Aluminum alloy sheet and method for producing aluminum alloy sheet

Abstract

An aluminum alloy sheet includes, by mass %, 0.3% to 1.5% of Si, 0.15% to 0.4% of Fe, 0.001% to 0.2% of Cu, 0.02% to 0.3% of Mn, 0.3% to 0.8% of Mg, 0.001% to 0.3% of Cr, 0.001% to 0.3% of Zn, and 0.001% to 0.3% of Ti with the remainder being Al and impurities. The aluminum alloy sheet has an area fraction of Cube orientation of 17% to 50%. A number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more in the aluminum alloy sheet is 2650 particles / mm2 or less, and a number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more in the aluminum alloy sheet is 225 particles / mm2 to 700 particles / mm2.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to Japanese Application No. 2025-059109 filed on Mar. 31, 2025. The content of each of these applications is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The present invention relates to an aluminum alloy sheet and a method for producing an aluminum alloy sheet.BACKGROUND ART

[0003] In recent years, social request for weight reduction in automobile bodies is increasing more and more due to considerations to the global environment or the like. In order to meet such requirements, aluminum alloy materials are being used in place of conventional steel materials such as steel sheets, to portions of automobile bodies such as panels (outer panels (external panels) such as hoods, doors, and roofs, and inner panels (internal panels)), bumper reinforcements (bumper R / F), and reinforcing materials such as door beams.

[0004] On the other hand, since automobiles are also required to have aesthetic design in terms of shape, formability is also important in addition to the weight reduction and rigidity for automobile applications as described above.

[0005] For example, a hood, a door, or the like, which is a portion to which an aluminum panel material for automobiles is applied, is constructed by press-forming an outer panel and an inner panel respectively, and then hemming (bending) the outer panel at 1800 to fasten the outer panel to the inner panel.

[0006] During the bending, a locally high strain is concentrated on a bending ridgeline portion, and thus cracking is likely to occur. Therefore, the outer panel is required to have high bendability.

[0007] Since the outer panel is also a portion that is most visible to consumers, it is necessary to ensure a good appearance. On the other hand, when the panel material is deformed, slight unevenness may occur on the surface, causing a poor appearance, and thus good surface condition after press molding is required.

[0008] On the other hand, Patent Literature 1 discloses that an aluminum alloy sheet excellent in bendability and surface quality can be obtained by setting an appropriate cooling rate after a homogenization heat treatment, together with holding a slab after the homogenization heat treatment in a certain temperature range before the start of hot rolling, and setting a cold rolling ratio in cold rolling to 65% or more.CITATION LISTPatent LiteraturePatent Literature 1: JP2018-16879ASUMMARY OF INVENTION

[0010] In the invention of Patent Literature 1, cooling is performed after the homogenization heat treatment, and then a heat treatment of holding in a certain temperature range is performed before the start of hot rolling.

[0011] On the other hand, as a result of studies by the present inventors, as described above, there is a concern that Mg—Si-based compounds may be coarsened by performing the cooling or heat treatment of holding in a certain temperature range between the homogenization heat treatment and the hot rolling. More specifically, as described in Patent Literature 1, it has been found that, when the cooling is performed after the homogenization heat treatment and then the heat treatment of holding in a certain temperature range is performed before the start of hot rolling, coarse Mg—Si-based compounds are precipitated in the middle of cooling, and improvement of bendability is limited. The step of cooling after the homogenization heat treatment and holding in a certain temperature range before the start of hot rolling requires time for cooling and an additional heat treatment, resulting in poor productivity.

[0012] With regard to the bendability, particularly in recent years, there has been a demand for a 6000 series aluminum alloy sheet to have higher bendability. This is due to a growing need for a sharper character line in automobiles and the like, and a need to ensure greater freedom in a combination of the respective thicknesses of an outer panel and an inner panel during bending at automobile manufacturers.

[0013] It is difficult for the aluminum alloy sheet disclosed in Patent Literature 1 to cope with a formability requirement for such strict bending, which is becoming increasingly demanding.

[0014] Therefore, an object of the present invention is to provide an aluminum alloy sheet satisfying both excellent bendability and excellent surface quality, which is capable of meeting the strict requirement for bending and the requirement for the good surface condition after press forming, and a method for producing the same.

[0015] With regard to the production method, an object of the present invention is to provide a method for producing an aluminum alloy sheet having excellent productivity in addition to the bendability and the surface quality.

[0016] As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved by setting an aluminum alloy sheet to have a specific composition range and performing a hot rolling step and a cold rolling step under specific conditions without performing cooling or an additional heat treatment between a homogenization heat treatment and hot rolling, and have completed the present invention.

[0017] That is, the present disclosure includes the following embodiments.[1] An aluminum alloy sheet comprising:0.3 mass % to 1.5 mass % of Si;

[0019] 0.15 mass % to 0.4 mass % of Fe;

[0020] 0.001 mass % to 0.2 mass % of Cu;

[0021] 0.02 mass % to 0.3 mass % of Mn;

[0022] 0.3 mass % to 0.8 mass % of Mg;

[0023] 0.001 mass % to 0.3 mass % of Cr;

[0024] 0.001 mass % to 0.3 mass % of Zn; and

[0025] 0.001 mass % to 0.3 mass % of Ti,

[0026] with the remainder being Al and impurities, wherein

[0027] the aluminum alloy sheet has an area fraction of Cube orientation of 17% to 50%,

[0028] a number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more in the aluminum alloy sheet is 2650 particles / mm2 or less, and

[0029] a number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more in the aluminum alloy sheet is 225 particles / mm2 to 700 particles / mm2.[2] The aluminum alloy sheet according to [1], which is used for an automobile.[3]A method for producing an aluminum alloy sheet, the method comprising: in the following order,

[0030] a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab;

[0031] a homogenization heat treatment step of subjecting the cast slab to a homogenization heat treatment;

[0032] a hot rough rolling step of subjecting the homogenized slab to hot rough rolling to obtain a hot rough rolled sheet;

[0033] a hot finish rolling step of subjecting the hot rough rolled sheet to hot finish rolling to obtain a hot finish rolled sheet;

[0034] a cold rolling step of subjecting the hot finish rolled sheet to cold rolling to obtain a cold rolled sheet; and

[0035] a solution treatment step of subjecting the cold rolled sheet to a solution treatment, wherein

[0036] in the homogenization heat treatment step, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C.,

[0037] no cooling and no additional heat treatment are performed between the homogenization heat treatment step and the hot rough rolling step,

[0038] in the hot rough rolling step, a final pass is performed under a condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011,Z=ε.⁢ exp⁡(Q / RT)wherein ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature,

[0040] the hot finish rolling step is performed such that an end temperature is 250° C. or higher and lower than 350° C.,

[0041] the cold rolling step is performed such that a cold rolling ratio is 74% to 95%, and

[0042] the obtained aluminum alloy sheet contains

[0043] 0.3 mass % to 1.5 mass % of Si,

[0044] 0.15 mass % to 0.4 mass % of Fe,

[0045] 0.001 mass % to 0.2 mass % of Cu,

[0046] 0.02 mass % to 0.3 mass % of Mn,

[0047] 0.3 mass % to 0.8 mass % of Mg,

[0048] 0.001 mass % to 0.3 mass % of Cr,

[0049] 0.001 mass % to 0.3 mass % of Zn, and

[0050] 0.001 mass % to 0.3 mass % of Ti,

[0051] with the remainder being Al and impurities.[4] The method according to [3], wherein

[0052] the homogenization heat treatment step is performed for 1 hour to 30 hours.[5] The method according to [3] or [4], wherein

[0053] the hot rough rolling step is performed such that an entry temperature is 400° C. to 570° C.[6] The method according to [3] or [4], further comprising:

[0054] a step of performing intermediate annealing before the cold rolling step or during the cold rolling step.[7] The method according to [6], wherein

[0055] the intermediate annealing is performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.[8] The method according to [3] or [4], wherein

[0056] the solution treatment step is performed in a temperature range of 520° C. to 580° C.[9] The method according to [3] or [4], wherein

[0057] cooling is performed at a cooling rate of 1° C. / sec to 100° C. / sec after the solution treatment.

[10] The method according to [3] or [4], wherein

[0058] a preliminary aging treatment is performed by holding the sheet at a temperature in a range of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

[0059] According to the present invention, an aluminum alloy sheet satisfying both excellent bendability and excellent surface quality, which is capable of meeting a strict requirement for surface condition after bending or press forming can be obtained. Since the aluminum alloy sheet as described above can be obtained by the homogenization heat treatment at a single temperature, a load on homogenizing equipment can be reduced, and this is also preferable from the viewpoint of productivity.BRIEF DESCRIPTION OF DRAWINGS

[0060] FIG. 1 is a schematic perspective view of a device used in a VDA bending test for evaluating bendability.

[0061] FIG. 2 is a diagram illustrating a punch, which is a pressing and bending jig used when applying a load to a test piece from above in the device in FIG. 1, in which a left side in FIG. 2 is a front view of the punch, and a right side in FIG. 2 is a side view of the punch.DESCRIPTION OF EMBODIMENTS

[0062] Hereinafter, an aluminum alloy sheet according to the present embodiment and a mode for carrying out a method for producing an aluminum alloy sheet will be described in detail. Although the present invention is described in detail with reference to specific embodiments, it is obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

[0063] In the present specification, “to” indicating a numerical range is used to mean that numerical values described therebefore and thereafter are included as a lower limit and an upper limit.<Method for Producing Aluminum Alloy Sheet>

[0064] The method for producing an aluminum alloy sheet according to the present embodiment includes the following steps in the following order.

[0065] Step 1: a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab.

[0066] Step 2: a homogenization heat treatment step of subjecting the slab cast in the step 1 to a homogenization heat treatment.

[0067] Step 3: a hot rough rolling step of subjecting the slab homogenized in the step 2 to hot rough rolling to obtain a hot rough rolled sheet.

[0068] Step 4: a hot finish rolling step of subjecting the hot rough rolled sheet obtained in the step 3 to hot finish rolling to obtain a hot finish rolled sheet.

[0069] Step 5: a cold rolling step of subjecting the hot finish rolled sheet obtained in the step 4 to cold rolling to obtain a cold rolled sheet.

[0070] Step 6: a solution treatment step of subjecting the cold rolled sheet obtained in the step 5 to a solution treatment.

[0071] A step of performing intermediate annealing may be further included before the cold rolling step in the step 5 or during the cold rolling step.

[0072] In the step 6, cooling may be performed at a predetermined cooling rate after the solution treatment.

[0073] After the solution treatment step in the step 6, a step of performing a preliminary aging treatment may be further included.

[0074] Cooling or an additional heat treatment is not performed between the homogenization heat treatment step in the step 2 and the hot rough rolling step in the step 3.

[0075] In the method for producing an aluminum alloy sheet according to the present embodiment, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C. in the homogenization heat treatment step in the step 2. In the hot rough rolling step in the step 3, a final pass is performed under a condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011. In the hot finish rolling step in the step 4, an end temperature is set to 250° C. or higher and lower than 350° C. Next, the cold rolling step in the step 5 is performed such that a cold rolling ratio is 74% to 95%.Z=ε.⁢ exp⁡(Q / RT)

[0076] In the above formula, ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature.

[0077] The obtained aluminum alloy sheet satisfies the following alloy composition:

[0078] 0.3 mass % to 1.5 mass % of Si,

[0079] 0.15 mass % to 0.4 mass % of Fe,

[0080] 0.001 mass % to 0.2 mass % of Cu,

[0081] 0.02 mass % to 0.3 mass % of Mn,

[0082] 0.3 mass % to 0.8 mass % of Mg,

[0083] 0.001 mass % to 0.3 mass % of Cr,

[0084] 0.001 mass % to 0.3 mass % of Zn, and

[0085] 0.001 mass % to 0.3 mass % of Ti,

[0086] with the remainder being Al and impurities.

[0087] The details of the above steps and the aluminum alloy sheet to be obtained will be described below.<Step 1: Casting Step>

[0088] The step 1 is a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab.

[0089] The melting of the raw materials is performed such that a composition of an aluminum alloy sheet to be obtained is adjusted to fall within a desired range. A method for casting the aluminum alloy molten metal which has been melted and adjusted into a slab is not particularly limited, and any common method can be used.

[0090] Examples of the casting method include a continuous casting method, a semi-continuous casting method (a DC casting method), and the like.

[0091] The raw material may be not only an ingot but also scrap produced during production processes such as the production of an aluminum alloy sheet and a pressing process. End of life vehicle (ELV) scrap such as an automobile panel, a wheel, a radiator, and a casting such as an engine block and a suspension may also be used. Further, an aluminum waste material such as a used beverage can (UBC), a home appliance, an aluminum printing plate, an electric wire, and sash scrap may also be used.<Step 2: Homogenization Heat Treatment Step>

[0092] The step 2 is a homogenization heat treatment step of subjecting the slab cast in the step 1 to a homogenization heat treatment.

[0093] The homogenization heat treatment is a heat treatment performed to make a microstructure uniform throughout, as a microstructure and components are non-uniform depending on a location only by casting.

[0094] In the homogenization heat treatment step, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C. After the homogenization heat treatment, the slab is directly subjected to the hot rough rolling step in the step 3 without being subjected to cooling or an additional heat treatment. The present step is advantageous in terms of productivity since the cooling or the additional heat treatment is not performed between the homogenization heat treatment step and the hot rough rolling step.

[0095] The temperature range of 520° C. to 580° C. means a homogenization heat treatment start temperature.

[0096] Here, from the viewpoint of rapidly homogenizing the microstructure, the temperature in the homogenization heat treatment is 520° C. or higher, preferably 525° C. or higher, and more preferably 530° C. or higher. From the viewpoint of preventing local melting due to a decrease in melting point of a segregated portion, the above temperature is 580° C. or lower, preferably 575° C. or lower, and more preferably 570° C. or lower.

[0097] In the step 2 in the present embodiment, a time for performing the homogenization heat treatment is preferably 1 hour to 30 hours. Here, from the viewpoint of favorably eliminating segregation, the time is preferably 1 hour or longer, more preferably 2 hours or longer, still more preferably 3 hours or longer, and may be 4 hours or longer. From the viewpoint of productivity, the time is preferably 30 hours or less, more preferably 20 hours or less, and still more preferably 10 hours or less.<Step 3: Hot Rough Rolling Step>

[0098] The step 3 is a hot rough rolling step of subjecting the slab homogenized in the step 2 to hot rough rolling to obtain a hot rough rolled sheet.

[0099] The slab after the homogenization heat treatment is made to have a predetermined thickness by the hot rough rolling. The hot rough rolling treatment is repeatedly performed using a reversible rolling mill until a predetermined thickness is obtained.

[0100] A start temperature of the hot rough rolling, that is, an entry temperature of the hot rough rolling step is preferably 400° C. to 570° C. Here, it is preferable that the hot rough rolling is performed to achieve a predetermined thickness with as few rolling passes (number of passes) as possible, and from the viewpoint of preventing an increase in the number of rolling passes due to high deformation resistance and difficulty in rolling, the temperature is preferably 400° C. or higher, and more preferably 500° C. or higher. From the viewpoint of preventing coarse recrystallized grains from being formed on the surface, which can cause a rough surface of a final product, the temperature is preferably 570° C. or lower.

