 7.0 7.7 −0.3 251 Poor 6000-series 0.4 −8.6 5.0 14 0.82 0.3 0.01 9.7 10.0 9.3 0.3 245 Poor reference material J 6.1 7.4 5.2 2.2 2.71 17 0.94 6.9 5.5 2.5 4.4 265 Good K 5.3 6.4 9.4 3.0 2.67 14 0.89 5.2 4.3 2.1 3.1 258 Good L 5.4 3.7 7.9 4.2 2.44 13 0.94 4.4 4.0 2.0 2.4 277 Good 5000-series 0.0 0.6 −1.7 2.3 2.73 0.0 0.00 3.5 4.1 3.7 −0.2 257 Poor reference material

INDUSTRIAL APPLICABILITY

The aluminum alloy sheet according to the present invention has a smaller springback amount after forming, and thus provides excellent formability.

REFERENCE SIGNS LIST

    • 1 aluminum alloy sheet for forming
    • 3 depth position at ½ the sheet thickness
    • 4 sheet surface position
    • 5 depth position at ¼ the sheet thickness
    • 6 sheet thickness direction
    • 7 sheet surface
    • p sheet thickness
    • q length that is ½ of the sheet thickness
    • r length that is ¼ of the sheet thickness

Claims

1. An aluminum alloy sheet for forming wherein, when hardness (Hv) is measured at intervals of 1/16 of a sheet thickness in a sheet thickness direction from a depth position at ½ the sheet thickness to a sheet surface, hardness distribution is plotted with the hardness (Hv) on a vertical axis and distance (mm) from the depth position at ½ the sheet thickness on a horizontal axis, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on the plotted hardness distribution, and a slope A of the linear function is determined by the least squares method, a product obtained by multiplying the slope A by the sheet thickness (mm) is 10 to 28.

2. The aluminum alloy sheet for forming according to claim 1, wherein, when a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on, among the plotted hardness distribution, the plotted hardness distribution from a depth position at ¼ the sheet thickness to the sheet surface, a slope B1 of the linear function is determined by the least squares method, a relation between the hardness (Hv) and the distance (mm) from the depth position at ½ the sheet thickness is approximated by a linear function based on, among the plotted hardness distribution, the plotted hardness distribution from the depth position at ½ the sheet thickness to the depth position at ¼ the sheet thickness, and a slope B2 of the linear function is determined by the least squares method, an absolute value of a difference (B1−B2) between the slope B1 and the slope B2 is 10 or less.

3. The aluminum alloy sheet for forming according to claim 1, wherein the aluminum alloy sheet has a tensile strength of 140.0 MPa or more.

4. The aluminum alloy sheet for forming according to claim 1, wherein the aluminum alloy sheet is made of a JIS 5000-series aluminum alloy.

5. The aluminum alloy sheet for forming according to claim 1, wherein the aluminum alloy sheet is made of a JIS 6000-series aluminum alloy.

6. A method for producing an aluminum alloy sheet for forming made of a JIS 5000-series aluminum alloy, the method comprising performing at least one of: (1a) setting a cold working rate at 70.0% or more; (2a) performing a skin pass at a reduction of 1.0 to 10.0% after stabilizing treatment, (3a) performing three or more passes of cold working at a reduction of 25.0% or less per pass; and (4a) performing treatment with a leveler after the stabilizing treatment.

7. A method for producing an aluminum alloy sheet for forming made of a JIS 6000-series aluminum alloy, the method comprising performing at least one of: (1b) setting a cold working rate at 70.0% or more; (2b) performing a skin pass at a reduction of 1.0 to 10.0% after artificial aging treatment; (3b) performing three or more passes of cold working at a reduction of 25.0% or less per pass; and (4b) performing treatment with a leveler after solution treatment or after the artificial aging treatment.

Patent History
Publication number: 20240279780
Type: Application
Filed: Jun 7, 2022
Publication Date: Aug 22, 2024
Applicant: UACJ CORPORATION (Tokyo)
Inventors: Sayuri Takemura (Tokyo), Wataru Narita (Tokyo), Makoto Yonemitsu (Tokyo)
Application Number: 18/567,498
Classifications
International Classification: C22C 21/08 (20060101); C22F 1/047 (20060101); C22F 1/05 (20060101);