US20170283913A1

US20170283913A1 - Aluminum alloy sheet having high formability

Info

Publication number: US20170283913A1
Application number: US15/457,386
Authority: US
Inventors: Yuuki KOSHINO; Yasuhiro Aruga
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2016-03-30
Filing date: 2017-03-13
Publication date: 2017-10-05
Also published as: JP2017179468A; CN107267816A

Abstract

Provided is a 6xxx-series aluminum sheet having high formability for automotive body panel use, which can be produced without significant changes in conventional chemical compositions and production conditions. The 6xxx-series aluminum alloy sheet is controlled in its microstructure as follows. The average grain size of the microstructure is controlled to be small; and the average proportion of small angle grain boundaries after application of tensile deformation to the sheet is controlled at two levels in a low strain region and a high strain region according to the levels of strain imparted by the tensile deformation. This restrains heterogeneous deformation from the high strain region leading to rupture upon press forming into an automotive body panel, and allows the sheet to offer good work hardening properties and to have high formability.

Description

FIELD OF INVENTION

The present invention relates to an Al—Mg—Si aluminum alloy sheet having excellent formability.
As used herein, the term “aluminum alloy sheet” refers to an aluminum alloy sheet which is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet and which is after being subjected to heat treatments (temper) such as solution treatment and quenching, but before being formed into a target article such as an automobile member and before being subjected to paint bake hardening. Hereinafter, “aluminum” is also simply represented by “Al”.

BACKGROUND OF INVENTION

Social demands for weight reduction of vehicles such as automobiles have recently increased more and more in consideration typically of global environment. To respond to these demands, aluminum alloy materials have been more and more applied as automobile materials instead of steel materials such as steel sheets. The aluminum alloy materials have formability and paint bake hardenability (BH; hereinafter also referred to as “bake hardenability”) at excellent levels and have lighter weights.
Representative, but non-limiting examples of aluminum alloy sheets for large-sized automotive body panels such as outer panels and inner panels include Al—Mg—Si AA (Aluminum Association) or JIS (Japanese Industrial Standards) 6xxx-series aluminum alloy sheets (hereinafter also simply referred to as “6xxx-series” aluminum alloy sheets). The 6xxx-series aluminum alloy sheets have chemical compositions essentially including Si and Mg. Upon forming, the 6xxx-series aluminum alloy sheets offer low yield strength (low strength) and have satisfactory formability, and, after forming, have higher yield strength (strength) by heating in artificial aging (hardening) treatments such as paint-bake treatment of panels, thereby ensure required strength, and have excellent paint bake hardenability.
Automotive outer panels require, in design, sharp character lines with a beautiful curved plane configuration without distortion and wrinkling, even when portions such as corner portions and character lines are designed to have sharpened or complicated shapes. In relation with the outer panels, automotive inner panels also require a curved plane configuration without distortion and wrinkling even when the inner panels are designed to have deeper (higher) concave and convex dimensions and thereby have complicated shapes.
The requirements for high formability as above become severer with widening applications of material aluminum alloy sheets.
However, it is considerably difficult issue to allow 6xxx-series aluminum alloy sheets to have such high formability as to be required in automotive body panels without significant changes in common (conventional) alloy chemical composition ranges and common production process and conditions, where the 6xxx-series aluminum alloy sheets resist working as compared with steel sheet materials.
In contrast, a variety of methods for controlling chemical compositions and microstructures in the material 6xxx-series aluminum alloy sheets for automotive body panels have been proposed so as to improve the formability and bake hardenability. The methods range from grain size control and texture control to atom duster (duster) control, as is well known.
Among these methods for controlling micorstructures, there has been also proposed control of the proportion of small angle grain boundaries a measured by SEM/EBSD (or EBSP) technique.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2009-173972 proposes a 6xxx-series aluminum alloy sheet for the panels, which less suffers from the roping upon press forming. The sheet is controlled so that the Goss orientation, which is a texture, is present in an area percentage of 4% or less and small angle grain boundaries with a tilt angle of 5° to 15° are present in a proportion of 5% or less in a texture of the sheet at a sheet thickness central part in a transverse direction (sheet width direction).
The literature JP-A No. 2009-173972 mentions that the sheet, if including the small angle grain boundaries in a large proportion at the sheet thickness central part (at a sheet thickness central position), has a long total length of grain boundaries, thereby has heterogeneous extensional deformability from a portion to another, in particular, from a portion to another in the sheet transverse direction upon press forming, and has lower extensional deformability over the sheet transverse direction.
On the basis of this, the technique disclosed in JP-A No. 2009-173972 lowers the proportion of the small angle grain boundaries so as to eliminate or minimize deterioration in formability and increase in susceptibility to roping under severer forming conditions and so as to give better resistance to roping.