[0101] The final pass of the hot rough rolling is performed under the condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011.Z=ε.⁢ exp⁡(Q / RT)

[0102] In the above formula, ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature.

[0103] In order to clarify the recrystallization behavior during the hot rolling in detail, a Zener-Hollomon parameter called a factor Z, which is a temperature-compensated strain rate, is used.

[0104] The larger the Z factor in the final pass of the hot rough rolling, the more recrystallization is promoted during the hot rough rolling and the hot finish rolling, and the more the promotion of the formation of Mg—Si-based compounds due to a processing strain can be prevented, and as a result, an amount of the Mg—Si-based compounds is reduced even after the solution treatment.

[0105] Even when the Mg—Si-based compounds are precipitated during the hot rough rolling, when a rolling reduction rate of the hot rough rolling is high, that is, the factor Z is large, the Mg—Si-based compounds are easily crushed and become fine, and are easily dissolved in solid after the solution treatment.

[0106] Therefore, as the Z factor in the final pass of the hot rough rolling is larger, a number density of the Mg—Si-based compounds after the solution treatment is reduced, and the bendability is improved.

[0107] In addition to the above, when the Z factor is large, an accumulated strain amount becomes large, and uniform and fine recrystallized grains are obtained after the hot rough rolling. When the uniform and fine recrystallized grains are obtained after the hot rough rolling, uniform and fine recrystallized grains are likely to be obtained after any subsequent intermediate annealing or solution treatment, and a rough surface can be prevented when the aluminum alloy sheet is press-formed.

[0108] That is, the Z factor of the final pass in the hot rough rolling is 1.0×108 or more, preferably 5.0×108 or more, and more preferably 1.0×109 or more, from the viewpoint of achieving satisfactory bendability by preventing the formation of the Mg—Si-based compounds and from the viewpoint of favorably preventing the rough surface by forming uniform and fine recrystallized grains. On the other hand, the factor Z is 5.0×1011 or less, preferably 1.0×1011 or less, and more preferably 5.0×1010 or less, from the viewpoint of preventing the rolling from becoming difficult due to the restriction of the rolling load when the temperature of the hot rough rolling is too low or the number of rolling passes in the hot rough rolling is too large.

[0109] In the above formula for determining the Z factor, ε′ represents the strain rate, Q represents the activation energy, R represents the gas constant, and T represents the temperature, and ε′indicating the strain rate is determined by the following formula of Orowan & Pascoe.ε. ? vRR′⁢ hl·1-0.75r1-r·r

[0110] In the above formula, vR represents a roll peripheral speed (mm / sec), R′ represents a flattened roll radius (mm), hl represents an entry sheet thickness (mm), and r represents a rolling reduction rate.

[0111] A thickness of the hot rough rolled sheet after the final pass is not particularly limited, and may be, for example, 10 mm to 100 mm.<Step 4: Hot Finish Rolling Step>

[0112] The step 4 is a hot finish rolling step of subjecting the hot rough rolled sheet obtained in the step 3 to hot finish rolling to obtain a hot finish rolled sheet.

[0113] The hot finish rolling is performed such that an end temperature is 250° C. or higher and lower than 350° C. Here, from the viewpoint of preventing a decrease in productivity due to a slow speed of finish rolling for coil cooling, the end temperature is 250° C. or higher, and may be 270° C. or higher. From the viewpoint of reducing the number density of the Mg—Si-based compounds and obtaining satisfactory bendability, the end temperature is lower than 350° C., preferably lower than 340° C., more preferably lower than 330° C., and still more preferably 310° C. or lower.

[0114] A thickness of the hot finish rolled sheet finally obtained by the step 4 is not particularly limited, and may be, for example, 3 mm to 10 mm.

[0115] A step of performing intermediate annealing may be included after the hot finish rolling in the step 4 and before the subsequent cold rolling step in the step 5. Here, in the case of an aluminum alloy sheet, generally, a crystallographic structure including many orientation factors (crystal grains having these orientations) is generally formed, and crystal planes corresponding thereto are present. Generally, the crystallographic structure in a rolled recrystallized sheet of an aluminum alloy is mainly composed of Cube orientation, Goss orientation, Brass orientation, S orientation, and Copper orientation. The expression of the crystallographic structure is expressed by a rolled plane and a rolling direction in the case of the crystallographic structure of a sheet material by rolling, the rolled plane is expressed by {hkl}, and the rolling direction is expressed by <uvw>. Based on such an expression, each direction is expressed as follows.

[0116] Cube orientation {001}<100>

[0117] Goss orientation {011}<100>

[0118] Brass orientation (B orientation) {011}<211>

[0119] Cu orientation (Copper orientation) {112}<111>

[0120] S orientation {123}<634>

[0121] On the other hand, there is a case where recrystallization does not occur in the hot finish rolled sheet obtained by the hot finish rolling. In this case, the intermediate annealing is preferably performed because the intermediate annealing releases the strain without applying any restraining force, resulting in uniform and fine recrystallized grains and satisfactory surface quality. When the recrystallization occurs, the intermediate annealing may not be performed.

[0122] By performing the subsequent cold rolling after the recrystallization by the intermediate annealing such that the cold rolling ratio is 74% to 95%, an area fraction of the Cube orientation can be increased through recrystallization in the solution treatment in the step 6, and more excellent bendability can be achieved.

[0123] The intermediate annealing is preferably performed in a temperature range of, for example, 300° C. to 450° C., and more preferably performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.

[0124] The temperature in the intermediate annealing is preferably 300° C. or higher, more preferably 325° C. or higher, and still more preferably 350° C. or higher, from the viewpoint of favorably obtaining recrystallized grains. The temperature is preferably 450° C. or lower, more preferably 425° C. or lower, and still more preferably 400° C. or lower, from the viewpoint of preventing the precipitation of the Mg—Si-based compounds and the decrease in bendability.

[0125] A time for the intermediate annealing is preferably 1 hour or longer, more preferably 2 hours or longer, and still more preferably 3 hours or longer, from the viewpoint of bringing the inside of the coil to a desired annealing temperature and suitably obtaining the effect of the annealing. The time is preferably 20 hours or shorter, more preferably 15 hours or shorter, and still more preferably 10 hours or shorter, from the viewpoint of preventing the coarsening of the Mg—Si-based compounds and the decrease in bendability.<Step 5: Cold Rolling Step>

[0126] The step 5 is a cold rolling step of subjecting the hot finish rolled sheet obtained in the step 4 to cold rolling to obtain a cold rolled sheet.

[0127] The cold rolling is a step performed once or a plurality of times to obtain the aluminum alloy sheet having a desired final thickness.

[0128] The step 5 in the present embodiment is performed such that the cold rolling ratio is 74% to 95%. Here, the cold rolling ratio is 74% or more, preferably 78% or more, and more preferably 80% or more, from the viewpoint of increasing the area fraction of Cube orientation after the recrystallization in the subsequent solution treatment step in the step 6 and achieving satisfactory bendability. The cold rolling ratio is 95% or less, preferably 93% or less, and more preferably 91% or less, from the viewpoint of suitably controlling the area fraction of Cube orientation and maintaining good surface condition.

[0129] A thickness of the cold rolled sheet is not particularly limited as long as the cold rolling ratio is 74% to 95%, and may be, for example, 0.7 mm to 2.5 mm.

[0130] The intermediate annealing may be performed during the cold rolling step. In this case, the cold rolling is preferably performed at a cold rolling ratio of 74% or more after the intermediate annealing.<Step 6: Solution Treatment Step>

[0131] The step 6 is a solution treatment step of subjecting the cold rolled sheet obtained in the step 5 to a solution treatment.

[0132] The solution treatment is a step of dissolving Mg or Si in the sheet to secure the proof stress after a bake hardening treatment.

[0133] The solution treatment is preferably performed at 520° C. or higher, and more preferably performed in a temperature range of 520° C. to 580° C. Here, the temperature is preferably 520° C. or higher, more preferably 525° C. or higher, and still more preferably 530° C. or higher, from the viewpoint of securing strength by ensuring a sufficient amount of solid solution and achieving better bendability by preventing an increase of the Mg—Si-based compounds. The temperature is preferably 580° C. or lower, more preferably 575° C. or lower, and still more preferably 570° C. or lower, from the viewpoint of preventing eutectic melting and recrystallized grain growth.

[0134] A holding time in the solution treatment is not particularly limited, and is preferably, for example, 1 second to 300 seconds.

[0135] Cooling is performed after the solution treatment, and a cooling rate at that time is preferably 1° C. / sec to 100° C. / sec. Here, the cooling rate is preferably 1° C. / sec or more, more preferably 5° C. / sec or more, and still more preferably 10° C. / sec or more, from the viewpoint of preventing a decrease in bendability due to grain boundary precipitation of coarse elemental Si phases, Mg—Si-based compounds, and the like. The cooling rate is preferably 100° C. / sec or less, more preferably 80° C. / sec or less, and still more preferably 60° C. / sec or less, from the viewpoint of preventing a decrease in flatness of the aluminum alloy sheet due to the cooling strain.

[0136] The cooling is performed to 100° C. or lower, for example, room temperature.

[0137] The method for producing an aluminum alloy sheet according to the present embodiment may further include a step of performing a preliminary aging treatment after performing the solution treatment and cooling in the step 6.

[0138] The step of performing the preliminary aging treatment is a step of performing a reheating treatment after the cooling following the solution treatment.

[0139] The preliminary aging treatment is preferably performed within 10 minutes after the solution treatment step, that is, after the solution treatment is completed and, for example, after cooling to room temperature, from the viewpoint of preventing a decrease in bake hardenability due to the formation of Si-rich Mg—Si-based clusters caused by room temperature aging.

[0140] The preliminary aging treatment is preferably performed in a temperature range of 50° C. to 100° C., and more preferably held in a temperature range of 50° C. to 100° C. for 1 hour to 50 hours.

[0141] Here, the temperature in the preliminary aging treatment is preferably 50° C. or higher, more preferably 55° C. or higher, and still more preferably 60° C. or higher, from the viewpoint of obtaining satisfactory bake hardenability. The temperature is preferably 100° C. or lower, more preferably 95° C. or lower, and still more preferably 90° C. or lower, from the viewpoint of preventing a decrease in bendability caused by the proof stress becoming excessively high.

[0142] A time for the preliminary aging treatment is preferably 1 hour or more, more preferably 2 hours or more, and still more preferably 4 hours or more, from the viewpoint of obtaining satisfactory bake hardenability. The time is preferably 50 hours or shorter, more preferably 40 hours or shorter, and still more preferably 30 hours or shorter, from the viewpoint of preventing a decrease in bendability caused by the proof stress becoming excessively high.

[0143] The preliminary aging treatment is more preferably performed at a temperature of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

[0144] Although each step for producing the aluminum alloy sheet according to the present embodiment has been described above, other steps may be performed between or before or after each step as long as each step is not adversely affected.<Aluminum Alloy Sheet; Composition>

[0145] The aluminum alloy sheet obtained by the production method according to the present embodiment has the following alloy composition:

[0146] 0.3 mass % to 1.5 mass % of Si,

[0147] 0.15 mass % to 0.4 mass % of Fe,

[0148] 0.001 mass % to 0.2 mass % of Cu,

[0149] 0.02 mass % to 0.3 mass % of Mn,

[0150] 0.3 mass % to 0.8 mass % of Mg,

[0151] 0.001 mass % to 0.3 mass % of Cr,

[0152] 0.001 mass % to 0.3 mass % of Zn, and

[0153] 0.001 mass % to 0.3 mass % of Ti,

[0154] with the remainder being Al and impurities.

[0155] Each component will be described in order.

[0156] Si: 0.3 mass % to 1.5 mass %

[0157] Si is a main alloy element in the aluminum alloy sheet, and is an element that contributes to improvement in strength of the aluminum alloy sheet together with Mg or Cu. A content of Si in the aluminum alloy sheet is 0.3 mass % to 1.5 mass %. Here, from the viewpoint of obtaining sufficient strength, the content is 0.3 mass % or more, preferably 0.4 mass % or more, and more preferably 0.5 mass % or more. From the viewpoint of preventing a decrease in bendability due to the generation of coarse elemental Si phases or Mg—Si-based compounds, the content is 1.5 mass % or less, preferably 1.25 mass % or less, and more preferably 1.0 mass % or less.

[0158] Fe: 0.15 mass % to 0.4 mass %

[0159] Fe is an element that contributes to grain refinement through a particle-stimulated nucleation (PSN) mechanism. A content of Fe in the aluminum alloy sheet is 0.15 mass % to 0.4 mass %. Here, the content is 0.15 mass % or more, preferably 0.2 mass % or more, and more preferably 0.25 mass % or more, from the viewpoint of preventing deterioration of surface condition due to coarsening of recrystallized grains during the treatment such as intermediate annealing by generating a sufficient amount of Al—Fe-based compounds serving as a recrystallized nucleus through the particle-stimulated nucleation mechanism (PSN mechanism). The content is 0.4 mass % or less, preferably 0.35 mass % or less, and more preferably 0.3 mass % or less, from the viewpoint of preventing a decrease in bendability due to an increase in Al—Fe-based compounds.

[0160] Cu: 0.001 mass % to 0.2 mass %

[0161] Cu is an element that contributes to improvement in strength by solid-solution hardening. A content of Cu in the aluminum alloy sheet is 0.001 mass % to 0.2 mass %. Here, allowing the presence of Cu in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Cu. However, as the content of Cu increases, the corrosion resistance deteriorates. Therefore, the content of Cu is preferably 0.001 mass % or more from an economical viewpoint, and the content is 0.2 mass % or less, preferably 0.13 mass % or less, and more preferably 0.05 mass % or less, from a viewpoint of obtaining excellent intergranular corrosion resistance.

[0162] Mn: 0.02 mass % to 0.3 mass %

[0163] Mn is an element that contributes to grain refinement through the PSN mechanism. A content of Mn in the aluminum alloy sheet is 0.02 mass % to 0.3 mass %. Here, the content is 0.02 mass % or more, preferably 0.04 mass % or more, more preferably 0.05 mass % or more, and still more preferably 0.06 mass % or more, from the viewpoint that grains are refined by the formation of Al—Fe—Si—Mn-based compounds or the like to obtain good surface condition. From the viewpoint of preventing a decrease in bendability due to an increase in Al—Fe—Si—Mn-based compounds or the like, the content is 0.3 mass % or less, preferably 0.25 mass % or less, and more preferably 0.2 mass % or less.

[0164] Mg: 0.3 mass % to 0.8 mass %

[0165] Mg is a main alloy element in the aluminum alloy sheet, and is an element that contributes to the improvement of the strength of the aluminum alloy sheet together with Si or Cu. A content of Mg in the aluminum alloy sheet is 0.3 mass % to 0.8 mass %. Here, the content is 0.3 mass % or more, preferably 0.35 mass % or more, and more preferably 0.4 mass % or more, from the viewpoint of obtaining sufficient strength. The content is 0.8 mass % or less from the viewpoint of preventing a decrease in bendability due to the generation of coarse Mg—Si-based compounds, and is more preferably less than 0.75 mass %, and still more preferably 0.70 mass % or less from the viewpoint of improving elongation in addition to satisfactory bendability.

