Aluminum alloy sheet excellent in formability and hardenability during baking of coating and method for production thereof

Abstract

A sheet of a 6000 type aluminum alloy containing Si and Mg as main alloy components and having excellent formability sufficient to allow flat hemming, excellent resistance to denting, and good hardenability during baking a coating, which exhibits an anisotropy of Lankford values of more than 0.4 or the strength ratio for cube orientations of the texture thereof of 20 or more, and exhibits a minimum bend radius of 0.5 mm or less at 180° bending even when the offset yield strength thereof exceeds 140 MPa through natural aging; and a method for producing the sheet of the aluminum alloy, which comprises subjecting an ingot to a homogenization treatment, cooling to a temperature lower than 350° C. at a cooling rate of 100° C./hr or more, optionally to room temperature, heating again to a temperature of 300 to 500° C. and subjecting it to hot rolling, cold rolling the hot rolled product, and subjecting the cold rolled sheet to a solution treatment at a temperature of 400° C. or higher, followed by quenching.

Description

TECHNICAL FIELD

[0001] The present invention relates to an aluminum alloy sheet with excellent formability and paint bake hardenability and suitable as a material for transportation parts, in particular, as an automotive outer panel, and a method for producing the same.

BACKGROUND ART

[0002] An automotive outer panel is required to have 1) formability, 2) shape fixability (shape of the press die is precisely transferred to the material by press working), 3) dent resistance, 4) corrosion resistance, 5) surface quality, and the like. Conventionally, 5000 series (Al—Mg) aluminum alloys and 6000 series (Al—Mg—Si) aluminum alloys have been applied to the automotive outer panel. The 6000 series aluminum alloy has attracted attention because high strength is obtained due to excellent paint bake hardenability, whereby further gage dawn and weight saving is expected. Therefore, various improvement have been made on the 6000 series aluminum alloy.

[0003] Among the properties required for the automotive outer panel, although the shape fixability prefer lower yield strength, the dent prefer higher yield strength. In order to solve this problem, press working are carried out lower yield strength for shape fixability and dent resistance are improved by excellent paint bake hardenability using 6000 series aluminum alloy (see JP. 5-247610, JP. 5-279822, JP. 6-17208, etc.).

[0004] The 6000 series aluminum alloy has problems relating to the surface quality after forming, such as occurrence of orange peel surfaces and ridging marks (long streak-shaped defects occurring in the rolling direction during plastic working). Surface quality defects can be solved by adjusting the alloy components, managing the production conditions, and the like. For example, a method of preventing formation of coarse precipitates by homogenizing the alloy at a temperature of 500° C. or more, cooling the homogenized product to 450-350° C., and starting hot rolling in this temperature range has been proposed in order to prevent occurrence of ridging marks (see JP. 7-228956). However, if the cooling rate is decreased when cooling the homogenized product from the homogenization temperature of 500° C. or more to the hot rolling temperature of 450° C., coarse Mg—Si compounds are formed. This makes it necessary to perform a solution treatment at a high temperature for a long time in the subsequent step, whereby production efficiency is decreased.

[0005] In the case of assembling an outer panel an inner panel material, 180° bending (flat hemming), in which working conditions are severe since the ratio (R/t) of the center bending radius (R) to the sheet thickness (t) is small, is performed. However, since the 6000 series aluminum alloy has inferior bendability in comparison with the 5000 series aluminum alloy, flat hemming can not be performed in high press working area.

DISCLOSURE OF THE INVENTION

[0006] The present inventors have examined for further improving formability, in particular, bendability of the 6000 series aluminum alloy. As a result, it has been found that bendability of the 6000 series alloy is affected by the precipitation state of Mg—Si compounds and misorientation of adjacent crystal grains, and also found that bendability has a correlation with the Lankford value, and it is necessary to increase anisotropy of the Lankford values in order to improve bendability. Furthermore, it has been found that bendability also has a correlation with the intensity ratio (random ratio) of cube orientation {100} <001>of the texture, and it is necessary to allow the texture to have a high degree of integration of cube orientation in order to improve bendability. In order to obtain the above properties, the present inventors have found that it is important to optimize the content of Si and Mg which are major elements of the 6000 series aluminum alloy, and to optimize the production steps, in particular, to appropriately control the cooling rate after homogenization of an ingot.

[0007] The present invention has been achieved based on the above findings. An object of the present invention is to provide an aluminum alloy sheet having excellent formability which allows flat hemming, showing no orange peel surfaces and ridging marks after forming, having excellent paint bake hardenability capable of solving the problems relating to shape fixability and dent resistance, and with excellent corrosion resistance, in particular, filiform corrosion resistance, and a method for producing the same.

[0008] An aluminum alloy sheet according to the present invention for achieving the above object is a 6000 series aluminum alloy sheet, with excellent bendability after a solution treatment and quenching, and has a minimum inner bending radius of 0.5 mm or less during 180° bending with 10% pre-stretch, even if the yield strength is further increased through natural aging. Specific embodiments of the aluminum alloy sheet are as follows.

[0009] (1) An aluminum alloy sheet comprising 0.5-1.5% of Si and 0.2-1.0% of Mg, with the balance consisting of Al and impurities, or comprising 0.8-1.2% of Si, 0.4-0.7% of Mg, and 0.1-0.3% of Zn, with the balance consisting of Al and impurities, in which the maximum diameter of Mg—Si compounds is 10 &mgr;m or less and the number of Mg—Si compounds having a diameter of 2-10 &mgr;m is 1000 per mm2 or less.

[0010] (2) An aluminum alloy sheet comprising 0.4-1.5% of Si, 0.2-1.2% of Mg, and 0.05-0.3% of Mn, with the balance consisting of Al and impurities, in which the percentage of crystal grain boundaries in which misorientation of adjacent crystal grains is 15° or less is 20% or more.

[0011] (3) An aluminum alloy sheet comprising 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2%, and Si %−0.58Mg %≧0.1% being satisfied and the balance consisting of Al and impurities, in which an anisotropy of Lankford values is more than 0.4. The Lankford value r is the ratio of the logarithmic strain inthedirectionof thewidthof the sheet to the logarithmic strain in the direction of the thickness of the sheet when applying a specific amount of tensile deformation, such as 15%, to a tensile specimen, specifically, r=(logarithmic strain in the sheet width direction)/(logarithmic strain in the sheet thickness direction). The anisotropy of Lankford values is (r0+r90−2×r45)/2 (r0: r value of a tensile specimen collected in a direction at 0° to the rolling direction, r90: r value of a tensile specimen collected in a direction at 90° to the rolling direction, and r45: r value of a tensile specimen collected in a direction at 45° to the rolling direction).

[0012] (4) An aluminum alloy sheet comprising 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2% being satisfied and the balance consisting of Al and impurities, inwhich an intensity ratio of cube orientation of crystallographic texture is 20 or more.

[0013] Specific embodiments of a method for producing the above aluminum alloy sheets are as follows.

[0014] (1) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of 350-500° C. at a cooling rate of 100° C./h or more, starting hot rolling of the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

[0015] (2) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

[0016] (3) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 100< C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

[0017] (4) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, hot rolling the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

[0018] (5) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

[0019] (6) A method for producing an aluminum alloy sheet comprising homogenizing an ingot of an aluminum alloy having the above composition at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

PREFERRED EMBODYMENTS

[0020] Effects and reasons for limitations of the alloy components in the Al—Mg—Si alloy sheet of the present invention are described below.

[0021] Si is necessary to obtain strength and high paint bake hardenability (BH), and increases strength by forming Mg—Si compounds. The Si content is preferably 0.5-2.0%. If the Si content is less than 0.5%, sufficient strength may not be obtained by heating during baking and formability may be decreased. If the Si content exceeds 2.0%, formability and shape fixability may be insufficient due to high yield strength during press working. Moreover, corrosion resistance may be decreased after painting. The Si content is more preferably 0.4-1.5%, still more preferably 0.5-1.5%, yet more preferably 0.6-1.3%, and particularly preferably 0.8-1.2%.

[0022] Mg increases strength in the same manner as Si. The Mg content is preferably 0.2-1.5%. If the Mg content is less than 0.2%, sufficient strength may not be obtained by heating during baking. If the Mg content exceeds 1.5%, yield strength may remain high after a solution heat treatment or additional heat treatment, whereby formability and spring-back properties may be insufficient. The Mg content is more preferably 0.2-1.2%, still more preferably 0.2-1.0%, yet more preferably 0.3-0.8%, and particularly preferably 0.4-0.7%.

[0023] Si and Mg are preferably added to satisfy the relations 0.7Si %+Mg %≦2.2%, and Si %−0.58Mg %≧0.1% so that anisotropy of the Lankford values is more than 0.4 and bendability is improved. In order to increase the intensity ratio of cube orientation of the texture to obtain good bendability, Si and Mg are preferably added to satisfy the relation 0.7Si %+Mg %≦2.2%.

[0024] Zn improves zinc phosphate treatment properties during the surface treatment. The Zn content is preferably 0.5% or less. If the Zn content exceeds 0.5%, corrosion resistance may be decreased. The Zn content is still more preferably 0.1-0.3%.

[0025] Cu improves strength and formability. The Cu content is preferably 1.0% or less. If the Cu content exceeds 1.0%, corrosion resistance may be decreased. The Cu content is still more preferably 0.3-0.8%. If corrosion resistance is an important, the Cu content is preferably limited to 0.1% or less.