SUMMARY OF INVENTION

However, the technique disclosed in JP-A No. 2009-173972 for higher formability has an object to provide better resistance to roping in press forming, but not to offer severe, high formability that is required for automotive body panels so as to realize the sharp character lines with a beautiful curved plane configuration.
In addition, the inventors of the present invention found that controls or restrainment of the area percentage of the Goss orientation, which is a texture, and the proportion of the small angle grain boundaries, such as controls in the technique in JP-A No. 2009-173972, fail to offer such high formability as to achieve the above-mentioned character lines.
Under present circumstances, therefore, there are only conventionally known measures to achieve such high formability. For example, the panel design and/or forming conditions are changed to lower the load upon forming, or 6xxx-series aluminum alloy sheets are controlled to have significantly lowered strength upon forming.
The present invention has been made to solve the problems and has an object to provide a 6xxx-series aluminum alloy sheet having high formability, which is used for automotive body panels and which can be produced without significant changes in chemical compositions and production conditions of conventional 6xxx-series aluminum alloy sheets.
To achieve the object, the present invention provides an aluminum alloy sheet having high formability. The aluminum alloy sheet is an Al—Mg—Si aluminum alloy sheet including, in mass percent, Si in a content of 0.30% to 2.0%, Mg in a content of 0.20% to 1.5%, Cu in a content of 0.05% to 1.0%, Mn in a content of greater than 0% to 1.0%, and Fe in a content of greater than 0% to 1.0% with the remainder consisting of Al and inevitable impurities. The aluminum alloy sheet has a microstructure at a sheet thickness central position, where the microstructure has properties as follows, as measured by SEM/EBSD technique. Specifically, the microstructure has an average grain size of 40 μm or less.
The microstructure includes small angle grain boundaries with a tilt angle of 2.0° to 15.0° in an average proportion of 12% to 30% after tensile deformation at a strain of 5% is imparted in the rolling direction of the sheet. The nanoscale structure includes small angle grain boundaries with a tilt angle of 2.0° to 15.0° in an average proportion of 50% to 70% after tensile deformation at a strain of 15% is imparted in the rolling direction of the sheet.
To achieve the high formability, the present invention allows a 6xxx-series aluminum alloy sheet to have a fine microstructure, restrains the local accumulation of intragranular strain introduced into the aluminum alloy sheet by tensile deformation upon forming into an automotive body panel, and allows the strain to accumulate uniformly (at relatively high level) in grains from a low strain region to a high strain region upon the tensile deformation.
To this end, the present invention controls of the microstructure of the 6xxx-series aluminum alloy sheet at a sheet thickness central position to have a small average grain sew. In addition, the present invention specifies the average proportion (average percentage) of small angle grain boundaries after application of tensile deformation to the sheet at two levels according to the levels of strain imparted by the tensile deformation.
This configuration restrains heterogeneous deformation from the high strain region leading to rupture in press forming of the sheet into an automotive body panel and allows the aluminum alloy sheet to offer work hardening properties at high levels.
In addition and advantageously, such high formability obtained by these microstructure controls can be achieved without significant changes in conventional aluminum alloy chemical compositions and production conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be illustrated in detail below with reference to embodiments on a condition or factor basis.
Chemical Composition
The Al—Mg—Si aluminum alloy sheet according to the present invention is hereinafter also referred to as “6xxx-series aluminum alloy sheet”. Initially, the chemical composition of the aluminum alloy sheet will be illustrated below. In the present invention, the chemical composition is specified so as to also o r properties required for the panel use, such as high formability, bake hardenability, strength, weldability, and corrosion resistance. However, this control is made on the precondition that conventional chemical composition and production conditions are not significantly changed.
To meet this requirement in terms of chemical composition, the 6xxx-series aluminum alloy sheet has a chemical composition including, in mass percent, Si in a content of 0.30% to aux, Mg in a content of 0.20% to 1.5%, Cu in a content of 0.05% to 1.0%, Mn in a content of greater than 0% to 1.0%, and Fe in a content of greater than 0% to 1.0%, with the remainder consisting of Al and inevitable impurities.
In addition, the aluminum alloy sheet may further include at least one element selected from the group consisting of Cr in a content of greater than 0% to 0.3%, Zr in a content of greater than 0% to 0.3%, V in a content of greater than 0% to 0.3%, Ti in a content of greater than 0%© to 0.1%, Zn in a content of greater than 0% to 1.0%, Ag in a content of greater than 0% to 0.2%, and Sn in a content of greater than 0% to 0.15%.
The content ranges and significance, or acceptable contents of elements in the 6xxx-series aluminum alloy sheet will be described below. All percentages in contents of elements are in mass percent.
Si: 0.30% to 2.0%
Silicon (Si), as with Mg, contributes to solid-solution strengthening. This element farms Mg-Si precipitates, which contribute to higher strength, upon artificial aging such as paint-bake treatment and offers artificial age hardenability (bake hardenability). Thus, the element is essential to give strength (yield strength) required for automotive body panels such as outer panels.
In addition, solute Si advantageously restrains dislocations from being localized and allows the dislocations to propagate uniformly from a low strain region to a high strain region in tensile deformation, where the dislocations are introduced into the material via press forming into an automotive body panel. This restrains heterogeneous deformation from the high strain region leading to rupture un press forming and allows the aluminum alloy sheet to elongation and work hardening properties at high levels.