[0166] Cr: 0.001 mass % to 0.3 mass %

[0167] Cr is an element that contributes to improvement in strength and ductility and improvement in surface condition such as a rough surface by grain refinement through compound formation. A content of Cr in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, allowing the presence of Cr in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Cr. Therefore, from an economical viewpoint, the content of Cr is preferably 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more. The content of Cr is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing a decrease in bendability due to the formation of coarse compounds.

[0168] Zn: 0.001 mass % to 0.3 mass %

[0169] A content of Zn in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, allowing the presence of Zn in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Zn. Therefore, from an economical viewpoint, the content of Zn is preferably 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more. On the other hand, the content of Zn is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing a decrease in pitting potential and a decrease in corrosion resistance.

[0170] Ti: 0.001 mass % to 0.3 mass %

[0171] Ti is an element that is actively introduced by addition of Al—Ti—B or Ti—B as a microstructure refining material and contributes to microstructure refinement. A content of Ti in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, the content is 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more, from the viewpoint of improving strength and ductility and improving surface condition such as a rough surface by microstructure refinement. The content is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing the formation of coarse compounds that adversely affect the required properties.

[0172] Remainder: Al and Impurities

[0173] The basic components of the component composition constituting the aluminum alloy sheet in the present embodiment are as described above, and the remainder is Al and impurities.

[0174] The impurities are components other than those described above and can be contained by selection of a melting raw material containing scrap for production of a slab, and are allowed to be contained within a range not impairing various properties of the aluminum alloy sheet.

[0175] Examples of the impurities include V, B, Ni, Na, Pb, Be, Ca, Sn, In, Cd, Bi, Ge, and the like.

[0176] The impurities include not only inevitable impurities that are inevitably contained unintentionally but also impurities that are intentionally added to the extent that the effects of the present invention are not impaired. The inevitable impurities are generally impurities that are inevitably mixed during melting of raw materials.

[0177] The elements described above as the examples of impurities may be contained as inevitable impurities, or may be intentionally added.

[0178] A content of each component serving as an impurity is preferably 0.05 mass % or less, and a total content of components serving as impurities is preferably 0.15 mass % or less.

[0179] As the remainder of the aluminum alloy sheet, a purity of Al, that is, a content of Al, is preferably 95 mass % or more, more preferably 96 mass % or more, and still more preferably 97 mass % or more.<Aluminum Alloy Sheet; Microstructure>

[0180] The aluminum alloy sheet obtained by the production method according to the present embodiment preferably has an area fraction of Cube orientation of 17% to 50%.

[0181] Here, it is common that the Cube orientation is a crystal orientation advantageous for bendability as described in Reference Literature 1 below. On the other hand, as described in Reference Literature 2 below, the Cube orientation also causes deterioration of surface condition. (Reference Literature 1: “Influence of Crystal Orientation on Bendability of Al—Mg—Si alloy”, Hirotaka Takeda et al., Light Metals, Vol. 60, No. 5, pp. 231-236 (2010); Reference Literature 2: “Crystal Plasticity Analysis of Ridging in Al—Mg—Si Series Sheet Alloy”, Haruyuki Konishi et al., R&D Kobe Steel Techniques, Vol. 62, No. 2, pp. 39-42 (2012))

[0182] On the other hand, the area fraction of the Cube orientation of the aluminum alloy in the present embodiment is preferably 17% or more, more preferably 20% or more, still more preferably 21% or more, and still more preferably 25% or more, from the viewpoint of achieving satisfactory bendability. The area fraction is preferably 50% or less, more preferably 45% or less, and still more preferably 40% or less, from the viewpoint of maintaining good surface condition.

[0183] As described above, the above range of the area fraction can be achieved by the intermediate annealing, the cold rolling step in the step 5, the solution treatment step in the step 6, or the like.

[0184] In the present embodiment, the area fraction of the Cube orientation of the aluminum alloy sheet can be measured by the following method.

[0185] A test piece is cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to the rolling direction can be observed, and the cross section is polished to be a mirror surface. The cross section is then subjected to ion etching using X-ray photoelectron spectroscopy (ESCA; electron spectroscopy for chemical analysis), and the area fraction of the Cube orientation is measured by irradiating the cross section parallel to the rolling direction with an electron beam at a pitch of 5 μm within a range of “1.6 μm in the rolling direction×a thickness length of the test piece” using electron back scattering diffraction (EBSD).

[0186] In the measurement by the EBSD method, for example, an FE-SEM7000F scanning electron microscope manufactured by JEOL Ltd. and a photodetector manufactured by TSL Solutions Co., Ltd. can be used, and an acceleration voltage of the measurement device is set to 20 kV.

[0187] Here, the Cube orientation refers to grains whose {100}plane is the sheet surface and <001> direction is parallel to the rolling direction, and grains with a misorientation angle of 15° or less from the ideal Cube orientation are considered to be Cube orientation, and the area fraction of the Cube orientation is determined.

[0188] In the aluminum alloy sheet obtained by the production method according to the present embodiment, a number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more is preferably 2650 particles / mm2 or less.

[0189] The present inventors have found that there is a correlation between the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more and bendability.

[0190] The Mg—Si-based compounds in the present specification mean an intermetallic compound containing Mg and Si as constituent elements.

[0191] From the viewpoint of achieving satisfactory bendability as described above, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 m or more is preferably 2650 particles / mm2 or less, more preferably 2400 particles / mm2 or less, and still more preferably 2250 particles / mm2 or less. The lower the number density, the better, and the number density may be, for example, 1 particle / mm2 or more.

[0192] As described above, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more can be within the above range by the Z factor of the final pass in the hot rough rolling in the step 3, the end temperature in the hot finish rolling in the step 4, or the like.

[0193] The number density of the Mg—Si-based compounds is determined by the following method.

[0194] A test piece is cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to the rolling direction can be observed, and the cross section is polished to be a mirror surface. Thereafter, in order to ensure conduction, gold vapor deposition is performed on the cross section, and then a vicinity of a sheet thickness center is observed with a scanning electron microscope FE-SEM (field emission scanning electron microscope).

[0195] The FE-SEM device used is, for example, a JSM-7001F manufactured by JEOL Ltd., and observation is performed in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view.

[0196] As a result of the above observation, black parts are Mg—Si-based compounds, and the number density of those having an equivalent circle diameter of 0.4 μm or more is determined.

[0197] In the aluminum alloy sheet obtained by the production method according to the present embodiment, a number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 225 particles / mm2 to 700 particles / mm2.

[0198] Here, the Al—Fe—Si-based compound in the present specification means an intermetallic compound that contains Al and Fe as constituent elements and may also contain Si, Mn, Cr, or the like.

[0199] As described above, when the area fraction of the Cube orientation is high, the surface condition may deteriorate. On the other hand, the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 225 particles / mm2 or more, more preferably 300 particles / mm2 or more, and still more preferably 400 particles / mm2 or more, from the viewpoint of achieving good surface condition by grain refinement. On the other hand, the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 700 particles / mm2 or less, more preferably 600 particles / mm2 or less, and still more preferably 500 particles / mm2 or less, from the viewpoint of preventing an increase in Al—Fe—Si compounds serving as a starting point of cracking and a decrease in bendability.

[0200] The number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more can be within the above range by adjusting the contents of Fe and Mn and the homogenization heat treatment conditions.

[0201] The number density of the Al—Fe—Si-based compounds is determined by observation using a scanning electron microscope FE-SEM in the same manner as the above-described number density of the Mg—Si-based compounds.

[0202] That is, in the same manner as the number density of the Mg—Si-based compounds, for example, observation is made using a JSM-7001F manufactured by JEOL Ltd. in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view. As a result of the above observation, white parts are Al—Fe—Si-based compounds, and the number density of those having an equivalent circle diameter of 2.0 μm or more is determined.<Aluminum Alloy Sheet; Properties>

[0203] The aluminum alloy sheet obtained by the production method according to the present embodiment has both excellent bendability and excellent ridging resistance.

[0204] The bendability can be evaluated by, for example, a VDA bending test.

[0205] The VDA bending test is a test performed in accordance with “VDA238-100 Plate bending test for metallic materials” standardized by the German Association of the Automotive Industry (VDA).

[0206] The test method will be described with reference to the drawings.

[0207] FIG. 1 is a schematic perspective view of a device used in a VDA bending test for evaluating bendability, a left side in FIG. 2 is a front view of the device, and a right side in FIG. 2 is a side view of the device.

[0208] First, as illustrated in FIG. 1, a sheet-shaped test piece obtained from the aluminum alloy sheet is placed horizontally on two rolls disposed in parallel to each other with a roll gap therebetween, with a length of the sheet-shaped test piece being equal on both sides, as illustrated by a dotted line in FIG. 1. Specifically, the sheet-shaped test piece is placed horizontally on the two rolls with the lengths of the sheet-shaped test piece on both sides being equal, the rolling direction of the sheet-shaped test piece and an extending direction of a sheet-shaped pressing and bending jig disposed vertically above are perpendicular to each other, and a central portion of the sheet-shaped test piece is positioned at a center of the roll gap.

[0209] The pressing and bending jig is pressed against the central portion of the sheet-shaped test piece from above to apply a load, the sheet-shaped test piece is pressed and bent (thrust and bent) toward a narrow roll gap, and the central portion of the bent and deformed sheet-shaped test piece is pressed into the narrow roll gap.

[0210] At this time, when a load F from the pressing and bending jig above is at a maximum, that is, immediately before a tip of the bent and deformed central portion of the sheet-shaped test piece is about to break, an angle at an outer side of the bending in the central portion of the sheet-shaped test piece is measured as a bending angle (°). A magnitude of the bending angle serves as an index for evaluation of bendability.

[0211] As test conditions of the VDA bending test, when symbols described in FIG. 1 are used, the sheet-shaped test piece has a rectangular shape with “width b: sheet width after 10% stretching×length 1: 60 mm”, a diameter D of each of the two rolls is 30 mm, and a roll gap L is (thickness of the sheet-shaped test piece×2+0.5) mm. S is an indentation depth of the central portion of the sheet-shaped test piece into the roll gap when the load F from the pressing and bending jig above is at the maximum. As illustrated in FIG. 2, a punch, which is a sheet-shaped pressing and bending jig, has a lower thin sheet-shaped blade pressed against the central portion of the sheet-shaped test piece, and a tip (lower end) of the blade is formed into a tapered shape with a radius r of 0.2 mm.

[0212] The sheet-shaped test piece used before the VDA bending test is prepared by the following procedure.

[0213] The aluminum alloy sheet is cut into strips of 250 mm in a rolling parallel direction ×30 mm in a rolling perpendicular direction, and subjected to an aging treatment in an air furnace under respective conditions of, for example, 90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h. Thereafter, the strips are subjected to 10% stretching in a direction parallel to the rolling direction using a tensile testing machine, and then cut out into a rectangle having a width b equal to the sheet width after stretching (after 10% stretching) and a length 1 of 60 mm to obtain a sheet-shaped test piece.

[0214] Using the sheet-shaped test piece, a VDA bending test in which a bending line is perpendicular to the rolling direction is performed in accordance with the above-described VDA238-100 standard.

[0215] A speed at which the pressing and bending jig is pressed in is 10 mm / min until the load reaches 30 N, and thereafter the speed is 20 mm / min.

[0216] The bending is set to stop when a crack occurs in the sheet-shaped test piece or when the load is reduced by 30 N from a maximum load, as a result of a reduction in sheet thickness of the sheet-shaped test piece. The bending test is performed twice for each aging treatment condition, and the bending angle (°) is an average value thereof.

[0217] In the VDA bending test in the present embodiment, the VDA bending angle at a proof stress of 150 MPa is obtained. The proof stress is adopted based on the fact that when automobile manufacturers perform hem bending, the proof stress may increase due to natural aging, which may result in a deterioration in bendability, and that the maximum proof stress expected during the hem bending is approximately 150 MPa.

[0218] The proof stress measured after 10% stretching is plotted on an x-axis and the VDA bending angle is plotted on a scatter diagram on a y-axis, and a linear approximation curve (y =ax+b) is obtained using a least squares method for each plot under each aging condition (for example, 90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h), and y (VDA bending angle) when x(proof stress)=150 is obtained.

[0219] Since the bendability varies depending on the sheet thickness, the VDA bending angle corresponding to the sheet thickness of 1.0 mm is obtained using a sheet thickness conversion formula of the VDA bending angle described in the VDA standard.

[0220] The sheet thickness conversion formula is as follows.αSoll=αref⁢treft

[0221] In the above formula, αSoll is a VDA bending angle conversion value, aref is a VDA bending angle measurement value, t is a sheet thickness to be converted, and tref is a sheet thickness measurement value.

[0222] Using the above formula, the VDA bending angle corresponding to a proof stress of 150 MPa (converted to sheet thickness of 1.0 mm) is obtained. When the value is 128° or more, it can be said that the bendability is satisfactory, and the value is preferably 136° or more, more preferably 144° or more, and still more preferably 150° or more.

[0223] The surface quality of the aluminum alloy sheet obtained by the production method according to the present embodiment can be evaluated using, for example, a surface roughness index as the surface texture.

[0224] The surface roughness index is a value obtained by the following method.

[0225] First, in order to simulate the surface quality after press forming, a strain is imparted to the aluminum alloy sheet by stretching with a tensile tester, and then the surface quality is evaluated. A pattern commonly seen in 6000 series alloys when the surface texture deteriorates is a ridging mark and a rough surface, but in the present description, the evaluation is made based on the rough surface, which shows a significant deterioration in properties when the rough surface is outside a specified range.

[0226] First, an aluminum alloy sheet is cut into a shape of “40 mm in the rolling parallel direction×200 mm in the rolling perpendicular direction”, and subjected to 10% stretching in a direction perpendicular to the rolling direction using a tensile tester to obtain a test piece.

[0227] Next, a rough surface degree of the test piece is defined and evaluated with reference to the method described in Kobe Steel Technical Report / Vol. 66 No. 2 (March 2017).

[0228] Specifically, a surface shape of the test piece is measured in a region of 30 mm×30 mm using a three-dimensional surface roughness measuring machine (Contour-GT, manufactured by Bruker Japan). Then, after removing waveform components having a wavelength of less than 1 mm or 5 mm or more, waveform components are classified according to orientations with respect to the rolling direction of the sheet material, and a total value of amplitudes of the respective waveform components is obtained, thereby quantitatively evaluating various surface defect forms appearing on the sheet material surface, such as a rough surface and ridging. Since the rough surface is a magnitude of unevenness regardless of orientation, the surface roughness degree can be expressed as a numerical index by summing the amplitudes of the waveform components in all orientations and dividing the sum by 360 (degrees).

[0229] In the present description, when the surface roughness index is 52 μm or less, the surface texture can be said to be satisfactory, and the surface roughness index is more preferably 48 μm or less, and still more preferably 42 μm or less.

[0230] The mechanical properties (strength) of the aluminum alloy sheet obtained by the production method according to the present embodiment can be evaluated using a 0.2% proof stress obtained by a tensile test.

[0231] The proof stress is a value obtained by the following method.