[0026] Mn, Cr, V, and Zr improve strength and refine crystal grains to prevent occurrence of orange peel surfaces during forming. The content of Mn, Cr, V, and Zr is preferably 1.0% or less, 0.3% or less, 0.29% or less, and 0.2% or less, respectively. If the content of Mn, Cr, V, and Zr exceeds the above upper limits, coarse intermetallic compounds may be formed, whereby formability may be decreased. The content of Mn and Zr is more preferably 0.3% or less and 0.15% or less, respectively. The content of Mn, Cr, V, and Zr is still more preferably 0.05-0.3%, 0.05-0.15%, 0.05-0.15%, and 0.05-0.15%, respectively.

[0027] In order to improve bendability by allowing percentage of crystal grain boundaries in which misorientation of adjacent crystal grains is 15° or less to be 20% or more, Mn is added in an amount of 0.05-0.3% as an essential component.

[0028] Ti and B refine a cast structure to improve formability. The content of Ti and B is preferably 0.1% or less and 50 ppm or less, respectively. If the content of Ti and B exceeds the above upper limits, the number of coarse intermetallic compounds may be increased, whereby formability may be decreased. It is preferable to limit the Fe content to 0.5% or less, and preferably 0.3% or less as another impurity.

[0029] The production steps of the aluminum alloy sheet of the present invention are described below.

[0030] Homogenization condition: Homogenization must be performed at a temperature of 450° C. or more. If the homogenization temperature is less than 450° C., removal of ingot segregation and homogenization may be insufficient. This results in insufficient dissolution of Mg2Si components which contribute to strength, whereby formability may be decreased. Homogenization is preferably performed at a temperature of 480° C. or more.

[0031] Cooling after homogenization: Good properties are obtained by cooling the homogenized product at a cooling rate of preferably 100° C./h or more, and still more preferably 300° C./h or more. Since large-scale equipment is necessary for increasing the cooling rate, it is preferable to manage the cooling rate in the range of 300-1000° C./h in practice. If the cooling rate is low, Mg—Si compounds are precipitated and coarsened. In a conventional cooling method, the cooling rate is about 30° C./h in the case of cooling a large slab. However, Mg—Si compounds are precipitated and coarsened during cooling at such a low cooling rate, whereby the material may not be provided with improved bendability after the solution heat treatment and quenching.

[0032] If the cooling rate is controlled in this manner, (1) appropriate distributions of Mg—Si compounds are obtained, (2) the percentage of crystal grain boundaries in which misorientation of adjacent crystal grains is 15° or less becomes 20% or more, (3) anisotropy of Lankford values is increased, and (4) the degree of integration of cube orientation is increased, whereby bendability is improved.

[0033] The cooling after homogenization must allow the temperature to be decreased to less than 350° C., and preferably less than 300° C. at a cooling rate of 100° C./h or more, preferably 150° C./h or more, and still more preferably at 300° C./h or more. The properties are affected if a region at 350° C. or more is partially present. Therefore, an ingot is cooled until the entire ingot is at 300° C. or less, and preferably 250° C. or less at the above cooling rate. There are no specific limitations to the method of cooling the homogenized ingot insofar as the necessary cooling rate is obtained. For example, water-cooling, fan cooling, mist cooling, or heat sink contact may be employed as the cooling method.

[0034] The cooling start temperature is not necessarily the homogenization temperature. The same effect can be obtained by allowing the ingot to be cooled to a temperature at which precipitation does not significantly occur, and starting cooling at a cooling rate of 100° C./h or more. For example, in the case where homogenization is performed at a temperature of 500° C. or more, the ingot may be slowly cooled to 500° C.

[0035] Hot rolling: The ingot is cooled to a specific temperature of 350-500° C. or 300-450° C. from the homogenization temperature, and hot rolling is started at the specific temperature. The ingot may be cooled to a specific temperature of 350° C. or less from the homogenization temperature, and hot rolling may be started at the specific temperature.

[0036] The ingot may be cooled to a temperature of 350° C. or less and heated to a temperature of 300-500° C., and hot rolling may be started at this temperature. The ingot may be cooled to a temperature of 350° C. or less, cooled to room temperature, heated to a temperature of 300-500° C., and hot-rolled at this temperature.

[0037] If the hot rolling start temperature is less than 300° C., deformation resistance is increased, whereby rolling efficiency is decreased. If the hot rolling start temperature exceeds 500° C., crystal grains coarsen during rolling, whereby ridging marks readily occur in the resulting material. Therefore, it is preferable to limit the hot rolling start temperature to 300-500° C. The hot rolling start temperature is still more preferably 380-450° C. taking into consideration deformation resistance and uniform microstructure.

[0038] The hot rolling finish temperature is preferably 300° C. or less. If the hot rolling finish temperature exceeds 300° C., precipitation of Mg—Si compounds easily occurs, whereby formability may be decreased. Moreover, recrystallized grains coarsen, thereby resulting in occurrence of ridging marks. Hot rolling is preferably finished at 200° C. or more taking into consideration deformation resistance during hot rolling and residual oil stains due to a coolant.

[0039] Cold rolling: The hot rolled sheet is cold rolled to the final gage.

[0040] Solution heat treatment: The solution heat treatment temperature is preferably 450° C. or more, and still more preferably 500° C. or more. If the solution heat treatment temperature is less than 500° C., dissolution of Mg—Si precipitates may be insufficient, whereby sufficient strength and formability cannot be obtained, or heat treatment for a considerably long time is needed to obtain necessary strength and formability. This is disadvantageous from the industrial point of view. There are no specific limitations to the solution heat treatment time insofar as necessary strength is obtained. The solution heat treatment time is usually 120 seconds or less from the industrial point of view.

[0041] Cooling rate during quenching: It is necessary to cool the sheet from the solution treatment temperature to 120° C. or less at a cooling rate of 5° C./s or more. It is preferable to cool the sheet at a cooling rate of 10° C./s or more. If the quenching cooling rate is too low, precipitation of eluted elements occurs, whereby strength, BH, formability, and corrosion resistance may be decreased.

[0042] Additional heat treatment: this heat treatment is performed at40-120° C. for 50 hours or less within 60 minutes after quenching. BH is improved by this treatment. If the temperature is less than 40° C., improvement of BH is insufficient. If the temperature exceeds 120° C. or the time exceeds 50 hours, the initial yield strength is excessively increased, whereby formability or paint bake hardenability is decreased.

[0043] Reversion treatment may be performed at a temperature of 170-230° C. for 60 seconds or less within seven days after final additional heat treatment. Paint bake harden ability is further improved by the reversion treatment.

[0044] A sheet material with excellent bendability after the solution heat treatment and quenching can be obtained by applying the above production steps to an aluminum alloy having the above composition. The aluminum alloy sheet is suitably used as a lightweight automotive member having a complicated shape which is subjected to hemming, such as a hood, trunk lid, and door. Moreover, in the case where the aluminum alloy sheet is applied to a fender, roof, and the like, which are not subjected to hemming, the aluminum alloy sheet can be subjected to severe working in which the bending radius is small due to its excellent bendability after pressing the sheet into a complicated shape. Therefore, the aluminum alloy sheet widens the range of application of aluminum materials to automotive materials, thereby contributing to a decrease in the weight of vehicles.

[0045] In order to securely improve formability, in particular, bendability, it is preferable to adjust the amount of alloy components, such as Si and Mg, and production conditions so that anisotropy of the Lankford values is 0.6 or more and the intensity ratio of cube orientation of the texture is 50 or more.

[0046] The present invention is described below by comparing examples of the present invention with comparative examples. The effects of the present invention will be demonstrated based on this comparison. The examples illustrate only one preferred embodiment of the present invention, which should not be construed as limiting the present invention.

EXAMPLE 1

[0047] Aluminum alloys having compositions shown in Table 1 were cast by using a DC casting method. The resulting ingots were homogenized at 540° C. for six hours and cooled to room temperature at a cooling rate of 300° C./h. The cooled ingots were heated to a temperature of 400° C., and hot rolling was started at this temperature. The ingots were rolled to a thickness of 4.0 mm, and cold-rolled to a thickness of 1.0 mm.

[0048] The cold-rolled sheets were subjected to a solution heat treatment at 540° C. for five seconds, quenched to a temperature of 120° C. at a cooling rate of 30° C./s, and additional heat treated at 100° C. for three hours after five minutes.

[0049] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability were evaluated when 10 days were passed after the final heat treatment, and the maximum diameter of Mg—Si compounds and the number of compounds having a diameter of 2-10 &mgr;m were measured according to the following methods. The tensile properties and a minimum bending radius for formability were also evaluated when four months were passed after the final heat treatment. The results are shown in Tables 2 and 3.

[0050] Tensile property: Tensile strength (&sgr;B), yield strength (&sgr;0.2), and elongation (&dgr;) were measured by performing a tensile test.

[0051] Formability: An Erichsen test (EV) was performed. A test material having a forming height of less than 10 mm was rejected. A 180° bending test for measuring the minimum bending radius after applying 10% tensile pre-strain was performed in order to evaluate hem workability. A test material having a minimum inner bending radius of 0.5 mm or less was accepted.

[0052] Corrosion resistance: The test material was subjected to a zinc phosphate treatment and electrodeposition coating using commercially available chemical treatment solutions. After painting crosscuts reaching the aluminum base material, a salt spray test was performed for 24 hours according to JIS Z2371. After allowing the test material to stand in a wet atmosphere at 50° C. and 95% for one month, the maximum length of filiform corrosion occurring from the crosscuts was measured. A test material having a maximum length of filiform corrosion of 4 mm or less was accepted.

[0053] Bake hardenability (BH): Yield strength (&sgr;0.2) was measured after applying 2% tensile deformation and performing heat treatment at 170° C. for 20 minutes. A test material having a yield strength of 200 MPa or more was accepted.

[0054] Measurement of Mg—Si compound: The maximum diameter of Mg—Si compounds was measured by observation using an optical microscope. The distribution of compounds having a diameter of 2-10 &mgr;m was examined using an image analyzer in the range of 1 square millimeter (1 mm2) in total provided that one pixel=0.25 &mgr;m. The Mg—Si compounds were distinguished from Al—Fe compounds by light and shade of the compounds. The detection conditions were selected at a level at which only the Mg—Si compounds were detected by confirming the compound particles in advance by point analysis. 1 TABLE 1 Composition (mass %) Alloy Si Mg Cu Mn Cr V Zr Fe Zn Ti B 1 1.0 0.5 — — — — — 0.17 0.02 0.02 5 2 0.8 0.6 0.02 0.08 — — — 0.17 0.02 0.02 5 3 1.1 0.5 0.01 0.08 — — — 0.17 0.02 0.02 5 4 1.0 0.6 0.7 0.1 — — — 0.17 0.02 0.02 5 5 1.2 0.4 0.01 — 0.1 — — 0.17 0.02 0.02 5 6 1.1 0.5 0.01 0.15 — 0.12 — 0.13 0.04 0.02 5 7 1.1 0.5 0.4 0.07 — — 0.08 0.15 0.03 0.02 5 Note: Unit for B is ppm.

[0055] 2 TABLE 2 Formability Corrosion BH Minimum resistance &sgr;0.2 Test Tensile properties inner Maximum length after mate- &sgr;B &sgr;0.2 &dgr; EV bending of filiform BH rial Alloy (MPa) (MPa) (%) (mm) radius (mm) corrosion (mm) (MPa) 1 1 242 125 31 10.8 0.1 0 211 2 2 245 131 30 10.4 0.2 1.5 220 3 3 243 127 32 10.6 0.1 0.5 214 4 4 274 134 31 10.5 0.2 3.5 221 5 5 257 135 32 10.6 0.2 1.0 217 6 6 259 132 30 10.2 0.3 1.0 208 7 7 268 136 30 10.3 0.2 2.5 223

[0056] 3 TABLE 3 Properties after Number of natural aging Maximum compounds for 4 months diameter with Minimum Test of Mg—Si diameter inner mate- compound of 2-10 &mgr;m &sgr;0.2 bending rial Alloy (&mgr;m) (/mm2) (MPa) radius (mm) 1 1 6 550 143 0.2 2 2 8 800 147 0.3 3 3 6 650 142 0.2 4 4 9 720 150 0.3 5 5 5 580 152 0.4 6 6 5 520 151 0.4 7 7 6 600 155 0.3

[0057] As shown in Tables 2 and 3, test materials Nos. 1 to 7 according to The present invention showed excellent BH of more than 200 MPa in the BH evaluation. The test materials Nos. 1 to 7 had excellent formability in which the forming height (EV) was more than 10 mm and the minimum inner bending radius was 0.5 mm or less. The test materials Nos. 1 to 7 exhibited excellent corrosion resistance in which the maximum length of filiform corrosion was 4 mm or less.

COMPARATIVE EXAMPLE 1

[0058] Aluminum alloys having compositions shown in Table 4 were cast by using a DC casting method. The resulting ingots were treated by the same steps as in Example 1 to obtain cold-rolled sheets with a thickness of 1 mm. The cold-rolled sheets were subjected to a solution heat treatment and quenching under the same conditions as in Example 1, and heat treatment at 100° C. for three hours after five minutes.

[0059] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and the maximum diameter of Mg—Si compounds and the number of compounds having a diameter of 2-10 &mgr;m were measured according to the same methods as in Example 1. The tensile properties and the minimum inner bending radius for formability evaluation were also evaluated when four months were passed after the final heat treatment. The results are shown in Tables 5 and 6. 4 TABLE 4 Composition (mass %) Alloy Si Mg Cu Mn Cr V Zr Fe Zn Ti B 8 0.3 0.6 0.01 0.05 0.01 — — 0.2 0.03 0.02 5 9 1.9 0.6 0.01 0.05 0.01 — — 0.2 0.03 0.02 5 10 1.1 0.1 0.01 0.05 0.01 — — 0.2 0.03 0.02 5 11 1.1 1.4 0.01 0.05 0.01 — — 0.2 0.03 0.02 5 12 1.1 0.5 1.5 0.05 0.01 — — 0.2 0.03 0.02 5 13 1.1 0.5 0.02 0.5 0.01 — — 0.2 0.03 0.02 5 14 1.1 0.5 0.02 0.02 0.4 — — 0.2 0.03 0.02 5 15 1.1 0.5 0.02 0.02 0.01 0.4 — 0.2 0.03 0.02 5 16 1.1 0.5 0.02 0.02 0.01 — 0.3 0.2 0.03 0.02 5 Note: Unit for B is ppm.

[0060] 5 TABLE 5 Corrosion resistance Formability Maximum BH Minimum length &sgr;0.2 Test Tensile properties inner of filiform after mate- &sgr;B &sgr;0.2 &dgr; EV bending corrosion BH rial Alloy (MPa) (MPa) (%) (mm) radius (mm) (mm) (MPa) 8 8 163 70 30 10.7 0 0.5 125 9 9 265 139 31 10.5 0.5 1.0 224 10 10 157 65 32 10.8 0 1.5 118 11 11 280 141 29 10.2 0.6 1.0 229 12 12 294 132 30 10.6 0.4 5.0 228 13 13 247 130 28 9.7 0.6 1.0 217 14 14 246 128 29 9.6 0.4 1.0 214 15 15 247 129 28 9.8 0.5 1.0 212 16 16 245 132 27 9.5 0.7 1.5 213

[0061] 6 TABLE 6 Properties after Number of natural aging Maximum compounds for 4 months diameter with Minimum Test of Mg—Si diameter inner mate- compound of 2-10 &mgr;m &sgr;0.2 bending rial Alloy (&mgr;m) (/mm2) (MPa) radius (mm) 8 8 4 300 85 0 9 9 15 1350 158 0.7 10 10 3 260 79 0 11 11 18 2430 159 0.7 12 12 9 880 154 0.5 13 13 12 1250 146 0.7 14 14 8 940 143 0.5 15 15 12 1120 146 0.6 16 16 14 1290 148 0.7

[0062] As shown in Tables 5 and 6, test material No. 8 and test material No. 10 showed insufficient BH due to low Si content and low Mg content, respectively. Test material No. 9 and test material No. 11 had insufficient bendability due to high Si content and high Mg content, respectively. Test material No. 12 had inferior filiform corrosion resistance due to high Cu content. Test materials Nos. 13 to 16 had a small forming height (EV) due to high Mn content, high Cr content, high V content, and high Zr content, respectively. Moreover, these test materials showed insufficient bendability.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

[0063] Ingots of the alloys Nos. 1 and 3 of Example 1 were homogenized at 540° C. for eight hours. The ingots were cooled to the hot rolling temperature after homogenization, and hot rolling was started at the temperatures shown in Table 7. The thickness of hot-rolled products was 4.5 mm. The hot-rolled products were cold-rolled to a thickness of 1 mm, subjected to a solution heat treatment under the conditions shown in Table 7, quenched to 120° C. at a cooling rate of 15° C./s, and additional heat treatment at 90° C. for five hours after 10 minutes. In Example 2 and Comparative Example 2, the ingots were cooled to the hot rolling temperature after homogenization, and hot rolling was performed at this temperature.

[0064] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and the maximum diameter of Mg—Si compounds and the number of compounds having a diameter of 2-10 ∥m were measured according to the same methods as in Example 1. The tensile properties and the minimum bending radius for formability evaluation were also evaluated when four months were passed after the final heat treatment. Electro deposition coating was performed after applying 10% tensile deformation in the direction at 90° to the rolling direction. The presence or absence of ridging marks was evaluated with the naked eye.

[0065] The results are shown in Tables 8 and 9. 7 TABLE 7 Cooling rate Hot rolling Solution heat after start treatment Test homogenization temperature condition material Alloy (° C./h) (° C.) (° C.) − (sec) 17 1 150 370 550 − 3 18 1 800 450 520 − 5 19 3 200 400 530 − 7 20 3 600 440 550 − 5 21 3 2000 470 560 − 3 22 1 30 420 550 − 3 23 1 70 400 550 − 3 24 1 200 550 520 − 7 25 3 150 410 450 − 3 26 3 20 450 520 − 5

[0066] 8 TABLE 8 Corrosion resistance Formability Maximum BH Minimum Occurrence length of &sgr;0.2 Test Tensile properties inner of filiform after mate- &sgr;B &sgr;0.2 &dgr; EV bending ridging corrosion BH rial Alloy (MPa) (MPa) (%) (mm) radius (mm) mark (mm) (MPa) 17 1 243 123 30 10.7 0.1 None 1.0 210 18 1 248 126 31 10.6 0 None 1.5 218 19 3 244 125 31 10.5 0 None 0.5 215 20 3 249 127 30 10.4 0 None 0.5 216 21 3 252 129 31 10.5 0.1 None 0.5 215 22 1 195 80 30 10.8 0 None 1.0 180 23 1 207 92 30 10.7 0 None 1.0 188 24 1 245 127 31 10.5 0.2 Observed 0.5 220 25 3 201 92 32 10.5 0 None 2.0 162 26 3 210 105 31 10.7 0 None 1.5 185

[0067] 9 TABLE 9 Properties after Number of natural aging Maximum compounds for 4 months diameter with Minimum Test of Mg—Si diameter inner mate- compound of 2-10 &mgr;m &sgr;0.2 bending rial Alloy (&mgr;m) (/mm2) (MPa) radius (mm) 17 1 8 470 141 0.2 18 1 7 630 143 0.1 19 3 6 570 142 0 20 3 6 660 142 0.1 21 3 6 750 142 0.1 22 1 22 1800 97 0 23 1 17 1520 108 0 24 1 8 1360 146 0.3 25 3 15 2520 106 0 26 3 26 2400 127 0

[0068] As shown in Tables 8 and 9, test materials Nos. 17 to 21 according to the present invention showed excellent tensile strength, BH, formability, and corrosion resistance, and maintained excellent bendability after natural aging for four months. Test materials Nos. 22, 23, and 26 had low tensile strength since the cooling rate after homogenization was low. Moreover, these test materials showed insufficient BH. Ridging marks occurred in test material No. 24 due to grain growth during hot rolling since the hot rolling temperature was high. Test material No.25 had a low tensile strength and inferior BH due to a low solution heat treatment temperature.