[0232] A 13A test piece conforming to JIS Z 2201: 1998 is collected and prepared from the aluminum alloy sheet such that a tensile direction is perpendicular to a rolling direction, and a tensile test is performed at room temperature.

[0233] The tensile test is performed at 5 mm / min until the 0.2% proof stress is measured, and then at 30 mm / min.

[0234] When the proof stress after 3 days to 14 days from the time of obtaining the test piece is 80 MPa or more and less than 150 MPa, it can be said that the mechanical properties are satisfactory.

[0235] The mechanical properties after a baked finish are also important for the aluminum alloy sheet.

[0236] Therefore, in order to simulate the baked finish, a test piece is given a pre-strain of 2% and then subjected to an aging treatment (a bake hardening treatment) at 185° C. for 20 minutes, and then a tensile test is performed on the test piece.

[0237] When the 0.2% proof stress after the bake hardening treatment is 190 MPa or more and less than 240 MPa, it can be said that the mechanical properties are satisfactory.<<Aluminum Alloy Sheet>>

[0238] The aluminum alloy sheet according to the present embodiment is an aluminum alloy sheet obtained by the above-described <<Method for Producing Aluminum Alloy Sheet>>.

[0239] An aspect of the aluminum alloy sheet according to the present embodiment satisfies the following alloy composition.

[0240] 0.3 mass % to 1.5 mass % of Si,

[0241] 0.15 mass % to 0.4 mass % of Fe,

[0242] 0.001 mass % to 0.2 mass % of Cu,

[0243] 0.02 mass % to 0.3 mass % of Mn,

[0244] 0.3 mass % to 0.8 mass % of Mg,

[0245] 0.001 mass % to 0.3 mass % of Cr,

[0246] 0.001 mass % to 0.3 mass % of Zn, and

[0247] 0.001 mass % to 0.3 mass % of Ti,

[0248] with the remainder being Al and impurities.

[0249] In addition to the above alloy composition, it is preferable to satisfy one or more, more preferably to satisfy two or more, and still more preferably to satisfy all three of the following microstructure items: the area fraction of Cube orientation is 17% to 50%, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 um or more is 2650 particles / mm2 or less, and the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is 225 particles / mm2 to 700 particles / mm2 or less.

[0250] The description and preferred aspect of the above alloy composition and microstructure items are the same as the preferred aspect described in the above <<Method for Producing Aluminum Alloy Sheet>>.

[0251] It can be said that the aluminum alloy sheet according to the present embodiment is an aluminum alloy sheet having both excellent bendability and excellent surface quality as long as the VDA bending angle corresponding to a proof stress of 150 MPa (converted to sheet thickness of 1.0 mm) is 128° or more and the average intensity value Aave, which is a surface roughness index, is 52 μm or less.

[0252] In addition to the above, it is preferable to satisfy one or more and more preferable to satisfy both of property items: the proof stress after 3 days to 14 days is 80 MPa or more and the 0.2% proof stress after the bake hardening treatment is 190 MPa or more.

[0253] The description and preferred aspect of the above properties are the same as the preferred aspect described in the above <<Method for Producing Aluminum Alloy Sheet>>.EXAMPLES

[0254] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples, and modifications can be made within the scope that can conform to the gist thereof, and all of them are included in the technical scope of the present invention.Test ExampleExample 1

[0255] A raw material was melted, and a molten metal whose component was adjusted to a component composition shown in an alloy A in Table 1 was cast into a slab. The slab was subjected to scalping in order to remove a segregation layer on the surface. (Step 1)

[0256] Next, the slab was subjected to a one-stage homogenization heat treatment at 560° C. for 6 hours. (Step 2)

[0257] Hot rough rolling was started immediately after the homogenization heat treatment, and an entry temperature was set to a range of 500° C. to 560° C., and a final pass was performed under a condition that a Z factor was 7.63×109. (Step 3)

[0258] Subsequently, hot finish rolling was performed. An end temperature of the hot finish rolling was 300° C., and a sheet thickness of a hot finish rolled sheet was 6.0 mm. (Step 4)

[0259] Thereafter, intermediate annealing was performed at 375° C. for 4 hours.

[0260] The hot finish rolled sheet after the intermediate annealing was cold rolled. The obtained cold rolled sheet had a sheet thickness of 1.0 mm and a cold rolling ratio of 83%. (Step 5)

[0261] Finally, a solution treatment was performed at 540° C., and after cooling to room temperature at a cooling rate in a range of 10° C. / sec to 60° C. / sec, a preliminary aging treatment was performed by heating to 80° C. after about 5 minutes and holding for 4 hours to 30 hours to obtain an aluminum alloy sheet. (Step 6)Comparative Example 1

[0262] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that a component composition was changed to that shown in an alloy B in Table 1 and conditions of hot finish rolling, intermediate annealing, and cold rolling were changed to those shown in Table 2.Comparative Example 2

[0263] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that conditions of hot finish rolling were changed to those shown in Table 2.Comparative Example 3

[0264] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that a component composition was changed to that shown in an alloy C in Table 1 and conditions of cold rolling and a solution treatment were changed to those shown in Table 2.TABLE 1Chemical composition (mass %)AlloySiFeCuMnMgCrZnTiAlA0.790.280.010.070.480.010.020.02RemainderB0.670.170.120.070.570.020.020.03RemainderC0.750.130.010.070.520.010.020.03RemainderTABLE 2Homogenization heat treatmentHomogenizationHomogenizationHotHot finish rollingheat treatmentheat treatmentroughEndThicknesstemperaturetimerollingtemperature(mm) afterAlloy(° C.)(h)Z factor(° C.)hot rollingExample 1A56067.63.E+093006.0ComparativeB56067.63.E+093003.5Example 1ComparativeA56067.63.E+093506.0Example 2ComparativeC56067.63.E+093006.0Example 3Cold rollingPreliminaryThicknessSolutionagingintermediate annealing(mm) aftertreatmenttreatmentAnnealingcold rollingColdSolutionReheatingtemperatureAnnealing(productrollingtemperaturetemperature(° C.)time (h)thickness)ratio(° C.)(° C.)Example 137541.083%54080Comparative42041.071%54080Example 1Comparative37541.083%54080Example 2Comparative37540.985%53580Example 3Evaluation<Microstructure; Number Density of Mg—Si-based Compounds and Al—Fe—Si-based Compounds>A test piece was cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to a rolling direction could be observed, and the cross section was polished to be a mirror surface. Thereafter, in order to ensure conduction, gold vapor deposition was performed on the cross section, and then a vicinity of a sheet thickness center was observed with a scanning electron microscope FE-SEM (field emission scanning electron microscope).

[0266] The FE-SEM device used was a JSM-7001F manufactured by JEOL Ltd., and observation was performed in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view.

[0267] As a result of the above observation, black parts were Mg—Si-based compounds, and the number density of those having an equivalent circle diameter of 0.4 μm or more was calculated to determine the number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more.

[0268] As a result of the above, white parts were Al—Fe—Si-based compounds, and the number density of those having an equivalent circle diameter of 2.0 μm or more was calculated to determine the number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more.

[0269] The results thereof are shown in Table 3.<Microstructure; Area Fraction of Cube Orientation>

[0270] A test piece was cut out from the obtained aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to a rolling direction could be observed, and the cross section was polished to be a mirror surface. The cross section was then subjected to ion etching using X-ray photoelectron spectroscopy (ESCA; Electron Spectroscopy for Chemical Analysis), and the area fraction of Cube orientation was measured by irradiating the cross section parallel to the rolling direction with an electron beam at a pitch of 5 μm within a range of “1.6 μm in the rolling direction×the thickness length of the test piece” using electron back scattering diffraction (EBSD).

[0271] In the measurement by the EBSD method, an FE-SEM7000F scanning electron microscope manufactured by JEOL Ltd. and a photodetector manufactured by TSL Solutions Co., Ltd. was used, and an acceleration voltage of the measurement device was set to 20 kV Here, the Cube orientation refers to grains whose {100}plane is parallel to a sheet surface and <001> direction is parallel to the rolling direction, and grains with a misorientation angle of 150 or less from the ideal Cube orientation were considered to be Cube orientation, and the area fraction of the Cube orientation was calculated.

[0272] The results thereof are shown in Table 3.<Properties; Mechanical Properties>

[0273] A 13A test piece conforming to JIS Z 2201: 1998 was collected and prepared from the obtained aluminum alloy sheet such that a tensile direction was perpendicular to the rolling direction, and a tensile test was performed at room temperature.

[0274] The tensile test was performed at 5 mm / min until the 0.2% proof stress was measured, and was thereafter performed at 30 mm / min.

[0275] The proof stress was determined after 4 days (Example 1, Comparative Example 1), 7 days (Comparative Example 2), or 8 days (Comparative Example 3) had elapsed since the test piece was obtained.

[0276] As mechanical properties of the aluminum alloy sheet after a baked finish, a test piece was given a pre-strain of 2% and then subjected to an aging treatment (a bake hardening treatment) at 185° C. for 20 minutes, and then a tensile test was performed on the test piece.

[0277] The results thereof are shown in Table 3.<Evaluation; Bendability>

[0278] The obtained aluminum alloy sheet was subjected to a VDA bending test using the device as described above. Specifically, the sheet-shaped test piece used before the VDA bending test was prepared by the following procedure.

[0279] The obtained aluminum alloy sheet was cut into strips 250 mm in a rolling parallel direction×30 mm in a rolling perpendicular direction, and subjected to an aging treatment in an air furnace under respective conditions of 90° C. for 5 hours, 100° C. for 5 hours, 110° C. for 5 hours, and 120° C. for 5 hours. Thereafter, the strips were subjected to 10% stretching in a direction parallel to the rolling direction using a tensile testing machine, and then cut out into a rectangle having a width b equal to the sheet width after stretching (after 10% stretching) and a length 1 of 60 mm to obtain a sheet-shaped test piece.

[0280] Using the sheet-shaped test piece, a VDA bending test in which a bending line was perpendicular to the rolling direction was performed in accordance with the above-described VDA238-100 standard.

[0281] A speed at which the pressing and bending jig was pressed in was 10 mm / min until the load reached 30 N, and thereafter the speed was 20 mm / min.

[0282] The bending was set to stop when a crack occurs in the sheet-shaped test piece or when the load is reduced by 30 N from a maximum load, as a result of a reduction in sheet thickness of the sheet-shaped test piece, and the bending angle θ after stopping was measured. The bending test was performed twice for each aging treatment condition, and the bending angle (deg.) was an average value thereof.

[0283] The 0.2% proof stress measured after 10% stretching was plotted on an x-axis and the VDA bending angle was plotted on a scatter diagram on a y-axis, and a linear approximation curve (y=ax+b) was obtained using a least squares method for each plot under each aging condition (90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h). By determining y (VDA bending angle) when x (proof stress)=150 (MPa) and converting y to a sheet thickness of 1.0 mm, the VDA bending angle corresponding to a proof stress of 150 MPa (converted to a sheet thickness of 1.0 mm) was determined.

[0284] The results are shown in Table 3.<Evaluation; Surface Quality>

[0285] First, the obtained aluminum alloy sheet was cut into a shape of “40 mm in the rolling parallel direction×200 mm in the rolling perpendicular direction”, and subjected to 10% stretching in a direction perpendicular to the rolling direction using a tensile tester to obtain a test piece.

[0286] Next, a power spectrum intensity distribution in a frequency region of 1 mm to 5 mm was extracted from a two-dimensional power spectrum intensity distribution of a two-dimensional surface roughness distribution. The power spectrum intensity distribution was divided into a plurality of regions, and absolute values of a power spectrum intensity in each divided region were summed up for each divided region to calculate an integrated intensity value. The calculated integrated intensity values for the plurality of divided regions were averaged to obtain an average intensity value Aave as a surface roughness index value.

[0287] Next, three-dimensional surface roughness of the sheet surface was measured, and after removing waveform components having a wavelength of less than 1 mm or 5 mm or more, waveform components were classified according to orientations with respect to the rolling direction of the sheet material, a sum of amplitudes of each waveform component was calculated, amplitudes of the waveform components in all orientations were summed and divided by 360 (degrees) to determine the surface roughness index.

[0288] The results thereof are shown in Table 3.TABLE 3MicrostructurePropertiesNumber densityNumber densityMechanical properties(particles / mm2) of(paricles / mm2) ofAreaProof stressBendabilityRidging resistanceMg—Si-basedAl—Fe—Si-basedfractionProof(MPa) afterVDA bending angleRough surfacecompounds (0.4compounds (2.0of CubeTimestressbake hardening(deg.) @ 150 MPaindex Aave (μm)μm or more)μm or more)orientation(days)(MPa)treatment(converted to 1 mm)(after 10% stretch)Example 1211143925%48821215435Comparative139126416%49021412249Example 1Comparative282445924%710021712943Example 2Comparative106620532%810122814555Example 3

[0289] From the above results, it was found that according to the present embodiment, an aluminum alloy sheet having both excellent bendability and excellent surface quality can be obtained.

Examples

examples

[0254]Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples, and modifications can be made within the scope that can conform to the gist thereof, and all of them are included in the technical scope of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to Japanese Application No. 2025-059109 filed on Mar. 31, 2025. The content of each of these applications is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The present invention relates to an aluminum alloy sheet and a method for producing an aluminum alloy sheet.BACKGROUND ART

[0003] In recent years, social request for weight reduction in automobile bodies is increasing more and more due to considerations to the global environment or the like. In order to meet such requirements, aluminum alloy materials are being used in place of conventional steel materials such as steel sheets, to portions of automobile bodies such as panels (outer panels (external panels) such as hoods, doors, and roofs, and inner panels (internal panels)), bumper reinforcements (bumper R / F), and reinforcing materials such as door beams.

[0004] On the other hand, since automobiles are also required to have aesthetic design in terms of shape, formability is also important in addition to the weight reduction and rigidity for automobile applications as described above.

[0005] For example, a hood, a door, or the like, which is a portion to which an aluminum panel material for automobiles is applied, is constructed by press-forming an outer panel and an inner panel respectively, and then hemming (bending) the outer panel at 1800 to fasten the outer panel to the inner panel.

[0006] During the bending, a locally high strain is concentrated on a bending ridgeline portion, and thus cracking is likely to occur. Therefore, the outer panel is required to have high bendability.

[0007] Since the outer panel is also a portion that is most visible to consumers, it is necessary to ensure a good appearance. On the other hand, when the panel material is deformed, slight unevenness may occur on the surface, causing a poor appearance, and thus good surface condition after press molding is required.

[0008] On the other hand, Patent Literature 1 discloses that an aluminum alloy sheet excellent in bendability and surface quality can be obtained by setting an appropriate cooling rate after a homogenization heat treatment, together with holding a slab after the homogenization heat treatment in a certain temperature range before the start of hot rolling, and setting a cold rolling ratio in cold rolling to 65% or more.CITATION LISTPatent LiteraturePatent Literature 1: JP2018-16879ASUMMARY OF INVENTION

[0010] In the invention of Patent Literature 1, cooling is performed after the homogenization heat treatment, and then a heat treatment of holding in a certain temperature range is performed before the start of hot rolling.