EXAMPLE 3 AND COMPARATIVE EXAMPLE 3

[0069] Aluminum alloys having compositions shown in Table 10 were cast by using a DC casting method. The resulting ingots were homogenized at 540° C. for six hours and cooled to room temperature at a cooling rate of 300° C./h. The ingots were then heated to a temperature of 400° C. Hot rolling was started at this temperature. The ingots were hot-rolled to a thickness of 4.0 mm, and cold-rolled to a thickness of 1.0 mm.

[0070] The cold-rolled sheets were subjected to a solution heat treatment at 540° C. for five seconds, quenched to a temperature of 120° C. at a cooling rate of 30° C./s, and additional heat treated at 90° C. for three hours after five minutes.

[0071] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and the maximum diameter of Mg—Si compounds and the number of compounds having a diameter of 2-10 &mgr;m were measured according to the same methods as in Example 1. The tensile properties and the minimum bending radius for formability evaluation were also evaluated when four months were passed after the final heat treatment. The results are shown in Tables 11 and 12. 10 TABLE 10 Composition (mass %) Alloy Si Mg Zn Cu Mn Cr V Zr Fe Ti B 17 1.0 0.5 0.18 — — — — — 0.17 0.02 5 18 0.9 0.6 0.28 — — — — — 0.17 0.02 5 19 1.1 0.45 0.2 0.01 0.01 — — — 0.14 0.02 5 20 1.0 0.5 0.15 0.03 0.04 0.1 — — 0.15 0.02 5 21 1.1 0.6 0.2 0.02 0.03 — 0.1 — 0.17 0.02 5 22 1.2 0.7 0.25 0.01 0.05 0.2 — 0.08 0.14 0.02 5 23 0.3 0.6 0.2 0.02 0.08 — — — 0.16 0.02 5 24 1.6 0.6 0.2 0.02 0.07 — — — 0.16 0.02 5 25 1.1 0.1 0.2 0.01 0.15 — — — 0.16 0.02 5 26 1.1 1.4 0.2 0.01 0.08 — — — 0.16 0.02 5 27 1.1 0.5 0.04 0.02 — — — — 0.16 0.02 5 28 1.1 0.5 0.6 0.01 0.1 0.1 — — 0.16 0.02 5 29 1.1 0.5 0.2 0.02 0.07 — — — 0.5 0.02 5 Note: Unit for B is ppm.

[0072] 11 TABLE 11 Corrosion resistance Formability Maximum BH Minimum length &sgr;0.2 Test Tensile properties inner of filiform after mate- &sgr;B &sgr;0.2 &dgr; EV bending corrosion BH rial Alloy (MPa) (MPa) (%) (mm) radius (mm) (mm) (MPa) 27 17 243 124 30 10.8 0 0.5 208 28 18 247 126 30 10.6 0.1 1.5 210 29 19 246 128 31 10.8 0 1.0 213 30 20 247 125 31 10.6 0 1.5 209 31 21 249 127 30 10.6 0.1 1.5 211 32 22 251 129 29 10.5 0.2 1.5 214 33 23 186 75 31 10.8 0 0 149 34 24 254 137 30 10.9 0.3 1.0 216 35 25 182 77 32 11 0 1 172 36 26 280 142 29 10.2 0.6 1.0 229 37 27 245 128 30 10.4 0 2.0 215 38 28 247 132 29 10.6 0 3.0 218 39 29 252 134 28 9.4 0.4 1.5 221

[0073] 12 TABLE 12 Properties after Number of natural aging for Maximum compounds 4 months diameter with Minimum Test of Mg—Si diameter inner mate- compound of 2-10 &mgr;m &sgr;0.2 bending rial Alloy (&mgr;m) (/mm2) (MPa) radius (mm) 27 17 8 560 142 0.1 28 18 9 820 144 0.2 29 19 7 540 145 0.1 30 20 8 810 145 0.1 31 21 8 820 144 0.1 32 22 9 830 146 0.2 33 23 6 380 93 0 34 24 12 890 156 0.5 35 25 5 250 94 0 36 26 18 2430 158 0.7 37 27 8 710 144 0.1 38 28 7 860 150 0.2 39 29 8 1140 150 0.5

[0074] As shown in Tables 11 and 12, test materials Nos. 27 to 32 according to the present invention showed excellent BH of more than 200 MPa in the BH evaluation. The test materials Nos. 27 to 32 had excellent formability in which the forming height (EV) was more than 10 mm and the minimum inner bending radius was 0.2 mm or less. The test materials Nos. 27 to 32 exhibited excellent corrosion resistance in which the maximum length of filiform corrosion was 2 mm or less.

[0075] On the contrary, test material No. 33 and test material No. 35 showed insufficient BH due to low Si content and low Mg content, respectively. Test material No. 34 and test material No. 36 exhibited insufficient bendability due to high Si content and high Mg content, respectively. Test materials Nos. 37 and 38 exhibited inferior filiform corrosion resistance due to low Zn content and high Zn content, respectively. Test material No.39 had a small forming height (EV) due to high Fe content. Moreover, the test material No.39 showed insufficient bendability.

EXAMPLE 4 AND COMPARATIVE EXAMPLE 4

[0076] Ingots of the alloy No.17 of Example 3 were homogenized at 540° C. for five hours. The ingots were cooled and hot-rolled to a thickness of 5.0 mm under conditions shown in Table 13. The hot-rolled products were cold-rolled to a thickness of 1.0 mm, subjected to a solution heat treatment under conditions shown in Table 13, quenched to 120° C. at a cooling rate of 150° C./s, and additional heat treated at 80° C. for two hours after five minutes. In Example 4 and Comparative Example 4, the ingots were cooled to the hot rolling temperature after homogenization, and hot rolling was started at this temperature.

[0077] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and the maximum diameter of Mg—Si compounds and the number of compounds having a diameter of 2-10 &mgr;m were measured according to the same methods as in Example 1. The tensile properties and the minimum bending radius for formability evaluation were also evaluated when four months were passed after the final heat treatment. Electrodeposition coating was performed after applying 10% tensile deformation in the direction at 90° to the rolling direction. The presence or absence of ridging marks was evaluated with the naked eye. The results are shown in Tables 14 and 15. 13 TABLE 13 Cooling rate Hot rolling Solution heat after start treatment Test homogenization temperature condition material Alloy (° C./h) (° C.) (° C.) − (sec) 40 17 300 400 550 − 5  41 17 200 470 530 − 10 42 17 600 440 540 − 10 43 17 40 450 550 − 5  44 17 300 540 520 − 10 45 17 250 420 450 − 10

[0078] 14 TABLE 14 Corrosion resistance Formability Maximum BH Inner Occurrence length &sgr;0.2 Test Tensile properties minimum of of filiform after mate- &sgr;B &sgr;0.2 &dgr; EV bending ridging corrosion BH rial Alloy (MPa) (MPa) (%) (mm) radius (mm) mark (mm) (MPa) 40 17 245 125 30 10.7 0 None 0.5 207 41 17 240 124 31 10.8 0 None 1.0 208 42 17 247 128 30 10.7 0 None 1.0 207 43 17 205 97 30 10.8 0 None 1.0 175 44 17 248 129 31 10.5 0.1 Observed 0.5 209 45 17 195 84 31 11.0 0 None 0.5 162

[0079] 15 TABLE 15 Properties after Number of natural aging Maximum compounds for 4 months diameter of with Minimum Test Mg—Si diameter inner mate- compound of 2-10 &mgr;m &sgr;0.2 bending rial Alloy (&mgr;m) (/mm2) (MPa) radius (mm) 40 17 7 620 141 0.1 41 17 8 750 140 0.1 42 17 7 580 144 0.1 43 17 15 1360 111 0 44 17 7 1550 146 0.2 45 17 18 2420 97 0

[0080] As shown in Tables 14 and 15, test materials Nos. 40 to 42 according to the present invention showed excellent tensile strength, BH, formability, and corrosion resistance, and maintained excellent bendability after natural aging for four months. Test material No. 43 had low tensile strength and insufficient BH since the cooling rate after homogenization was low. Ridging marks occurred in test material No. 44due to texture growth during hot rolling, since the hot rolling temperature was high. Test material No. 45 had a low tensile strength and inferior BH due to a low solution treatment temperature.

EXAMPLE 5

[0081] Aluminum alloys having compositions shown in Table 16 were cast by using a DC casting method. The resulting ingots were homogenized at 540° C. for six hours and cooled to room temperature at a cooling rate of 300° C./h. The ingots were heated to a temperature of 400° C., and hot rolling was started at this temperature. The ingots were hot-rolled to a thickness of 4.0 mm, and cold-rolled to a thickness of 1.0 mm.

[0082] The cold-rolled sheets were subjected to a solution heat treatment at 540° C. for five seconds, quenched to a temperature of 120° C. at a cooling rate of 30° C./s, and additional heat treated at 100° C. for three hours after five minutes.

[0083] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated according to the same methods as in Example 1 when 10 days were passed after final heat treatment. In addition, misorientation distributions of crystal grain boundaries were measured according to the following method. The results are shown in Table 17.

[0084] Measurement of misorientation distribution of crystal grain boundaries: The surface of the test material was ground using emery paper and mirror-ground by electrolytic grinding. The test material was set in a scanning electron microscope (SEM). The tilt angle distributions of the crystal grain boundaries were measured by measuring the crystal grain orientation at a pitch of 10 &mgr;m using an EBSP device installed in the SEM at an observation magnification of 100 times to calculate the percentage of crystal grain boundaries at 15° or less. 16 TABLE 16 Composition (mass %) Alloy Si Mg Cu Mn Cr V Zr Fe Zn Ti B 30 1.0 0.5 — 0.05 — — — 0.13 0.01 0.02 5 31 0.8 0.6 0.02 0.08 — — — 0.15 0.01 0.03 7 32 1.2 0.4 0.01 0.08 — — — 0.16 0.02 0.02 6 33 1.1 0.5 0.01 0.08 — — — 0.19 0.28 0.02 4 34 1.0 0.5 0.7 0.10 — — — 0.16 0.02 0.03 5 35 1.1 0.4 0.01 0.05 0.10 — — 0.17 0.02 0.03 6 36 1.1 0.5 0.01 0.15 — 0.13 — 0.13 0.04 0.02 5 37 1.1 0.5 0.5 0.07 — — 0.08 0.15 0.03 0.02 4 Note: Unit for B is ppm.

[0085] 17 TABLE 17 Corrosion Percentage resistance of crystal Formability Maximum BH grain Minimum length &sgr;0.2 Test boundaries Tensile properties inner of filiform after mate- at 15° &sgr;B &sgr;0.2 &dgr; EV bending corrosion BH rial Alloy or less (%) (MPa) (MPa) (%) (mm) radius (mm) (mm) (MPa) 46 30 38 242 125 32 10.5 0.1 0 213 47 31 35 247 134 31 10.2 0.2 1.3 222 48 32 42 242 125 32 10.7 0.1 0.4 213 49 33 41 242 126 30 10.5 0.1 0 216 50 34 36 278 139 30 10.4 0.1 3.2 225 51 35 43 261 136 32 10.5 0.2 1.2 218 52 36 46 258 129 29 10.4 0.2 1.1 210 53 37 42 265 135 30 10.5 0.2 2.7 222

[0086] As shown in Table 17, test materials Nos. 46 to 53 according to the conditions of the present invention showed excellent BH of more than 200 MPa in the BH evaluation. The test materials Nos. 46 to 53 had excellent formability in which the forming height (EV) was more than 10 mm and the minimum inner bending radius was 0.2 mm or less. The test materials Nos. 46 to 53 exhibited excellent corrosion resistance in which the maximum length of filiform corrosion was 4 mm or less.

COMPARATIVE EXAMPLE 5

[0087] Aluminum alloys having compositions shown in Table 18 were cast by using a DC casting method. The resulting ingots were treated by the same steps as in Example 5 to obtain cold-rolled sheets with a thickness of 1.0 mm. The cold-rolled sheets were subjected to a solution heat treatment and quenched under the same conditions as in Example 1. The quenched products were additional heat treated at 100° C. for three hours after five minutes.

[0088] The final heat treated sheets were used as test materials. Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and misorientation distributions of crystal grain boundaries were measured according to the same methods as in Example 5. The results are shown in Table 19. 18 TABLE 18 Composition (mass %) Alloy Si Mg Cu Mn Cr V Zr Fe Zn Ti B 38 0.3 0.5 0.02 0.06 0.01 — — 0.15 0.02 0.03 5 39 1.7 0.5 0.02 0.05 0.01 — — 0.14 0.03 0.02 6 40 1.0 0.1 0.02 0.04 0.01 — — 0.17 0.02 0.03 4 41 1.1 1.5 0.02 0.05 0.01 — — 0.16 0.03 0.03 5 42 1.0 0.5 0.02 0.06 0.01 — — 0.13 0.6 0.02 4 43 1.1 0.6 1.3 0.05 0.01 — — 0.15 0.03 0.02 6 44 1.0 0.5 0.01 0.5 0.01 — — 0.17 0.03 0.03 4 45 1.0 0.5 0.01 0.06 0.4 — — 0.16 0.02 0.02 5 46 1.1 0.6 0.01 0.05 0.01 0.4 — 0.14 0.02 0.03 4 47 1.1 0.6 0.01 0.06 0.01 — 0.23 0.16 0.03 0.02 5 48 1.0 0.6 0.02 0.02 0.01 — — 0.14 0.02 0.03 5 Note: Unit for B is ppm.

[0089] 19 TABLE 19 Percentage of crystal Corrosion BH grain Formability resistance &sgr;0.2 boundaries Tensile properties Minimum inner Maximum length after Test at 15° or &sgr;B &sgr;0.2 &dgr; EV bending radius of filiform BH material Alloy less (%) (MPa) (MPa) (%) (mm) (mm) corrosion (mm) (MPa) 54 38 27 161 68 29 10.8 0 0.4 123 55 39 42 268 142 31 10.6 0.6 1.1 226 56 40 31 160 68 32 10.7 0 1.6 119 57 41 39 279 140 30 10.2 0.7 1.1 228 58 42 41 248 125 31 10.6 0.2 6.8 220 59 43 35 291 129 29 10.5 0.4 5.5 226 60 44 46 245 128 27 9.5 0.7 0.9 215 61 45 51 244 126 29 9.6 0.8 1.1 213 62 46 48 251 131 28 9.8 0.8 1.0 214 63 47 43 244 130 27 9.5 0.7 1.3 214 64 48 17 243 124 30 10.3 0.8 0.4 210

[0090] As shown in Table 19, test material No. 54 and test material No. 56 exhibited insufficient BH due to low Si content and low Mg content, respectively. Test material No.55 and test material No. 57 exhibited insufficient bendability due to high Si content and high Mg content, respectively. Test material No. 58 and test material No. 59 showed inferior filiform corrosion resistance due to high Zn content and high Cu content, respectively. Test materials Nos. 60 to 63 had a small forming height (EV) and insufficient bendability due to high Mn content, high Cr content, high V content, and high Zr content, respectively. Test material No. 64 exhibited insufficient bendability since the percentage of crystal grain boundaries in which misorientation of adjacent crystal grains was 15° or less was less than 20% due to low Mn content.

EXAMPLE 6

[0091] Ingots of the alloy No. 30 shown in Table 16 used in Example 5 were subjected to homogenization, hot rolling, cold rolling, solution heat treatment, additional heat treatment, and reversion treatment under conditions shown in Table 20 to obtain test materials Nos. 65 to 71. In this example, the ingots were cooled to the hot rolling temperature after homogenization, and hot rolling was started at this temperature. Moreover, the homogenization time was six hours, the thickness of the hot-rolled sheet was 4.0 mm, the thickness of the cold-rolled sheet was 1.0 mm, and the period of time between quenching and additional heat treatment was five minutes. The test material No. 65 was subjected to the reversion treatment at 200° C. for three seconds after the additional heat treatment. The reversion treatment was performed when one day was passed after the additional heat treatment.

[0092] Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and misorientation distributions of crystal grain boundaries were measured according to the same methods as in Example 5. The results are shown in Table 21. Electrodeposition coating was performed after applying 10% tensile deformation in the direction at 90° to the rolling direction. The presence or absence of ridging marks was evaluated with the naked eye. As a result, occurrence of ridging marks was not observed at all. 20 TABLE 20 Homogenization Solution heat Cooling rate Hot rolling treatment Additional heat after Start Cooling treatment Test Temp. homogenization temperature Temp. Time rate Temp. Time material Alloy (° C.) (° C./h) (° C.) (° C.) (s) (° C./s) (° C.) (h) 65 30 540 300 400 550 5 30 100 3 66 30 520 300 400 550 5 30 100 3 67 30 540 200 400 550 5 30 100 3 68 30 540 300 450 550 5 30 100 3 69 30 540 300 400 520 30 30 100 3 70 30 540 300 400 550 5 10 100 3 71 30 540 300 400 550 10 30 60 5

[0093] 21 TABLE 21 Percentage of crystal Corrosion BH grain Formability resistance &sgr;0.2 boundaries Tensile properties Minimum inner Maximum length after Test at 15° or &sgr;B &sgr;0.2 &dgr; EV bending radius of filiform BH material Alloy less (%) (MPa) (MPa) (%) (mm) (mm) corrosion (mm) (MPa) 65 30 41 237 122 31 10.8 0.1 0.3 226 66 30 47 238 117 30 10.4 0.3 0.6 206 67 30 24 241 124 31 10.7 0.3 0.5 206 68 30 27 245 126 31 10.9 0 0.2 215 69 30 48 235 118 31 10.6 0 0.4 207 70 30 37 239 122 31 10.7 0.2 0.6 208 71 30 35 245 126 31 10.7 0.1 0.2 204

[0094] As shown in Table 21, the test materials Nos. 65 to 71 according to The present invention showed excellent tensile strength, BH, formability, and corrosion resistance. Moreover, occurrence of ridging marks was not observed at all.