[0011] On the other hand, as a result of studies by the present inventors, as described above, there is a concern that Mg—Si-based compounds may be coarsened by performing the cooling or heat treatment of holding in a certain temperature range between the homogenization heat treatment and the hot rolling. More specifically, as described in Patent Literature 1, it has been found that, when the cooling is performed after the homogenization heat treatment and then the heat treatment of holding in a certain temperature range is performed before the start of hot rolling, coarse Mg—Si-based compounds are precipitated in the middle of cooling, and improvement of bendability is limited. The step of cooling after the homogenization heat treatment and holding in a certain temperature range before the start of hot rolling requires time for cooling and an additional heat treatment, resulting in poor productivity.

[0012] With regard to the bendability, particularly in recent years, there has been a demand for a 6000 series aluminum alloy sheet to have higher bendability. This is due to a growing need for a sharper character line in automobiles and the like, and a need to ensure greater freedom in a combination of the respective thicknesses of an outer panel and an inner panel during bending at automobile manufacturers.

[0013] It is difficult for the aluminum alloy sheet disclosed in Patent Literature 1 to cope with a formability requirement for such strict bending, which is becoming increasingly demanding.

[0014] Therefore, an object of the present invention is to provide an aluminum alloy sheet satisfying both excellent bendability and excellent surface quality, which is capable of meeting the strict requirement for bending and the requirement for the good surface condition after press forming, and a method for producing the same.

[0015] With regard to the production method, an object of the present invention is to provide a method for producing an aluminum alloy sheet having excellent productivity in addition to the bendability and the surface quality.

[0016] As a result of intensive studies on the above problems, the present inventors have found that the above problems can be solved by setting an aluminum alloy sheet to have a specific composition range and performing a hot rolling step and a cold rolling step under specific conditions without performing cooling or an additional heat treatment between a homogenization heat treatment and hot rolling, and have completed the present invention.

[0017] That is, the present disclosure includes the following embodiments.[1] An aluminum alloy sheet comprising:0.3 mass % to 1.5 mass % of Si;

[0019] 0.15 mass % to 0.4 mass % of Fe;

[0020] 0.001 mass % to 0.2 mass % of Cu;

[0021] 0.02 mass % to 0.3 mass % of Mn;

[0022] 0.3 mass % to 0.8 mass % of Mg;

[0023] 0.001 mass % to 0.3 mass % of Cr;

[0024] 0.001 mass % to 0.3 mass % of Zn; and

[0025] 0.001 mass % to 0.3 mass % of Ti,

[0026] with the remainder being Al and impurities, wherein

[0027] the aluminum alloy sheet has an area fraction of Cube orientation of 17% to 50%,

[0028] a number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more in the aluminum alloy sheet is 2650 particles / mm2 or less, and

[0029] a number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more in the aluminum alloy sheet is 225 particles / mm2 to 700 particles / mm2.[2] The aluminum alloy sheet according to [1], which is used for an automobile.[3]A method for producing an aluminum alloy sheet, the method comprising: in the following order,

[0030] a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab;

[0031] a homogenization heat treatment step of subjecting the cast slab to a homogenization heat treatment;

[0032] a hot rough rolling step of subjecting the homogenized slab to hot rough rolling to obtain a hot rough rolled sheet;

[0033] a hot finish rolling step of subjecting the hot rough rolled sheet to hot finish rolling to obtain a hot finish rolled sheet;

[0034] a cold rolling step of subjecting the hot finish rolled sheet to cold rolling to obtain a cold rolled sheet; and

[0035] a solution treatment step of subjecting the cold rolled sheet to a solution treatment, wherein

[0036] in the homogenization heat treatment step, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C.,

[0037] no cooling and no additional heat treatment are performed between the homogenization heat treatment step and the hot rough rolling step,

[0038] in the hot rough rolling step, a final pass is performed under a condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011,Z=ε.⁢ exp⁡(Q / RT)wherein ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature,

[0040] the hot finish rolling step is performed such that an end temperature is 250° C. or higher and lower than 350° C.,

[0041] the cold rolling step is performed such that a cold rolling ratio is 74% to 95%, and

[0042] the obtained aluminum alloy sheet contains

[0043] 0.3 mass % to 1.5 mass % of Si,

[0044] 0.15 mass % to 0.4 mass % of Fe,

[0045] 0.001 mass % to 0.2 mass % of Cu,

[0046] 0.02 mass % to 0.3 mass % of Mn,

[0047] 0.3 mass % to 0.8 mass % of Mg,

[0048] 0.001 mass % to 0.3 mass % of Cr,

[0049] 0.001 mass % to 0.3 mass % of Zn, and

[0050] 0.001 mass % to 0.3 mass % of Ti,

[0051] with the remainder being Al and impurities.[4] The method according to [3], wherein

[0052] the homogenization heat treatment step is performed for 1 hour to 30 hours.[5] The method according to [3] or [4], wherein

[0053] the hot rough rolling step is performed such that an entry temperature is 400° C. to 570° C.[6] The method according to [3] or [4], further comprising:

[0054] a step of performing intermediate annealing before the cold rolling step or during the cold rolling step.[7] The method according to [6], wherein

[0055] the intermediate annealing is performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.[8] The method according to [3] or [4], wherein

[0056] the solution treatment step is performed in a temperature range of 520° C. to 580° C.[9] The method according to [3] or [4], wherein

[0057] cooling is performed at a cooling rate of 1° C. / sec to 100° C. / sec after the solution treatment.

[10] The method according to [3] or [4], wherein

[0058] a preliminary aging treatment is performed by holding the sheet at a temperature in a range of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

[0059] According to the present invention, an aluminum alloy sheet satisfying both excellent bendability and excellent surface quality, which is capable of meeting a strict requirement for surface condition after bending or press forming can be obtained. Since the aluminum alloy sheet as described above can be obtained by the homogenization heat treatment at a single temperature, a load on homogenizing equipment can be reduced, and this is also preferable from the viewpoint of productivity.BRIEF DESCRIPTION OF DRAWINGS

[0060] FIG. 1 is a schematic perspective view of a device used in a VDA bending test for evaluating bendability.

[0061] FIG. 2 is a diagram illustrating a punch, which is a pressing and bending jig used when applying a load to a test piece from above in the device in FIG. 1, in which a left side in FIG. 2 is a front view of the punch, and a right side in FIG. 2 is a side view of the punch.DESCRIPTION OF EMBODIMENTS

[0062] Hereinafter, an aluminum alloy sheet according to the present embodiment and a mode for carrying out a method for producing an aluminum alloy sheet will be described in detail. Although the present invention is described in detail with reference to specific embodiments, it is obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

[0063] In the present specification, “to” indicating a numerical range is used to mean that numerical values described therebefore and thereafter are included as a lower limit and an upper limit.<Method for Producing Aluminum Alloy Sheet>

[0064] The method for producing an aluminum alloy sheet according to the present embodiment includes the following steps in the following order.

[0065] Step 1: a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab.

[0066] Step 2: a homogenization heat treatment step of subjecting the slab cast in the step 1 to a homogenization heat treatment.

[0067] Step 3: a hot rough rolling step of subjecting the slab homogenized in the step 2 to hot rough rolling to obtain a hot rough rolled sheet.

[0068] Step 4: a hot finish rolling step of subjecting the hot rough rolled sheet obtained in the step 3 to hot finish rolling to obtain a hot finish rolled sheet.

[0069] Step 5: a cold rolling step of subjecting the hot finish rolled sheet obtained in the step 4 to cold rolling to obtain a cold rolled sheet.

[0070] Step 6: a solution treatment step of subjecting the cold rolled sheet obtained in the step 5 to a solution treatment.

[0071] A step of performing intermediate annealing may be further included before the cold rolling step in the step 5 or during the cold rolling step.

[0072] In the step 6, cooling may be performed at a predetermined cooling rate after the solution treatment.

[0073] After the solution treatment step in the step 6, a step of performing a preliminary aging treatment may be further included.

[0074] Cooling or an additional heat treatment is not performed between the homogenization heat treatment step in the step 2 and the hot rough rolling step in the step 3.

[0075] In the method for producing an aluminum alloy sheet according to the present embodiment, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C. in the homogenization heat treatment step in the step 2. In the hot rough rolling step in the step 3, a final pass is performed under a condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011. In the hot finish rolling step in the step 4, an end temperature is set to 250° C. or higher and lower than 350° C. Next, the cold rolling step in the step 5 is performed such that a cold rolling ratio is 74% to 95%.Z=ε.⁢ exp⁡(Q / RT)

[0076] In the above formula, ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature.

[0077] The obtained aluminum alloy sheet satisfies the following alloy composition:

[0078] 0.3 mass % to 1.5 mass % of Si,

[0079] 0.15 mass % to 0.4 mass % of Fe,

[0080] 0.001 mass % to 0.2 mass % of Cu,

[0081] 0.02 mass % to 0.3 mass % of Mn,

[0082] 0.3 mass % to 0.8 mass % of Mg,

[0083] 0.001 mass % to 0.3 mass % of Cr,

[0084] 0.001 mass % to 0.3 mass % of Zn, and

[0085] 0.001 mass % to 0.3 mass % of Ti,

[0086] with the remainder being Al and impurities.

[0087] The details of the above steps and the aluminum alloy sheet to be obtained will be described below.<Step 1: Casting Step>

[0088] The step 1 is a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab.

[0089] The melting of the raw materials is performed such that a composition of an aluminum alloy sheet to be obtained is adjusted to fall within a desired range. A method for casting the aluminum alloy molten metal which has been melted and adjusted into a slab is not particularly limited, and any common method can be used.

[0090] Examples of the casting method include a continuous casting method, a semi-continuous casting method (a DC casting method), and the like.

[0091] The raw material may be not only an ingot but also scrap produced during production processes such as the production of an aluminum alloy sheet and a pressing process. End of life vehicle (ELV) scrap such as an automobile panel, a wheel, a radiator, and a casting such as an engine block and a suspension may also be used. Further, an aluminum waste material such as a used beverage can (UBC), a home appliance, an aluminum printing plate, an electric wire, and sash scrap may also be used.<Step 2: Homogenization Heat Treatment Step>

[0092] The step 2 is a homogenization heat treatment step of subjecting the slab cast in the step 1 to a homogenization heat treatment.

[0093] The homogenization heat treatment is a heat treatment performed to make a microstructure uniform throughout, as a microstructure and components are non-uniform depending on a location only by casting.

[0094] In the homogenization heat treatment step, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C. After the homogenization heat treatment, the slab is directly subjected to the hot rough rolling step in the step 3 without being subjected to cooling or an additional heat treatment. The present step is advantageous in terms of productivity since the cooling or the additional heat treatment is not performed between the homogenization heat treatment step and the hot rough rolling step.

[0095] The temperature range of 520° C. to 580° C. means a homogenization heat treatment start temperature.

[0096] Here, from the viewpoint of rapidly homogenizing the microstructure, the temperature in the homogenization heat treatment is 520° C. or higher, preferably 525° C. or higher, and more preferably 530° C. or higher. From the viewpoint of preventing local melting due to a decrease in melting point of a segregated portion, the above temperature is 580° C. or lower, preferably 575° C. or lower, and more preferably 570° C. or lower.

[0097] In the step 2 in the present embodiment, a time for performing the homogenization heat treatment is preferably 1 hour to 30 hours. Here, from the viewpoint of favorably eliminating segregation, the time is preferably 1 hour or longer, more preferably 2 hours or longer, still more preferably 3 hours or longer, and may be 4 hours or longer. From the viewpoint of productivity, the time is preferably 30 hours or less, more preferably 20 hours or less, and still more preferably 10 hours or less.<Step 3: Hot Rough Rolling Step>

[0098] The step 3 is a hot rough rolling step of subjecting the slab homogenized in the step 2 to hot rough rolling to obtain a hot rough rolled sheet.

[0099] The slab after the homogenization heat treatment is made to have a predetermined thickness by the hot rough rolling. The hot rough rolling treatment is repeatedly performed using a reversible rolling mill until a predetermined thickness is obtained.

[0100] A start temperature of the hot rough rolling, that is, an entry temperature of the hot rough rolling step is preferably 400° C. to 570° C. Here, it is preferable that the hot rough rolling is performed to achieve a predetermined thickness with as few rolling passes (number of passes) as possible, and from the viewpoint of preventing an increase in the number of rolling passes due to high deformation resistance and difficulty in rolling, the temperature is preferably 400° C. or higher, and more preferably 500° C. or higher. From the viewpoint of preventing coarse recrystallized grains from being formed on the surface, which can cause a rough surface of a final product, the temperature is preferably 570° C. or lower.

[0101] The final pass of the hot rough rolling is performed under the condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011.Z=ε.⁢ exp⁡(Q / RT)

[0102] In the above formula, ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature.

[0103] In order to clarify the recrystallization behavior during the hot rolling in detail, a Zener-Hollomon parameter called a factor Z, which is a temperature-compensated strain rate, is used.

[0104] The larger the Z factor in the final pass of the hot rough rolling, the more recrystallization is promoted during the hot rough rolling and the hot finish rolling, and the more the promotion of the formation of Mg—Si-based compounds due to a processing strain can be prevented, and as a result, an amount of the Mg—Si-based compounds is reduced even after the solution treatment.

[0105] Even when the Mg—Si-based compounds are precipitated during the hot rough rolling, when a rolling reduction rate of the hot rough rolling is high, that is, the factor Z is large, the Mg—Si-based compounds are easily crushed and become fine, and are easily dissolved in solid after the solution treatment.

[0106] Therefore, as the Z factor in the final pass of the hot rough rolling is larger, a number density of the Mg—Si-based compounds after the solution treatment is reduced, and the bendability is improved.

[0107] In addition to the above, when the Z factor is large, an accumulated strain amount becomes large, and uniform and fine recrystallized grains are obtained after the hot rough rolling. When the uniform and fine recrystallized grains are obtained after the hot rough rolling, uniform and fine recrystallized grains are likely to be obtained after any subsequent intermediate annealing or solution treatment, and a rough surface can be prevented when the aluminum alloy sheet is press-formed.

[0108] That is, the Z factor of the final pass in the hot rough rolling is 1.0×108 or more, preferably 5.0×108 or more, and more preferably 1.0×109 or more, from the viewpoint of achieving satisfactory bendability by preventing the formation of the Mg—Si-based compounds and from the viewpoint of favorably preventing the rough surface by forming uniform and fine recrystallized grains. On the other hand, the factor Z is 5.0×1011 or less, preferably 1.0×1011 or less, and more preferably 5.0×1010 or less, from the viewpoint of preventing the rolling from becoming difficult due to the restriction of the rolling load when the temperature of the hot rough rolling is too low or the number of rolling passes in the hot rough rolling is too large.

[0109] In the above formula for determining the Z factor, ε′ represents the strain rate, Q represents the activation energy, R represents the gas constant, and T represents the temperature, and ε′indicating the strain rate is determined by the following formula of Orowan & Pascoe.ε. ? vRR′⁢ hl·1-0.75r1-r·r

[0110] In the above formula, vR represents a roll peripheral speed (mm / sec), R′ represents a flattened roll radius (mm), hl represents an entry sheet thickness (mm), and r represents a rolling reduction rate.

[0111] A thickness of the hot rough rolled sheet after the final pass is not particularly limited, and may be, for example, 10 mm to 100 mm.<Step 4: Hot Finish Rolling Step>

[0112] The step 4 is a hot finish rolling step of subjecting the hot rough rolled sheet obtained in the step 3 to hot finish rolling to obtain a hot finish rolled sheet.

[0113] The hot finish rolling is performed such that an end temperature is 250° C. or higher and lower than 350° C. Here, from the viewpoint of preventing a decrease in productivity due to a slow speed of finish rolling for coil cooling, the end temperature is 250° C. or higher, and may be 270° C. or higher. From the viewpoint of reducing the number density of the Mg—Si-based compounds and obtaining satisfactory bendability, the end temperature is lower than 350° C., preferably lower than 340° C., more preferably lower than 330° C., and still more preferably 310° C. or lower.

[0114] A thickness of the hot finish rolled sheet finally obtained by the step 4 is not particularly limited, and may be, for example, 3 mm to 10 mm.

[0115] A step of performing intermediate annealing may be included after the hot finish rolling in the step 4 and before the subsequent cold rolling step in the step 5. Here, in the case of an aluminum alloy sheet, generally, a crystallographic structure including many orientation factors (crystal grains having these orientations) is generally formed, and crystal planes corresponding thereto are present. Generally, the crystallographic structure in a rolled recrystallized sheet of an aluminum alloy is mainly composed of Cube orientation, Goss orientation, Brass orientation, S orientation, and Copper orientation. The expression of the crystallographic structure is expressed by a rolled plane and a rolling direction in the case of the crystallographic structure of a sheet material by rolling, the rolled plane is expressed by {hkl}, and the rolling direction is expressed by <uvw>. Based on such an expression, each direction is expressed as follows.

[0116] Cube orientation {001}<100>

[0117] Goss orientation {011}<100>

[0118] Brass orientation (B orientation) {011}<211>

[0119] Cu orientation (Copper orientation) {112}<111>

[0120] S orientation {123}<634>

[0121] On the other hand, there is a case where recrystallization does not occur in the hot finish rolled sheet obtained by the hot finish rolling. In this case, the intermediate annealing is preferably performed because the intermediate annealing releases the strain without applying any restraining force, resulting in uniform and fine recrystallized grains and satisfactory surface quality. When the recrystallization occurs, the intermediate annealing may not be performed.

[0122] By performing the subsequent cold rolling after the recrystallization by the intermediate annealing such that the cold rolling ratio is 74% to 95%, an area fraction of the Cube orientation can be increased through recrystallization in the solution treatment in the step 6, and more excellent bendability can be achieved.

[0123] The intermediate annealing is preferably performed in a temperature range of, for example, 300° C. to 450° C., and more preferably performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.

[0124] The temperature in the intermediate annealing is preferably 300° C. or higher, more preferably 325° C. or higher, and still more preferably 350° C. or higher, from the viewpoint of favorably obtaining recrystallized grains. The temperature is preferably 450° C. or lower, more preferably 425° C. or lower, and still more preferably 400° C. or lower, from the viewpoint of preventing the precipitation of the Mg—Si-based compounds and the decrease in bendability.

[0125] A time for the intermediate annealing is preferably 1 hour or longer, more preferably 2 hours or longer, and still more preferably 3 hours or longer, from the viewpoint of bringing the inside of the coil to a desired annealing temperature and suitably obtaining the effect of the annealing. The time is preferably 20 hours or shorter, more preferably 15 hours or shorter, and still more preferably 10 hours or shorter, from the viewpoint of preventing the coarsening of the Mg—Si-based compounds and the decrease in bendability.<Step 5: Cold Rolling Step>

[0126] The step 5 is a cold rolling step of subjecting the hot finish rolled sheet obtained in the step 4 to cold rolling to obtain a cold rolled sheet.

[0127] The cold rolling is a step performed once or a plurality of times to obtain the aluminum alloy sheet having a desired final thickness.

[0128] The step 5 in the present embodiment is performed such that the cold rolling ratio is 74% to 95%. Here, the cold rolling ratio is 74% or more, preferably 78% or more, and more preferably 80% or more, from the viewpoint of increasing the area fraction of Cube orientation after the recrystallization in the subsequent solution treatment step in the step 6 and achieving satisfactory bendability. The cold rolling ratio is 95% or less, preferably 93% or less, and more preferably 91% or less, from the viewpoint of suitably controlling the area fraction of Cube orientation and maintaining good surface condition.

[0129] A thickness of the cold rolled sheet is not particularly limited as long as the cold rolling ratio is 74% to 95%, and may be, for example, 0.7 mm to 2.5 mm.

[0130] The intermediate annealing may be performed during the cold rolling step. In this case, the cold rolling is preferably performed at a cold rolling ratio of 74% or more after the intermediate annealing.<Step 6: Solution Treatment Step>

[0131] The step 6 is a solution treatment step of subjecting the cold rolled sheet obtained in the step 5 to a solution treatment.

[0132] The solution treatment is a step of dissolving Mg or Si in the sheet to secure the proof stress after a bake hardening treatment.

[0133] The solution treatment is preferably performed at 520° C. or higher, and more preferably performed in a temperature range of 520° C. to 580° C. Here, the temperature is preferably 520° C. or higher, more preferably 525° C. or higher, and still more preferably 530° C. or higher, from the viewpoint of securing strength by ensuring a sufficient amount of solid solution and achieving better bendability by preventing an increase of the Mg—Si-based compounds. The temperature is preferably 580° C. or lower, more preferably 575° C. or lower, and still more preferably 570° C. or lower, from the viewpoint of preventing eutectic melting and recrystallized grain growth.

[0134] A holding time in the solution treatment is not particularly limited, and is preferably, for example, 1 second to 300 seconds.

[0135] Cooling is performed after the solution treatment, and a cooling rate at that time is preferably 1° C. / sec to 100° C. / sec. Here, the cooling rate is preferably 1° C. / sec or more, more preferably 5° C. / sec or more, and still more preferably 10° C. / sec or more, from the viewpoint of preventing a decrease in bendability due to grain boundary precipitation of coarse elemental Si phases, Mg—Si-based compounds, and the like. The cooling rate is preferably 100° C. / sec or less, more preferably 80° C. / sec or less, and still more preferably 60° C. / sec or less, from the viewpoint of preventing a decrease in flatness of the aluminum alloy sheet due to the cooling strain.

[0136] The cooling is performed to 100° C. or lower, for example, room temperature.

[0137] The method for producing an aluminum alloy sheet according to the present embodiment may further include a step of performing a preliminary aging treatment after performing the solution treatment and cooling in the step 6.

[0138] The step of performing the preliminary aging treatment is a step of performing a reheating treatment after the cooling following the solution treatment.

[0139] The preliminary aging treatment is preferably performed within 10 minutes after the solution treatment step, that is, after the solution treatment is completed and, for example, after cooling to room temperature, from the viewpoint of preventing a decrease in bake hardenability due to the formation of Si-rich Mg—Si-based clusters caused by room temperature aging.

[0140] The preliminary aging treatment is preferably performed in a temperature range of 50° C. to 100° C., and more preferably held in a temperature range of 50° C. to 100° C. for 1 hour to 50 hours.

[0141] Here, the temperature in the preliminary aging treatment is preferably 50° C. or higher, more preferably 55° C. or higher, and still more preferably 60° C. or higher, from the viewpoint of obtaining satisfactory bake hardenability. The temperature is preferably 100° C. or lower, more preferably 95° C. or lower, and still more preferably 90° C. or lower, from the viewpoint of preventing a decrease in bendability caused by the proof stress becoming excessively high.

[0142] A time for the preliminary aging treatment is preferably 1 hour or more, more preferably 2 hours or more, and still more preferably 4 hours or more, from the viewpoint of obtaining satisfactory bake hardenability. The time is preferably 50 hours or shorter, more preferably 40 hours or shorter, and still more preferably 30 hours or shorter, from the viewpoint of preventing a decrease in bendability caused by the proof stress becoming excessively high.

[0143] The preliminary aging treatment is more preferably performed at a temperature of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

[0144] Although each step for producing the aluminum alloy sheet according to the present embodiment has been described above, other steps may be performed between or before or after each step as long as each step is not adversely affected.<Aluminum Alloy Sheet; Composition>

[0145] The aluminum alloy sheet obtained by the production method according to the present embodiment has the following alloy composition:

[0146] 0.3 mass % to 1.5 mass % of Si,

[0147] 0.15 mass % to 0.4 mass % of Fe,

[0148] 0.001 mass % to 0.2 mass % of Cu,

[0149] 0.02 mass % to 0.3 mass % of Mn,

[0150] 0.3 mass % to 0.8 mass % of Mg,

[0151] 0.001 mass % to 0.3 mass % of Cr,

[0152] 0.001 mass % to 0.3 mass % of Zn, and

[0153] 0.001 mass % to 0.3 mass % of Ti,

[0154] with the remainder being Al and impurities.

[0155] Each component will be described in order.

[0156] Si: 0.3 mass % to 1.5 mass %

[0157] Si is a main alloy element in the aluminum alloy sheet, and is an element that contributes to improvement in strength of the aluminum alloy sheet together with Mg or Cu. A content of Si in the aluminum alloy sheet is 0.3 mass % to 1.5 mass %. Here, from the viewpoint of obtaining sufficient strength, the content is 0.3 mass % or more, preferably 0.4 mass % or more, and more preferably 0.5 mass % or more. From the viewpoint of preventing a decrease in bendability due to the generation of coarse elemental Si phases or Mg—Si-based compounds, the content is 1.5 mass % or less, preferably 1.25 mass % or less, and more preferably 1.0 mass % or less.

[0158] Fe: 0.15 mass % to 0.4 mass %

[0159] Fe is an element that contributes to grain refinement through a particle-stimulated nucleation (PSN) mechanism. A content of Fe in the aluminum alloy sheet is 0.15 mass % to 0.4 mass %. Here, the content is 0.15 mass % or more, preferably 0.2 mass % or more, and more preferably 0.25 mass % or more, from the viewpoint of preventing deterioration of surface condition due to coarsening of recrystallized grains during the treatment such as intermediate annealing by generating a sufficient amount of Al—Fe-based compounds serving as a recrystallized nucleus through the particle-stimulated nucleation mechanism (PSN mechanism). The content is 0.4 mass % or less, preferably 0.35 mass % or less, and more preferably 0.3 mass % or less, from the viewpoint of preventing a decrease in bendability due to an increase in Al—Fe-based compounds.

[0160] Cu: 0.001 mass % to 0.2 mass %

[0161] Cu is an element that contributes to improvement in strength by solid-solution hardening. A content of Cu in the aluminum alloy sheet is 0.001 mass % to 0.2 mass %. Here, allowing the presence of Cu in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Cu. However, as the content of Cu increases, the corrosion resistance deteriorates. Therefore, the content of Cu is preferably 0.001 mass % or more from an economical viewpoint, and the content is 0.2 mass % or less, preferably 0.13 mass % or less, and more preferably 0.05 mass % or less, from a viewpoint of obtaining excellent intergranular corrosion resistance.

[0162] Mn: 0.02 mass % to 0.3 mass %

[0163] Mn is an element that contributes to grain refinement through the PSN mechanism. A content of Mn in the aluminum alloy sheet is 0.02 mass % to 0.3 mass %. Here, the content is 0.02 mass % or more, preferably 0.04 mass % or more, more preferably 0.05 mass % or more, and still more preferably 0.06 mass % or more, from the viewpoint that grains are refined by the formation of Al—Fe—Si—Mn-based compounds or the like to obtain good surface condition. From the viewpoint of preventing a decrease in bendability due to an increase in Al—Fe—Si—Mn-based compounds or the like, the content is 0.3 mass % or less, preferably 0.25 mass % or less, and more preferably 0.2 mass % or less.

[0164] Mg: 0.3 mass % to 0.8 mass %

[0165] Mg is a main alloy element in the aluminum alloy sheet, and is an element that contributes to the improvement of the strength of the aluminum alloy sheet together with Si or Cu. A content of Mg in the aluminum alloy sheet is 0.3 mass % to 0.8 mass %. Here, the content is 0.3 mass % or more, preferably 0.35 mass % or more, and more preferably 0.4 mass % or more, from the viewpoint of obtaining sufficient strength. The content is 0.8 mass % or less from the viewpoint of preventing a decrease in bendability due to the generation of coarse Mg—Si-based compounds, and is more preferably less than 0.75 mass %, and still more preferably 0.70 mass % or less from the viewpoint of improving elongation in addition to satisfactory bendability.

[0166] Cr: 0.001 mass % to 0.3 mass %

[0167] Cr is an element that contributes to improvement in strength and ductility and improvement in surface condition such as a rough surface by grain refinement through compound formation. A content of Cr in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, allowing the presence of Cr in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Cr. Therefore, from an economical viewpoint, the content of Cr is preferably 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more. The content of Cr is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing a decrease in bendability due to the formation of coarse compounds.

[0168] Zn: 0.001 mass % to 0.3 mass %

[0169] A content of Zn in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, allowing the presence of Zn in the aluminum alloy is economically advantageous in that it enables recycling and reuse of aluminum scrap containing Zn. Therefore, from an economical viewpoint, the content of Zn is preferably 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more. On the other hand, the content of Zn is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing a decrease in pitting potential and a decrease in corrosion resistance.

[0170] Ti: 0.001 mass % to 0.3 mass %

[0171] Ti is an element that is actively introduced by addition of Al—Ti—B or Ti—B as a microstructure refining material and contributes to microstructure refinement. A content of Ti in the aluminum alloy sheet is 0.001 mass % to 0.3 mass %. Here, the content is 0.001 mass % or more, may be 0.002 mass % or more, and may be 0.004 mass % or more, from the viewpoint of improving strength and ductility and improving surface condition such as a rough surface by microstructure refinement. The content is 0.3 mass % or less, preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, from the viewpoint of preventing the formation of coarse compounds that adversely affect the required properties.

[0172] Remainder: Al and Impurities

[0173] The basic components of the component composition constituting the aluminum alloy sheet in the present embodiment are as described above, and the remainder is Al and impurities.

[0174] The impurities are components other than those described above and can be contained by selection of a melting raw material containing scrap for production of a slab, and are allowed to be contained within a range not impairing various properties of the aluminum alloy sheet.

[0175] Examples of the impurities include V, B, Ni, Na, Pb, Be, Ca, Sn, In, Cd, Bi, Ge, and the like.

[0176] The impurities include not only inevitable impurities that are inevitably contained unintentionally but also impurities that are intentionally added to the extent that the effects of the present invention are not impaired. The inevitable impurities are generally impurities that are inevitably mixed during melting of raw materials.

[0177] The elements described above as the examples of impurities may be contained as inevitable impurities, or may be intentionally added.

[0178] A content of each component serving as an impurity is preferably 0.05 mass % or less, and a total content of components serving as impurities is preferably 0.15 mass % or less.

[0179] As the remainder of the aluminum alloy sheet, a purity of Al, that is, a content of Al, is preferably 95 mass % or more, more preferably 96 mass % or more, and still more preferably 97 mass % or more.<Aluminum Alloy Sheet; Microstructure>

[0180] The aluminum alloy sheet obtained by the production method according to the present embodiment preferably has an area fraction of Cube orientation of 17% to 50%.