COMPARATIVE EXAMPLE 6

[0095] Ingots of the alloy No. 30 shown in Table 16 used in Example 5 were subjected to homogenization, hot rolling, cold rolling, solution heat treatment, additional heat treatment, and reversion treatment under conditions shown in Table 22 to obtain test materials Nos. 72 to 80. In this example, the ingots were cooled to the hot rolling temperature after homogenization, and hot rolling was started at this temperature. Moreover, the homogenization time was six hours, the thickness of the hot-rolled sheet was 4.0 mm, the thickness of the cold-rolled sheet was 1.0 mm, and the period of time between quenching and additional heat treatment was five minutes. The test material No. 80 was subjected to the reversion treatment at 300° C. for 30 seconds. The reversion treatment was performed when one day was passed after the additional heat treatment.

[0096] Tensile properties, formability, corrosion resistance, and bake hardenability of the test materials were evaluated when 10 days were passed after final heat treatment, and misorientation distributions of crystal grain boundaries were measured according to the same methods as in Example 5. The results are shown in Table 23. Electrodeposition coating was performed after applying 10% tensile deformation in the direction at 90° to the rolling direction. The presence or absence of ridging marks was evaluated with the naked eye. As a result, occurrence of ridging marks was observed in the test material No. 74. 22 TABLE 22 Homogenization Solution heat Cooling rate Hot rolling treatment Additional heat after Start Cooling treatment Test Temp. homogenization temperature Temp. Time rate Temp. Time material Alloy (° C.) (° C./h) (° C.) (° C.) (s) (° C./s) (° C.) (h) 72 30 450 300 400 550 5 30 100 3 73 30 540 100 400 560 10 30 100 3 74 30 540 50 400 560 20 30 100 3 75 30 540 300 500 550 5 30 100 3 76 30 540 300 400 470 10 30 100 3 77 30 540 300 400 550 5 1 100 3 78 30 540 300 400 550 5 30 — — 79 30 540 300 400 550 5 30 140 72 80 30 540 300 400 550 5 30 100 3

[0097] 23 TABLE 23 Percentage of crystal Corrosion BH grain Formability resistance &sgr;0.2 boundaries Tensile properties Minimum inner Maximum length after Test at 15° or &sgr;B &sgr;0.2 &dgr; EV bending radius of filiform BH material Alloy less (%) (MPa) (MPa) (%) (mm) (mm) corrosion (mm) (MPa) 72 30 18 215 102 30 9.3 0.8 1.3 172 73 30 15 225 110 31 10.3 0.7 0.7 195 74 30 11 221 107 31 10.4 0.8 0.8 191 75 30 16 243 127 32 10.6 0.7 0.4 218 76 30 43 209 96 27 9.4 0 1.2 164 77 30 35 213 99 28 9.4 0.7 6.2 183 78 30 32 241 124 31 10.8 0.1 0.3 175 79 30 38 281 165 29 9.6 0.4 0.4 228 80 30 36 181 82 30 9.8 0.2 0.2 153

[0098] As shown in Table 23, the test material No. 72 had low EV and insufficient bendability due to a low homogenization temperature. Moreover, the test material No.72 showed inferior BH. The test materials Nos. 73 and 74 showed insufficient bendability and inferior BH due to a low cooling rate after homogenization. Ridging marks occurred in the test material No. 75 due to inferior bendability since the hot rolling start temperature was high. The test material No.76 had low strength and low EV due to a low solution treatment temperature. Moreover, the test material No. 76 had low BH. The test material No. 77 showed insufficient EV, bendability, and corrosion resistance due to a low quenching rate after the solution heat treatment. Moreover, the test material No. 77 showed insufficient strength and BH. The test material No. 78 had low BH since additional heat treatment was not performed. The test material No. 79 had low EV since the additional heat treatment was performed at a high temperature for a long period of time. The test material No. 80 had low strength and low BH since the reversion treatment temperature was high. Moreover, the test material No. 80 had low EV.

EXAMPLE 7

[0099] Aluminum alloys having compositions shown in Table 24 were cast by using a DC casting method. The resulting ingots were homogenized at 550° C. for six hours and cooled to 200° C. at a cooling rate of 600° C./h. The ingots were cooled to room temperature, heated to 420° C., and hot-rolled to a thickness of 4.5 mm. The hot rolling finish temperature was 250° C.

[0100] The hot-rolled products were cold-rolled to a thickness of 1.0 mm. The cold-rolled sheets were subjected to a solution heat treatment at 540° C. for 20 seconds and quenched to 120° C. at a cooling rate of 30° C./s. The quenched sheets were additional heat treated at 100° C. for three hours after three minutes.

[0101] Tensile performance, anisotropy of Lankford values, bake hardenability (BH), and bendability of the aluminum alloy sheets were evaluated according to the following methods when 10 days were passed after the final heat treatment. The results are shown in Table 25.

[0102] Tensile performance: Tensile specimens were collected in three directions (at 0°, 45°, and 90° to the rolling direction), and subjected to a tensile test to determine average values of tensile strength, yield strength, and elongation as the tensile performance.

[0103] Anisotropy of Lankford values: Tensile specimens were collected in three directions (at 0°, 45°, and 90° to the rolling direction), and subjected to a tensile test to determine the Lankford values rat 15% deformation, and to calculate anisotropy of the Lankford values.

[0104] Bake hardenability (BH): Yield strength was measured after applying 2% tensile deformation in the rolling direction and performing heat treatment at 170° C. for 20 minutes. A test material having a yield strength of 200 MPa or more was accepted.

[0105] Bendability: A 180° bending test for measuring the minimum bending radius was performed after applying 15% tensile prestrain. A test material having a minimum inner bending radius of 0.1 mm or less was accepted. 24 TABLE 24 Composition (wt %) Alloy Si Mg Zn Cu Mn Cr V Zr Fe Ti B 49 1.0 0.65 — — — — — — 0.25 0.03 10 50 1.0 0.48 — 0.02 0.09 — — — 0.17 0.02 5 51 0.91 0.53 0.18 0.01 0.1  — — — 0.18 0.02 5 52 1.0 0.4 0.02 0.72 0.1  — — — 0.18 0.02 5 53 1.6 0.34 — — — 0.05 — — 0.18 0.02 5 54 1.1 0.54 0.02 — 0.05 — 0.08 — 0.13 0.01 7 55 0.8 1.1 0.01 0.02 0.07 — — 0.08 0.15 0.02 5 Note: Unit for B is ppm.

[0106] 25 TABLE 25 Tensile performance Yield Tensile Yield strength Anisotropy Minimum inner Test strength strength Elongation after BH of Lankford bending radius material Alloy (MPa) (MPa) (%) (MPa) values r (mm) 81 49 246 132 30 212 0.66 0.0 82 50 237 122 31 206 0.73 0.0 83 51 241 130 30 210 0.70 0.0 84 52 266 127 31 220 0.45 0.1 85 53 252 141 31 223 0.62 0.1 86 54 239 132 30 219 0.66 0.0 87 55 254 138 29 226 0.57 0.1

[0107] As shown in Table 25, test materials Nos. 81 to 87 according to the present invention excelled in strength and BH, had anisotropy of the Lankford values of more than 0.4, and showed excellent minimum bending properties. Bendability after natural aging for four months was evaluated. As a result, the test materials of all the alloys had a minimum bending radius of 0.0˜0.1.

COMPARATIVE EXAMPLE 7

[0108] Aluminum alloys having compositions shown in Table 26 were cast by using a DC casting method. The resulting ingots were treated by the same steps as in Example 7. Tensile performance, anisotropy of Lankford values, bake hardenability (BH), and bendability of the aluminum alloy sheets were evaluated according to the same methods as in Example 7 when 10 days were passed after the final heat treatment. The results are shown in Table 27. 26 TABLE 26 Composition (wt %) Alloy Si Mg Zn Cu Mn Cr V Zr Fe Ti B 56 0.34 0.6 — 0.01 0.06 0.01 — — 0.2  0.02 5 57 2.4 0.5 — 0.01 0.06 — — — 0.18 0.02 5 58 1.1 0.14 — 0.01 — 0.05 — — 0.15 0.02 5 59 0.7 1.4 0.1 0.01 — 0.05 — — 0.15 0.02 5 60 1.7 1.3 — 0.01 0.06 — — — 0.18 0.02 5 61 1.1 0.48 — 1.5 — — — 0.1 0.18 0.02 5 62 1.1 0.53 — 0.02 1.2 — — — 0.15 0.02 5 63 1.1 0.53 — 0.03 — 0.4  — — 0.17 0.02 5 64 1.1 0.45 — 0.02 — 0.01 0.4 — 0.22 0.02 5 65 1.1 0.61 — 0.01 — — — 0.3 0.14 0.02 5 Note: Unit for B is ppm.

[0109] 27 TABLE 27 Tensile performance Yield Tensile Yield strength Anisotropy Minimum inner Test strength strength Elongation after BH of Lankford bending radius material Alloy (MPa) (MPa) (%) (MPa) values r (mm) 88 56 152 83 29 123 0.62 0.0 89 57 263 148 31 231 0.34 0.6 90 58 162 85 30 132 0.62 0.0 91 59 249 138 29 194 0.26 0.6 92 60 270 154 28 230 0.31 0.6 93 61 283 147 30 243 0.38 0.7 94 62 253 141 29 227 0.26 0.6 95 63 242 133 28 218 0.32 0.5 96 64 239 135 29 217 0.22 0.6 97 65 242 141 28 220 0.15 0.7

[0110] As shown in Table 27, test material No. 88 and test material No. 90 exhibited low strength and insufficient BH due to low Si content and low Mg content, respectively. Test material No. 89 had high strength due to high Si content, whereby anisotropy of Lankford values was decreased and bendability was insufficient Test material No. 91 had a small anisotropy of Lankford values since the value for (Si %−0.58Mg %) was smaller than 0.1%, whereby minimum bendability was insufficient.

[0111] Test material No. 92 had a small anisotropy of Lankford values since (0.7Si %+Mg %) exceeded 2.2%, whereby bendability was insufficient. Test materials No. 93 to 97 had a small anisotropy of Lankford values due to high Cu content, high Mn content, high Cr content, high V content, and high Zr content, respectively, whereby bendability was insufficient.

EXAMPLE 8 AND COMPARATIVE EXAMPLE 8

[0112] The alloy No. 50 shown in Table 24 was cast by using a DC casting method. The resulting ingots were homogenized at 540° C. for 10 hours and cooled to 250° C. at cooling rates shown in Table 28. The ingots were then cooled to room temperature. The ingots were heated to the temperatures shown in Table 28 and hot-rolled to a thickness of 4.2 mm. The hot rolling finish temperature was 280° C. The hot-rolled products were cold-rolled to obtain sheets with a thickness of 1.0 mm. only test material No. 107 was cold-rolled to a thickness of 3.0 mm and subjected to process annealing at 450° C. for 30 seconds.

[0113] The cold-rolled sheets were subjected to a solution heat treatment at 550° C. for 10 seconds and quenched to 120° C. at a cooling rate of 30° C./s. The quenched sheets were additional heat treated at 100° C. for three hours after three minutes. Tensile performance, anisotropy of Lankford values, BH, and bendability of the aluminum alloy sheets obtained by these steps were evaluated according to the same methods as in Example 7.

[0114] For the evaluation of ridging marks, tensile specimens were collected in the direction at 90° to the rolling direction and subjected to 10% tensile deformation and electrodeposition coating. The presence or absence of ridging marks was then evaluated. The results are shown in Table 29. 28 TABLE 28 Cooling rate after Hot rolling start Condition homogenization (° C./h) temperature (° C.) a 550 420 b 200 400 c 3000 430 d 480 480 e 480 360 f 380 550 g 3000 530 h 50 400 i 30 520 j 550 420

[0115] 29 TABLE 29 Minimum Tensile performance Yield Anisotropy inner Tensile Yield strength of bending Test strength strength Elongation after Lankford radius Occurrence of material Condition (MPa) (MPa) (%) BH (MPa) values r (mm) ridging mark 98 a 230 121 30 210 0.55 0.0 None 99 b 218 118 31 207 0.62 0.0 None 100 c 234 132 30 226 0.58 0.1 None 101 d 241 130 31 230 0.51 0.1 None 102 e 225 123 32 219 0.67 0.0 None 103 f 236 127 31 227 0.45 0.3 Observed 104 g 238 131 29 222 0.33 0.3 Observed 105 h 212 107 31 193 0.25 0.5 None 106 i 231 125 30 214 0.18 0.6 Observed 107 j 224 118 29 204 0.1 0.4 None

[0116] As shown in Table 29, test materials Nos. 98 to 102 according to The present invention excelled in strength and BH, had an anisotropy of Lankford values of more than 0.4, and showed excellent minimum bending properties.

[0117] On the contrary, ridging marks occurred in test materials Nos. 103 and 104 due to a high hot rolling temperature. Test material No. 105 had a small anisotropy of Lankford values due to a low cooling rate after homogenization, whereby bendability was insufficient. Ridging marks occurred in test material No. 106 due to a high hot rolling temperature and a low cooling rate after homogenization. Moreover, the test material No. 106 had a small anisotropy of Lankford values, whereby bendability was insufficient. Test material No. 107 had a small anisotropy of Lankford values since process annealing was performed, whereby bendability was insufficient.

EXAMPLE 9

[0118] The alloy No. 50 shown in Table 24 was cast by using a DC casting method. The resulting ingots were homogenized at 550° C. for eight hours and cooled to 200° C. at a cooling rate of 500° C./h. The ingots were cooled to room temperature, heated to 400° C., and hot-rolled to a thickness of 4.2 mm. The hot rolling finish temperature was 260° C.

[0119] The hot-rolled products were cold-rolled to obtain sheets with a thickness of 1.0 mm. The cold-rolled sheets were subjected to a solution heat treatment at 550° C. for four seconds and quenched to 120° C. at a cooling rate of 40° C./s. The quenched sheets were additional heat treated at 100° C. for two hours after two minutes.

[0120] The aluminum alloy sheets obtained by these steps were subjected to measurements of tensile strength, yield strength, elongation, Lankford value r, yield strength after BH, and minimum bending radius in the directions at 0°, 45°, and 90° to the rolling direction by using the same methods as in Example 7 when seven days were passed after the final heat treatment. Anisotropy of Lankford values r was calculated and the presence or absence of ridging marks was evaluated. The results are shown in Table 30. As shown in Table 30, excellent properties were obtained in all the directions. 30 TABLE 30 Minimum Tensile performance Yield inner Angle to Tensile strength Anisotropy bending Occurrence rolling strength Yield Elongation after BH n r of Lankford radius of ridging direction (MPa) (MPa) (%) (MPa) value value values r (mm) mark  0° 241 128 23 227 0.26 0.66 0.61 0.0 None 45° 225 112 37 205 0.29 0.18 0.0 None 90° 234 122 30 221 0.27 0.92 0.0 None

EXAMPLE 10

[0121] Aluminum alloys having compositions shown in Table 31 were cast by using a DC casting method. The resulting ingots were homogenized at 550° C. for six hours and cooled to 200° C. at a cooling rate of 450° C./h. The ingots were then cooled to room temperature, heated to 420° C., and hot-rolled to a thickness of 4.5 mm. The hot rolling finish temperature was 250° C.

[0122] The hot-rolled products were cold-rolled to obtain sheets with a thickness of 1.0 mm. The cold-rolled sheets were subjected to a solution heat treatment at 540° C. for 20 seconds and quenched to 120° C. at a cooling rate of 30° C./s. The sheets were additional heat treated at 100° C. for three hours after three minutes.

[0123] The aluminum alloy sheets were subjected to a tensile test when 10 days were passed after the final heat treatment. Bake hardenability (BH), intensity ratio (random ratio) of cube orientation, and bendability were evaluated according to the following methods. The results are shown in Table 32.

[0124] Intensity ratio of cube orientation: The intensity ratio of cube orientation was calculated by a series expansion method proposed by Bunge using an ODF analysis device in which the expansion order of even-numbered terms was 22 and the expansion order of odd-numbered terms was 19.

[0125] Bake hardenability (BH): Yield strength was measured after applying 2% tensile deformation and performing heat treatment at 170° C. for 20 minutes. A test material having a yield strength of 200 MPa or more was accepted.

[0126] Bendability: A 180° bending test for measuring the minimum bending radius was performed after applying 15% tensileprestrain. A test material having a minimum inner bending radius of 0.2 mm or less was accepted. 31 TABLE 31 Composition (wt %) Alloy Si Mg Zn Cu Mn Cr V Zr Fe Ti B 66 1.0 0.62 — — — — — — 0.24 0.03 10 67 1.0 0.46 — 0.01 0.08 — — — 0.16 0.02 5 68 0.94 0.53 0.18 0.01 0.10 — — — 0.15 0.02 5 69 1.0 0.42 0.04 0.75 0.10 — — — 0.15 0.02 5 70 1.6 0.36 — — — 0.06 — — 0.15 0.02 5 71 1.1 0.54 0.02 — 0.05 — 0.09 — 0.12 0.01 7 72 0.9 1.1 0.01 0.02 0.07 — — 0.07 0.14 0.02 5 Note: Unit for B is ppm.

[0127] 32 TABLE 32 Tensile performance Yield Intensity Tensile Yield strength ratio of Minimum inner Test strength strength Elongation after BH cube bending radius material Alloy (MPa) (MPa) (%) (MPa) orientation (mm) 108 66 244 130 31 208 63 0.1 109 67 238 123 31 207 82 0.0 110 68 239 128 31 212 57 0.1 111 69 263 125 30 222 38 0.2 112 70 252 147 31 226 44 0.2 113 71 241 134 30 221 78 0.1 114 72 253 136 30 228 27 0.2

[0128] As shown in Table 32, test materials Nos.108 to 114 according to The present invention excelled in strength and BH, had an intensity ratio of cube orientation of more than 20, and showed excellent minimum bending properties. Bendability after natural aging for four months was measured. As a result, the test materials of all the alloys had a minimum bending radius of 0.4 or less although the yield strength exceeded 160 MPa.

COMPARATIVE EXAMPLE 9

[0129] Aluminum alloys having compositions shown in Table 33 were cast by using a DC casting method. The resulting ingots were treated by the same steps as in Example 10. Tensile performance, bake hardenability (BH), intensity ratio of cube orientation, and bendability of the aluminum alloy sheets were evaluated according to the same methods as in Example 10 when 10 days were passed after the final heat treatment. The results are shown in Table 34. 33 TABLE 33 Composition (wt %) Alloy Si Mg Zn Cu Mn Cr V Zr Fe Ti B 73 0.37 0.62 — 0.01 0.06 0.01 — — 0.22 0.02 5 74 2.4 0.61 — 0.01 0.06 — — — 0.17 0.02 5 75 1.1 0.13 — 0.01 — 0.05 — — 0.14 0.02 5 76 0.7 1.8  0.1 0.01 — 0.05 — — 0.14 0.02 5 77 1.7 0.46 — 1.5  — — — 0.12 0.17 0.02 5 78 1.1 0.55 — 0.02 1.3  — — — 0.14 0.02 5 79 1.1 0.54 — 0.03 — 0.4  — — 0.17 0.02 5 80 1.1 0.47 — 0.02 — 0.01 0.4 — 0.24 0.02 5 81 1.1 0.63 — 0.01 — — — 0.3  0.13 0.02 5 Note: Unit for B is ppm.