[0181] Here, it is common that the Cube orientation is a crystal orientation advantageous for bendability as described in Reference Literature 1 below. On the other hand, as described in Reference Literature 2 below, the Cube orientation also causes deterioration of surface condition. (Reference Literature 1: “Influence of Crystal Orientation on Bendability of Al—Mg—Si alloy”, Hirotaka Takeda et al., Light Metals, Vol. 60, No. 5, pp. 231-236 (2010); Reference Literature 2: “Crystal Plasticity Analysis of Ridging in Al—Mg—Si Series Sheet Alloy”, Haruyuki Konishi et al., R&D Kobe Steel Techniques, Vol. 62, No. 2, pp. 39-42 (2012))

[0182] On the other hand, the area fraction of the Cube orientation of the aluminum alloy in the present embodiment is preferably 17% or more, more preferably 20% or more, still more preferably 21% or more, and still more preferably 25% or more, from the viewpoint of achieving satisfactory bendability. The area fraction is preferably 50% or less, more preferably 45% or less, and still more preferably 40% or less, from the viewpoint of maintaining good surface condition.

[0183] As described above, the above range of the area fraction can be achieved by the intermediate annealing, the cold rolling step in the step 5, the solution treatment step in the step 6, or the like.

[0184] In the present embodiment, the area fraction of the Cube orientation of the aluminum alloy sheet can be measured by the following method.

[0185] A test piece is cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to the rolling direction can be observed, and the cross section is polished to be a mirror surface. The cross section is then subjected to ion etching using X-ray photoelectron spectroscopy (ESCA; electron spectroscopy for chemical analysis), and the area fraction of the Cube orientation is measured by irradiating the cross section parallel to the rolling direction with an electron beam at a pitch of 5 μm within a range of “1.6 μm in the rolling direction×a thickness length of the test piece” using electron back scattering diffraction (EBSD).

[0186] In the measurement by the EBSD method, for example, an FE-SEM7000F scanning electron microscope manufactured by JEOL Ltd. and a photodetector manufactured by TSL Solutions Co., Ltd. can be used, and an acceleration voltage of the measurement device is set to 20 kV.

[0187] Here, the Cube orientation refers to grains whose {100}plane is the sheet surface and <001> direction is parallel to the rolling direction, and grains with a misorientation angle of 15° or less from the ideal Cube orientation are considered to be Cube orientation, and the area fraction of the Cube orientation is determined.

[0188] In the aluminum alloy sheet obtained by the production method according to the present embodiment, a number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more is preferably 2650 particles / mm2 or less.

[0189] The present inventors have found that there is a correlation between the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more and bendability.

[0190] The Mg—Si-based compounds in the present specification mean an intermetallic compound containing Mg and Si as constituent elements.

[0191] From the viewpoint of achieving satisfactory bendability as described above, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 m or more is preferably 2650 particles / mm2 or less, more preferably 2400 particles / mm2 or less, and still more preferably 2250 particles / mm2 or less. The lower the number density, the better, and the number density may be, for example, 1 particle / mm2 or more.

[0192] As described above, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more can be within the above range by the Z factor of the final pass in the hot rough rolling in the step 3, the end temperature in the hot finish rolling in the step 4, or the like.

[0193] The number density of the Mg—Si-based compounds is determined by the following method.

[0194] A test piece is cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to the rolling direction can be observed, and the cross section is polished to be a mirror surface. Thereafter, in order to ensure conduction, gold vapor deposition is performed on the cross section, and then a vicinity of a sheet thickness center is observed with a scanning electron microscope FE-SEM (field emission scanning electron microscope).

[0195] The FE-SEM device used is, for example, a JSM-7001F manufactured by JEOL Ltd., and observation is performed in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view.

[0196] As a result of the above observation, black parts are Mg—Si-based compounds, and the number density of those having an equivalent circle diameter of 0.4 μm or more is determined.

[0197] In the aluminum alloy sheet obtained by the production method according to the present embodiment, a number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 225 particles / mm2 to 700 particles / mm2.

[0198] Here, the Al—Fe—Si-based compound in the present specification means an intermetallic compound that contains Al and Fe as constituent elements and may also contain Si, Mn, Cr, or the like.

[0199] As described above, when the area fraction of the Cube orientation is high, the surface condition may deteriorate. On the other hand, the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 225 particles / mm2 or more, more preferably 300 particles / mm2 or more, and still more preferably 400 particles / mm2 or more, from the viewpoint of achieving good surface condition by grain refinement. On the other hand, the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is preferably 700 particles / mm2 or less, more preferably 600 particles / mm2 or less, and still more preferably 500 particles / mm2 or less, from the viewpoint of preventing an increase in Al—Fe—Si compounds serving as a starting point of cracking and a decrease in bendability.

[0200] The number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more can be within the above range by adjusting the contents of Fe and Mn and the homogenization heat treatment conditions.

[0201] The number density of the Al—Fe—Si-based compounds is determined by observation using a scanning electron microscope FE-SEM in the same manner as the above-described number density of the Mg—Si-based compounds.

[0202] That is, in the same manner as the number density of the Mg—Si-based compounds, for example, observation is made using a JSM-7001F manufactured by JEOL Ltd. in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view. As a result of the above observation, white parts are Al—Fe—Si-based compounds, and the number density of those having an equivalent circle diameter of 2.0 μm or more is determined.<Aluminum Alloy Sheet; Properties>

[0203] The aluminum alloy sheet obtained by the production method according to the present embodiment has both excellent bendability and excellent ridging resistance.

[0204] The bendability can be evaluated by, for example, a VDA bending test.

[0205] The VDA bending test is a test performed in accordance with “VDA238-100 Plate bending test for metallic materials” standardized by the German Association of the Automotive Industry (VDA).

[0206] The test method will be described with reference to the drawings.

[0207] FIG. 1 is a schematic perspective view of a device used in a VDA bending test for evaluating bendability, a left side in FIG. 2 is a front view of the device, and a right side in FIG. 2 is a side view of the device.

[0208] First, as illustrated in FIG. 1, a sheet-shaped test piece obtained from the aluminum alloy sheet is placed horizontally on two rolls disposed in parallel to each other with a roll gap therebetween, with a length of the sheet-shaped test piece being equal on both sides, as illustrated by a dotted line in FIG. 1. Specifically, the sheet-shaped test piece is placed horizontally on the two rolls with the lengths of the sheet-shaped test piece on both sides being equal, the rolling direction of the sheet-shaped test piece and an extending direction of a sheet-shaped pressing and bending jig disposed vertically above are perpendicular to each other, and a central portion of the sheet-shaped test piece is positioned at a center of the roll gap.

[0209] The pressing and bending jig is pressed against the central portion of the sheet-shaped test piece from above to apply a load, the sheet-shaped test piece is pressed and bent (thrust and bent) toward a narrow roll gap, and the central portion of the bent and deformed sheet-shaped test piece is pressed into the narrow roll gap.

[0210] At this time, when a load F from the pressing and bending jig above is at a maximum, that is, immediately before a tip of the bent and deformed central portion of the sheet-shaped test piece is about to break, an angle at an outer side of the bending in the central portion of the sheet-shaped test piece is measured as a bending angle (°). A magnitude of the bending angle serves as an index for evaluation of bendability.

[0211] As test conditions of the VDA bending test, when symbols described in FIG. 1 are used, the sheet-shaped test piece has a rectangular shape with “width b: sheet width after 10% stretching×length 1: 60 mm”, a diameter D of each of the two rolls is 30 mm, and a roll gap L is (thickness of the sheet-shaped test piece×2+0.5) mm. S is an indentation depth of the central portion of the sheet-shaped test piece into the roll gap when the load F from the pressing and bending jig above is at the maximum. As illustrated in FIG. 2, a punch, which is a sheet-shaped pressing and bending jig, has a lower thin sheet-shaped blade pressed against the central portion of the sheet-shaped test piece, and a tip (lower end) of the blade is formed into a tapered shape with a radius r of 0.2 mm.

[0212] The sheet-shaped test piece used before the VDA bending test is prepared by the following procedure.

[0213] The aluminum alloy sheet is cut into strips of 250 mm in a rolling parallel direction ×30 mm in a rolling perpendicular direction, and subjected to an aging treatment in an air furnace under respective conditions of, for example, 90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h. Thereafter, the strips are subjected to 10% stretching in a direction parallel to the rolling direction using a tensile testing machine, and then cut out into a rectangle having a width b equal to the sheet width after stretching (after 10% stretching) and a length 1 of 60 mm to obtain a sheet-shaped test piece.

[0214] Using the sheet-shaped test piece, a VDA bending test in which a bending line is perpendicular to the rolling direction is performed in accordance with the above-described VDA238-100 standard.

[0215] A speed at which the pressing and bending jig is pressed in is 10 mm / min until the load reaches 30 N, and thereafter the speed is 20 mm / min.

[0216] The bending is set to stop when a crack occurs in the sheet-shaped test piece or when the load is reduced by 30 N from a maximum load, as a result of a reduction in sheet thickness of the sheet-shaped test piece. The bending test is performed twice for each aging treatment condition, and the bending angle (°) is an average value thereof.

[0217] In the VDA bending test in the present embodiment, the VDA bending angle at a proof stress of 150 MPa is obtained. The proof stress is adopted based on the fact that when automobile manufacturers perform hem bending, the proof stress may increase due to natural aging, which may result in a deterioration in bendability, and that the maximum proof stress expected during the hem bending is approximately 150 MPa.

[0218] The proof stress measured after 10% stretching is plotted on an x-axis and the VDA bending angle is plotted on a scatter diagram on a y-axis, and a linear approximation curve (y =ax+b) is obtained using a least squares method for each plot under each aging condition (for example, 90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h), and y (VDA bending angle) when x(proof stress)=150 is obtained.

[0219] Since the bendability varies depending on the sheet thickness, the VDA bending angle corresponding to the sheet thickness of 1.0 mm is obtained using a sheet thickness conversion formula of the VDA bending angle described in the VDA standard.

[0220] The sheet thickness conversion formula is as follows.αSoll=αref⁢treft

[0221] In the above formula, αSoll is a VDA bending angle conversion value, aref is a VDA bending angle measurement value, t is a sheet thickness to be converted, and tref is a sheet thickness measurement value.

[0222] Using the above formula, the VDA bending angle corresponding to a proof stress of 150 MPa (converted to sheet thickness of 1.0 mm) is obtained. When the value is 128° or more, it can be said that the bendability is satisfactory, and the value is preferably 136° or more, more preferably 144° or more, and still more preferably 150° or more.

[0223] The surface quality of the aluminum alloy sheet obtained by the production method according to the present embodiment can be evaluated using, for example, a surface roughness index as the surface texture.

[0224] The surface roughness index is a value obtained by the following method.

[0225] First, in order to simulate the surface quality after press forming, a strain is imparted to the aluminum alloy sheet by stretching with a tensile tester, and then the surface quality is evaluated. A pattern commonly seen in 6000 series alloys when the surface texture deteriorates is a ridging mark and a rough surface, but in the present description, the evaluation is made based on the rough surface, which shows a significant deterioration in properties when the rough surface is outside a specified range.

[0226] First, an aluminum alloy sheet is cut into a shape of “40 mm in the rolling parallel direction×200 mm in the rolling perpendicular direction”, and subjected to 10% stretching in a direction perpendicular to the rolling direction using a tensile tester to obtain a test piece.

[0227] Next, a rough surface degree of the test piece is defined and evaluated with reference to the method described in Kobe Steel Technical Report / Vol. 66 No. 2 (March 2017).

[0228] Specifically, a surface shape of the test piece is measured in a region of 30 mm×30 mm using a three-dimensional surface roughness measuring machine (Contour-GT, manufactured by Bruker Japan). Then, after removing waveform components having a wavelength of less than 1 mm or 5 mm or more, waveform components are classified according to orientations with respect to the rolling direction of the sheet material, and a total value of amplitudes of the respective waveform components is obtained, thereby quantitatively evaluating various surface defect forms appearing on the sheet material surface, such as a rough surface and ridging. Since the rough surface is a magnitude of unevenness regardless of orientation, the surface roughness degree can be expressed as a numerical index by summing the amplitudes of the waveform components in all orientations and dividing the sum by 360 (degrees).

[0229] In the present description, when the surface roughness index is 52 μm or less, the surface texture can be said to be satisfactory, and the surface roughness index is more preferably 48 μm or less, and still more preferably 42 μm or less.

[0230] The mechanical properties (strength) of the aluminum alloy sheet obtained by the production method according to the present embodiment can be evaluated using a 0.2% proof stress obtained by a tensile test.

[0231] The proof stress is a value obtained by the following method.

[0232] A 13A test piece conforming to JIS Z 2201: 1998 is collected and prepared from the aluminum alloy sheet such that a tensile direction is perpendicular to a rolling direction, and a tensile test is performed at room temperature.

[0233] The tensile test is performed at 5 mm / min until the 0.2% proof stress is measured, and then at 30 mm / min.

[0234] When the proof stress after 3 days to 14 days from the time of obtaining the test piece is 80 MPa or more and less than 150 MPa, it can be said that the mechanical properties are satisfactory.

[0235] The mechanical properties after a baked finish are also important for the aluminum alloy sheet.

[0236] Therefore, in order to simulate the baked finish, a test piece is given a pre-strain of 2% and then subjected to an aging treatment (a bake hardening treatment) at 185° C. for 20 minutes, and then a tensile test is performed on the test piece.

[0237] When the 0.2% proof stress after the bake hardening treatment is 190 MPa or more and less than 240 MPa, it can be said that the mechanical properties are satisfactory.<<Aluminum Alloy Sheet>>

[0238] The aluminum alloy sheet according to the present embodiment is an aluminum alloy sheet obtained by the above-described <<Method for Producing Aluminum Alloy Sheet>>.

[0239] An aspect of the aluminum alloy sheet according to the present embodiment satisfies the following alloy composition.

[0240] 0.3 mass % to 1.5 mass % of Si,

[0241] 0.15 mass % to 0.4 mass % of Fe,

[0242] 0.001 mass % to 0.2 mass % of Cu,

[0243] 0.02 mass % to 0.3 mass % of Mn,

[0244] 0.3 mass % to 0.8 mass % of Mg,

[0245] 0.001 mass % to 0.3 mass % of Cr,

[0246] 0.001 mass % to 0.3 mass % of Zn, and

[0247] 0.001 mass % to 0.3 mass % of Ti,

[0248] with the remainder being Al and impurities.

[0249] In addition to the above alloy composition, it is preferable to satisfy one or more, more preferably to satisfy two or more, and still more preferably to satisfy all three of the following microstructure items: the area fraction of Cube orientation is 17% to 50%, the number density of the Mg—Si-based compounds having an equivalent circle diameter of 0.4 um or more is 2650 particles / mm2 or less, and the number density of the Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more is 225 particles / mm2 to 700 particles / mm2 or less.

[0250] The description and preferred aspect of the above alloy composition and microstructure items are the same as the preferred aspect described in the above <<Method for Producing Aluminum Alloy Sheet>>.

[0251] It can be said that the aluminum alloy sheet according to the present embodiment is an aluminum alloy sheet having both excellent bendability and excellent surface quality as long as the VDA bending angle corresponding to a proof stress of 150 MPa (converted to sheet thickness of 1.0 mm) is 128° or more and the average intensity value Aave, which is a surface roughness index, is 52 μm or less.