[0130] 34 TABLE 34 Tensile performance Yield Intensity Tensile Yield strength ratio of Minimum inner Test strength strength Elongation after BH cube bending radius material Alloy (MPa) (MPa) (%) (MPa) orientation (mm) 115 73 148  79 30 119 51 0.0 116 74 261 147 31 228 16 0.6 117 75 155  75 29 127 66 0.0 118 76 270 149 29 283 14 0.6 119 77 281 145 29 244  8 0.7 120 78 251 140 29 228 14 0.6 121 79 243 132 27 220 15 0.6 122 80 236 133 29 218 12 0.6 123 81 238 139 29 222 17 0.7

[0131] As shown in Table 34, test material No. 115 and test material No. 117 had low strength and insufficient BH due to low Si content and low Mg content, respectively. Test material No. 116 and test material No. 118 showed high strength since (0.7Si %+Mg %) exceeded 2.2% due to high Si content and high Mg content, respectively. As a result, the degree of integration of cube orientation was decreased, whereby bendability was insufficient.

[0132] The degree of integration of cube orientation was decreased in test materials Nos. 119 to 123 due to high Cu content, high Mn content, high Cr content, high V content, and high Zr content, respectively, whereby bendability was insufficient.

EXAMPLE 11 AND COMPARATIVE EXAMPLE 10

[0133] The alloy No. 67 shown in Table 31 was cast by using a DC casting method. The resulting ingots were homogenized at 550° C. for five hours and cooled to 250° C. at a cooling rate shown in Table 35. The ingots were heated to a temperature shown in Table 35 and hot-rolled to a thickness of 4.4 mm. The hot rolling finish temperature was 250° C. The hot-rolled products were cold-rolled to obtain sheets with a thickness of 1.0 mm. Annealing process was performed at 400° C. for two hours after hot rolling under a condition “t”.

[0134] The sheets were subjected to a solution heat treatment at 550° C. for five seconds and quenched to 120° C. at a cooling rate of 30° C./s. The quenched sheets were additional heat treated at 100° C. for three hours after three minutes. Tensile performance, BH, intensity ratio of cube orientation, and bendability of the aluminum alloy sheets obtained by these steps were evaluated according to the same methods as in Example 10.

[0135] For the evaluation of ridging marks, tensile specimens were collected in the direction at 90° to the rolling direction and subjected to 10% tensile deformation and electrodeposition coating. The presence or absence of ridging marks was then evaluated.

[0136] The results are shown in Table 36. 35 TABLE 35 Cooling rate after Hot rolling start Condition homogenization (° C./h) temperature (° C.) k 550 420 l 200 430 m 3500 410 n 500 470 o 450 350 p 360 540 q 2000 520 r 50 410 s 25 530 t 500 420

[0137] 36 TABLE 36 Tensile performance Yield Intensity Minimum Occurrence Tensile Yield strength ratio of inner of Test strength strength Elongation after BH cube bending radius ridging material Condition (MPa) (MPa) (%) (MPa) orientation (mm) mark 124 k 232 122 29 213 77 0.0 None 125 l 224 120 31 206 85 0.0 None 126 m 232 131 30 227 73 0.1 None 127 n 241 131 31 232 70 0.1 None 128 o 225 123 31 220 83 0.0 None 129 p 235 126 30 224 35 0.3 Observed 130 q 230 126 28 218 28 0.3 Observed 131 r 214 109 30 190 11 0.5 None 132 s 233 123 30 213  7 0.6 Observed 133 t 226 118 30 208 15 0.4 None

[0138] As shown in Table 36, test materials Nos. 124 to 128 according to The present invention excelled in strength and BH, had an intensity ratio of cube orientation of more than 20, and showed excellent minimum bending properties.

[0139] On the contrary, ridging marks occurred in test materials Nos. 129 and 130 due to a high hot rolling temperature. Test material No. 131 had a small degree of integration of cube orientation due to a low cooling rate after homogenization, whereby bendability was insufficient. Ridging marks occurred in test material No. 132 due to a high hot rolling temperature and a low cooling rate after homogenization. Moreover, the test material No. 132 had a small degree of integration of cube orientation, whereby bendability was insufficient. Test material No. 133 had a small degree of integration of cube orientation since process annealing was performed, whereby bendability was insufficient.

Industrial Applicability

[0140] According to The present invention, an aluminum alloy sheet having excellent bendability which allows flat hemming, excellent bake hardenability, and excellent corrosion resistance, and a method for producing the same can be provided. The aluminum alloy sheet is suitably used as a lightweight automotive member having a complicated shape which is subjected to hemming, such as an automotive hood, trunk lid, and door.

Claims

1. An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, the aluminum alloy sheet having a minimum inner bending radius of 0.5 mm or less by 180° bending after 10% tensile stretching, even if yield strength of the aluminum alloy sheet exceeds 140 MPa through natural aging after a solution heat treatment and quenching.

2. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 1, which comprises 0.5-1.5% (mass %, hereinafter the same) of Si and 0.2-1.0% of Mg, with the balance consisting of Al and impurities.

3. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 1, which comprises 0.5-1.5% of Si and 0.2-1.0% of Mg, with the balance consisting of Al and impurities, in which the maximum diameter of Mg—Si compounds is 10 &mgr;m or less and the number of Mg—Si compounds having a diameter of 2-10 &mgr;m is 1000 per mm2 or less.

4. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 2 or 3, which further comprises 0.1-0.3% of Zn.

5. An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises 0.8-1.2% of Si, 0.4-0.7% of Mg, and 0.1-0.3% of Zn, with the balance consisting of Al and impurities, in which the maximum diameter of Mg—Si compounds is 10 &mgr;m or less and the number of Mg—Si compounds having a diameter of 2-10 &mgr;m is 1000 per mm2 or less, the aluminum alloy sheet having a minimum inner bending radius of 0.2 mm or less by 180° bending after 10% tensile stretching, even if yield strength of the aluminum alloy sheet exceeds 140 MPa through natural aging after a solution heat treatment and quenching.

6. The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 1-5, which further comprises at least one of 0.3% or less (excluding 0%, hereinafter the same) of Mn, 0.3% or less of Cr, 0.2% or less of V, and 0.15% or less of Zr.

7. An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises 0.4-1.5% of Si, 0.2-1.2% of Mg, and 0.05-0.3% of Mn, with the balance consisting of Al and impurities, in which the percentage of crystal grain boundaries in which misorientation of adjacent crystal grains is 15° or less is 20% or more.

8. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 7, which further comprises 0.5% or less of Zn.

9. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 7 or 8, which further comprises at least one of 0.3% or less of Cr, 0.2% or less of V, and 0.15% or less of Zr.

10. An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, in which an anisotropy of Lankford values is more than 0.4.

11. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 10, which comprises 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2%, and Si %−0.58Mg %≧0.1% being satisfied and the balance consisting of Al and impurities.

12. An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, in which an intensity ratio of cube orientation of crystal graphic texture is 20 or more.

13. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 12, which comprises 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2% being satisfied and the balance consisting of Al and impurities.

14. The aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 11 or 13, which further comprises 0.5% or less of Zn.

15. The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 11, 13, and 14, which further comprises at least one of 1.0% or less of Mn, 0.3% or less of Cr, 0.2% or less of V, and 0.2% or less of Zr.

16. The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 2-9, 11, and 13-15, which further comprises 1.0% or less of Cu.

17. The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 2-9, 11, and 13-16, which further comprises at least one of 0.1% or less of Ti and 50 ppm or less of B.

18. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 2-6, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 2-6, 16, and 17 at a temperature of 450° C. or more, cooling the ingot to a temperature of 350-500° C. at a cooling rate of 100° C./h or more, starting hot rolling of the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

19. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 2-6, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 2-6, 16, and 17 at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

20. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 2-6, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 2-6, 16, and 17 at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

21. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 7-9, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 7-9, 16, and 17 at a temperature of 480° C. or more, cooling the ingot to a temperature of 300-450° C. at a cooling rate of 150° C./h or more, starting hot rolling of the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

22. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 7-9, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 7-9, 16, and 17 at a temperature of 480° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 150° C./h or more, heating the ingot to a temperature of 300-450° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

23. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 7-9, 16, and 17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 7-9, 16, and 17 at a temperature of 480° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 150° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 300-450° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.

24. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 11 and 13-17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 11 and 13-17 at a temperature of 450° C. or more, cooling the ingot to a specific temperature of less than 350° C. at a cooling rate of 100° C./h or more, hot rolling the ingot at the specific temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

25. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 11 and 13-17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 11 and 13-17 at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

26. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 11 and 13-17, the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims 11 and 13-17 at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.

27. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 18-26, wherein the hot rolling is finished at a temperature of 300° C. or less.

28. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims 18-27, comprising quenching the solution-treated product to 120° C. at a cooling rate of 5° C./s or more, and subjecting the quenched product to a heat treatment at a temperature of 40-120° C. for 50 hours or less within 60 minutes after the quenching.

29. A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to claim 28, comprising subjecting the heat-treated product to a reversion treatment at a temperature of 170-230° C. for 60 seconds or less within seven days after the heat treatment.