[0252] In addition to the above, it is preferable to satisfy one or more and more preferable to satisfy both of property items: the proof stress after 3 days to 14 days is 80 MPa or more and the 0.2% proof stress after the bake hardening treatment is 190 MPa or more.

[0253] The description and preferred aspect of the above properties are the same as the preferred aspect described in the above <<Method for Producing Aluminum Alloy Sheet>>.EXAMPLES

[0254] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples, and modifications can be made within the scope that can conform to the gist thereof, and all of them are included in the technical scope of the present invention.Test ExampleExample 1

[0255] A raw material was melted, and a molten metal whose component was adjusted to a component composition shown in an alloy A in Table 1 was cast into a slab. The slab was subjected to scalping in order to remove a segregation layer on the surface. (Step 1)

[0256] Next, the slab was subjected to a one-stage homogenization heat treatment at 560° C. for 6 hours. (Step 2)

[0257] Hot rough rolling was started immediately after the homogenization heat treatment, and an entry temperature was set to a range of 500° C. to 560° C., and a final pass was performed under a condition that a Z factor was 7.63×109. (Step 3)

[0258] Subsequently, hot finish rolling was performed. An end temperature of the hot finish rolling was 300° C., and a sheet thickness of a hot finish rolled sheet was 6.0 mm. (Step 4)

[0259] Thereafter, intermediate annealing was performed at 375° C. for 4 hours.

[0260] The hot finish rolled sheet after the intermediate annealing was cold rolled. The obtained cold rolled sheet had a sheet thickness of 1.0 mm and a cold rolling ratio of 83%. (Step 5)

[0261] Finally, a solution treatment was performed at 540° C., and after cooling to room temperature at a cooling rate in a range of 10° C. / sec to 60° C. / sec, a preliminary aging treatment was performed by heating to 80° C. after about 5 minutes and holding for 4 hours to 30 hours to obtain an aluminum alloy sheet. (Step 6)Comparative Example 1

[0262] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that a component composition was changed to that shown in an alloy B in Table 1 and conditions of hot finish rolling, intermediate annealing, and cold rolling were changed to those shown in Table 2.Comparative Example 2

[0263] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that conditions of hot finish rolling were changed to those shown in Table 2.Comparative Example 3

[0264] An aluminum alloy sheet was obtained in the same manner as in Example 1 except that a component composition was changed to that shown in an alloy C in Table 1 and conditions of cold rolling and a solution treatment were changed to those shown in Table 2.TABLE 1Chemical composition (mass %)AlloySiFeCuMnMgCrZnTiAlA0.790.280.010.070.480.010.020.02RemainderB0.670.170.120.070.570.020.020.03RemainderC0.750.130.010.070.520.010.020.03RemainderTABLE 2Homogenization heat treatmentHomogenizationHomogenizationHotHot finish rollingheat treatmentheat treatmentroughEndThicknesstemperaturetimerollingtemperature(mm) afterAlloy(° C.)(h)Z factor(° C.)hot rollingExample 1A56067.63.E+093006.0ComparativeB56067.63.E+093003.5Example 1ComparativeA56067.63.E+093506.0Example 2ComparativeC56067.63.E+093006.0Example 3Cold rollingPreliminaryThicknessSolutionagingintermediate annealing(mm) aftertreatmenttreatmentAnnealingcold rollingColdSolutionReheatingtemperatureAnnealing(productrollingtemperaturetemperature(° C.)time (h)thickness)ratio(° C.)(° C.)Example 137541.083%54080Comparative42041.071%54080Example 1Comparative37541.083%54080Example 2Comparative37540.985%53580Example 3Evaluation<Microstructure; Number Density of Mg—Si-based Compounds and Al—Fe—Si-based Compounds>A test piece was cut out from the aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to a rolling direction could be observed, and the cross section was polished to be a mirror surface. Thereafter, in order to ensure conduction, gold vapor deposition was performed on the cross section, and then a vicinity of a sheet thickness center was observed with a scanning electron microscope FE-SEM (field emission scanning electron microscope).

[0266] The FE-SEM device used was a JSM-7001F manufactured by JEOL Ltd., and observation was performed in COMPO mode at 1000× magnification with 50 fields of view or at 500× magnification with 20 fields of view.

[0267] As a result of the above observation, black parts were Mg—Si-based compounds, and the number density of those having an equivalent circle diameter of 0.4 μm or more was calculated to determine the number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more.

[0268] As a result of the above, white parts were Al—Fe—Si-based compounds, and the number density of those having an equivalent circle diameter of 2.0 μm or more was calculated to determine the number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more.

[0269] The results thereof are shown in Table 3.<Microstructure; Area Fraction of Cube Orientation>

[0270] A test piece was cut out from the obtained aluminum alloy sheet, embedded in a polishing resin such that a cross section parallel to a rolling direction could be observed, and the cross section was polished to be a mirror surface. The cross section was then subjected to ion etching using X-ray photoelectron spectroscopy (ESCA; Electron Spectroscopy for Chemical Analysis), and the area fraction of Cube orientation was measured by irradiating the cross section parallel to the rolling direction with an electron beam at a pitch of 5 μm within a range of “1.6 μm in the rolling direction×the thickness length of the test piece” using electron back scattering diffraction (EBSD).

[0271] In the measurement by the EBSD method, an FE-SEM7000F scanning electron microscope manufactured by JEOL Ltd. and a photodetector manufactured by TSL Solutions Co., Ltd. was used, and an acceleration voltage of the measurement device was set to 20 kV Here, the Cube orientation refers to grains whose {100}plane is parallel to a sheet surface and <001> direction is parallel to the rolling direction, and grains with a misorientation angle of 150 or less from the ideal Cube orientation were considered to be Cube orientation, and the area fraction of the Cube orientation was calculated.

[0272] The results thereof are shown in Table 3.<Properties; Mechanical Properties>

[0273] A 13A test piece conforming to JIS Z 2201: 1998 was collected and prepared from the obtained aluminum alloy sheet such that a tensile direction was perpendicular to the rolling direction, and a tensile test was performed at room temperature.

[0274] The tensile test was performed at 5 mm / min until the 0.2% proof stress was measured, and was thereafter performed at 30 mm / min.

[0275] The proof stress was determined after 4 days (Example 1, Comparative Example 1), 7 days (Comparative Example 2), or 8 days (Comparative Example 3) had elapsed since the test piece was obtained.

[0276] As mechanical properties of the aluminum alloy sheet after a baked finish, a test piece was given a pre-strain of 2% and then subjected to an aging treatment (a bake hardening treatment) at 185° C. for 20 minutes, and then a tensile test was performed on the test piece.

[0277] The results thereof are shown in Table 3.<Evaluation; Bendability>

[0278] The obtained aluminum alloy sheet was subjected to a VDA bending test using the device as described above. Specifically, the sheet-shaped test piece used before the VDA bending test was prepared by the following procedure.

[0279] The obtained aluminum alloy sheet was cut into strips 250 mm in a rolling parallel direction×30 mm in a rolling perpendicular direction, and subjected to an aging treatment in an air furnace under respective conditions of 90° C. for 5 hours, 100° C. for 5 hours, 110° C. for 5 hours, and 120° C. for 5 hours. Thereafter, the strips were subjected to 10% stretching in a direction parallel to the rolling direction using a tensile testing machine, and then cut out into a rectangle having a width b equal to the sheet width after stretching (after 10% stretching) and a length 1 of 60 mm to obtain a sheet-shaped test piece.

[0280] Using the sheet-shaped test piece, a VDA bending test in which a bending line was perpendicular to the rolling direction was performed in accordance with the above-described VDA238-100 standard.

[0281] A speed at which the pressing and bending jig was pressed in was 10 mm / min until the load reached 30 N, and thereafter the speed was 20 mm / min.

[0282] The bending was set to stop when a crack occurs in the sheet-shaped test piece or when the load is reduced by 30 N from a maximum load, as a result of a reduction in sheet thickness of the sheet-shaped test piece, and the bending angle θ after stopping was measured. The bending test was performed twice for each aging treatment condition, and the bending angle (deg.) was an average value thereof.

[0283] The 0.2% proof stress measured after 10% stretching was plotted on an x-axis and the VDA bending angle was plotted on a scatter diagram on a y-axis, and a linear approximation curve (y=ax+b) was obtained using a least squares method for each plot under each aging condition (90° C.×5 h, 100° C.×5 h, 110° C.×5 h, and 120° C.×5 h). By determining y (VDA bending angle) when x (proof stress)=150 (MPa) and converting y to a sheet thickness of 1.0 mm, the VDA bending angle corresponding to a proof stress of 150 MPa (converted to a sheet thickness of 1.0 mm) was determined.

[0284] The results are shown in Table 3.<Evaluation; Surface Quality>

[0285] First, the obtained aluminum alloy sheet was cut into a shape of “40 mm in the rolling parallel direction×200 mm in the rolling perpendicular direction”, and subjected to 10% stretching in a direction perpendicular to the rolling direction using a tensile tester to obtain a test piece.

[0286] Next, a power spectrum intensity distribution in a frequency region of 1 mm to 5 mm was extracted from a two-dimensional power spectrum intensity distribution of a two-dimensional surface roughness distribution. The power spectrum intensity distribution was divided into a plurality of regions, and absolute values of a power spectrum intensity in each divided region were summed up for each divided region to calculate an integrated intensity value. The calculated integrated intensity values for the plurality of divided regions were averaged to obtain an average intensity value Aave as a surface roughness index value.

[0287] Next, three-dimensional surface roughness of the sheet surface was measured, and after removing waveform components having a wavelength of less than 1 mm or 5 mm or more, waveform components were classified according to orientations with respect to the rolling direction of the sheet material, a sum of amplitudes of each waveform component was calculated, amplitudes of the waveform components in all orientations were summed and divided by 360 (degrees) to determine the surface roughness index.

[0288] The results thereof are shown in Table 3.TABLE 3MicrostructurePropertiesNumber densityNumber densityMechanical properties(particles / mm2) of(paricles / mm2) ofAreaProof stressBendabilityRidging resistanceMg—Si-basedAl—Fe—Si-basedfractionProof(MPa) afterVDA bending angleRough surfacecompounds (0.4compounds (2.0of CubeTimestressbake hardening(deg.) @ 150 MPaindex Aave (μm)μm or more)μm or more)orientation(days)(MPa)treatment(converted to 1 mm)(after 10% stretch)Example 1211143925%48821215435Comparative139126416%49021412249Example 1Comparative282445924%710021712943Example 2Comparative106620532%810122814555Example 3

[0289] From the above results, it was found that according to the present embodiment, an aluminum alloy sheet having both excellent bendability and excellent surface quality can be obtained.

Examples

examples

[0254]Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples, and modifications can be made within the scope that can conform to the gist thereof, and all of them are included in the technical scope of the present invention.

Claims

1. An aluminum alloy sheet comprising:0.3 mass % to 1.5 mass % of Si;0.15 mass % to 0.4 mass % of Fe;0.001 mass % to 0.2 mass % of Cu;0.02 mass % to 0.3 mass % of Mn;0.3 mass % to 0.8 mass % of Mg;0.001 mass % to 0.3 mass % of Cr;0.001 mass % to 0.3 mass % of Zn; and0.001 mass % to 0.3 mass % of Ti,with the remainder being Al and impurities, whereinthe aluminum alloy sheet has an area fraction of Cube orientation of 17% to 50%,a number density of Mg—Si-based compounds having an equivalent circle diameter of 0.4 μm or more in the aluminum alloy sheet is 2650 particles / mm2 or less, anda number density of Al—Fe—Si-based compounds having an equivalent circle diameter of 2.0 μm or more in the aluminum alloy sheet is 225 particles / mm2 to 700 particles / mm2.

2. A method for producing an aluminum alloy sheet, the method comprising: in the following order,a casting step of melting raw materials to obtain a molten metal having a predetermined chemical composition and casting the molten metal into a slab;a homogenization heat treatment step of subjecting the cast slab to a homogenization heat treatment;a hot rough rolling step of subjecting the homogenized slab to hot rough rolling to obtain a hot rough rolled sheet;a hot finish rolling step of subjecting the hot rough rolled sheet to hot finish rolling to obtain a hot finish rolled sheet;a cold rolling step of subjecting the hot finish rolled sheet to cold rolling to obtain a cold rolled sheet; anda solution treatment step of subjecting the cold rolled sheet to a solution treatment, whereinin the homogenization heat treatment step, the homogenization heat treatment is performed in a temperature range of 520° C. to 580° C.,no cooling and no additional heat treatment are performed between the homogenization heat treatment step and the hot rough rolling step,in the hot rough rolling step, a final pass is performed under a condition that a Z factor, as expressed by the following formula, is 1.0×108 to 5.0×1011,Z=ε.⁢ exp⁡(Q / RT)wherein ε′ represents a strain rate, Q represents activation energy, R represents a gas constant, and T represents a temperature,the hot finish rolling step is performed such that an end temperature is 250° C. or higher and lower than 350° C.,the cold rolling step is performed such that a cold rolling ratio is 74% to 95%, andthe obtained aluminum alloy sheet contains0.3 mass % to 1.5 mass % of Si,0.15 mass % to 0.4 mass % of Fe,0.001 mass % to 0.2 mass % of Cu,0.02 mass % to 0.3 mass % of Mn,0.3 mass % to 0.8 mass % of Mg,0.001 mass % to 0.3 mass % of Cr,0.001 mass % to 0.3 mass % of Zn, and0.001 mass % to 0.3 mass % of Ti,with the remainder being Al and impurities.

3. The method according to claim 2, whereinthe homogenization heat treatment step is performed for 1 hour to 30 hours.

4. The method according to claim 2, whereinthe hot rough rolling step is performed such that an entry temperature is 400° C. to 570° C.

5. The method according to claim 3, whereinthe hot rough rolling step is performed such that an entry temperature is 400° C. to 570° C.

6. The method according to claim 2, further comprising:a step of performing intermediate annealing before the cold rolling step or during the cold rolling step.

7. The method according to claim 3, further comprising:a step of performing intermediate annealing before the cold rolling step or during the cold rolling step.

8. The method according to claim 6, whereinthe intermediate annealing is performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.

9. The method according to claim 7, whereinthe intermediate annealing is performed in a temperature range of 300° C. to 450° C. for 1 hour to 20 hours.

10. The method according to claim 2, whereinthe solution treatment step is performed in a temperature range of 520° C. to 580° C.

11. The method according to claim 3, whereinthe solution treatment step is performed in a temperature range of 520° C. to 580° C.

12. The method according to claim 2, whereincooling is performed at a cooling rate of 1° C. / sec to 100° C. / sec after the solution treatment.

13. The method according to claim 3, whereincooling is performed at a cooling rate of 1° C. / sec to 100° C. / sec after the solution treatment.

14. The method according to claim 2, whereina preliminary aging treatment is performed by holding the sheet at a temperature in a range of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

15. The method according to claim 3, whereina preliminary aging treatment is performed by holding the sheet at a temperature in a range of 50° C. to 100° C. for 1 hour to 50 hours within 10 minutes after the solution treatment step.

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