The aluminum alloy sheet, if containing Si in an excessively low content, may include a smaller amount of solute Si and have elongation and work hardening properties at lower levels upon press firming. This may cause dislocations to less propagate after the application of tensile deformation at a strain of 5%. In addition, the aluminum alloy sheet in this case may offer lower bake hardenability and has significantly lower strength after paint-bake treatment, because of insufficient amount of formed Mg—Si precipitates.
In contrast, the aluminum alloy sheet, if containing Si in an excessively high content, may undergo remarkable sheet cracking during hot rolling, due to the formation of coarse particles and precipitates.
In consideration of these, the Si content is controlled within the range of 0.30% to 2.0%. The preferred lower limit and the preferred upper limit of the Si content are respectively 0.50% and 1.5%.
Mg: 0.20% to 1.5%
Magnesium (Mg), as with Si, contributes to solid-solution strengthening. This element forms Mg—Si precipitates, which contribute to higher strength, upon artificial aging such as paint-bake treatment, and offers artificial age hardenability (bake hardenability). Thus, this element is essential to give yield strength required for panels.
In addition, solute Mg, as with solute Si, advantageously restrains dislocations from being localized and allows the dislocations to propagate uniformly from a low strain region to a high strain region in tensile deformation, where the dislocations are introduced into the material via press forming into an automotive body panel. This restrains heterogeneous deformation in press forming from the high strain region leading to rupture and allows the aluminum alloy sheet to offer elongation and work hardening properties at high levels.
The aluminum alloy sheet, if containing Mg in an excessively low content, may include a smaller amount of solute Mg and may have lower work hardening properties. This may cause dislocations to less propagate after the application of tensile deformation at a strain of 5%. In addition, the aluminum alloy sheet in this case may have lower bake hardenability and may have lower strength after paint-bake treatment due to insufficient amount of formed Mg—Si precipitates.
In contrast, the aluminum alloy sheet, if containing Mg in an excessively high content, may undergo significant sheet cracking during hot rolling, due to the formation of coarse particles and precipitates.
In consideration of these, the Mg content is controlled within the range of 0.20% to 1.5%. The preferred lower limit and preferred upper limit of the Mg content are respectively 0.30% and 1.2%.
Cu: 0.05% to 1.0%
Copper (Cu) contributes to higher strength and better formability. Solute Cu improves work hardening properties and allows the aluminum alloy sheet to have strength and formability in better balance, as with solute Si.
The aluminum alloy sheet, if containing Cu in a content 0.05%, may receive smaller advantageous effects by Cu itself and may enjoy insufficient advantageous effects by the solute Cu, due to insufficient amount of the solute Cu.
In contrast, the aluminum alloy sheet, if containing Cu in a content greater than 1.0%, may suffer from significant deterioration in filiform rust resistance after painting and in stress corrosion cracking resistance. To eliminate or minimize this, the Cu content is preferably controlled to 0.80% or less when the aluminum alloy sheet is to be used in applications that attach importance to corrosion resistance.
Mn: greater than 0% to 1.0%
Manganese (Mn) offers solid-solution strengthening and grain refinement effects and allows the aluminum alloy to have higher strength.
However, the aluminum alloy sheet, if containing Mn in an excessively high content of greater than 1.0%, may contain Al—Mn intermetallic compounds in a higher content and may often have lower elongation, because the Al—Mn intermetallic compounds act as fracture origins. The aluminum alloy sheet in this case may also offer lower work hardening properties because dislocations are localized around the Al—Mn intermetallic compounds upon the application of a low strain of about 5% to the sheet.
In consideration of these, the Mn content is controlled within the range of greater than 0% to 1.0%.
Fe: greater than 0% to 1.0%
Iron (Fe) forms Al—Fe intermetallic compounds in the aluminum alloy. The intermetallic compounds increase in amount and act as fracture origins with an increasing Fe content This tends to cause the aluminum alloy sheet to have lower elongation. In addition, the Al—Fe intermetallic compounds often also include Si, and this causes solute Si to be decreased in amount by the amount in which Si is incorporated into the intermetallic compounds.
Fe, which is a base metal impurity, is contaminated in the aluminum alloy and increases in content with an increasing amount of aluminum alloy scrap (proportion to the aluminum base metal) as a melting material. Thus, the lower the Fe content is, the better. However, reduction of Fe content typically to a level of detection limit or less causes increased cost To minimize this disadvantage, a certain amount of Fe is allowed to be contained.
In consideration of these, the Fe content is controlled within the range of greater than 0% to 1.0%, and preferably greater than 0% to 0.5%.
Other Elements
The aluminum alloy sheet according to the present invention may further include at least one element selected from the group consisting of Cr in a content of greater than 0% to 0.3%, Zr in a content of greater than 0% to 0.3%, V in a content of greater than 0% to 0.3%, Ti in a content of greater than 0% to 0.1%, Zn in a content of greater than 0% to 1.0%, Ag in a content of greater than 0% to 0 2 %, and Sn in a content of greater than 0% to 0.15%.
These elements, in common, effectively allow the sheet to have higher strength and are considered as equieffective elements for higher strength. However, concrete mechanisms of these elements include not only a common, identical portion, but also different portions.
Chromium (Cr), zirconium (Zr), and vanadium (V) form dispersed particles (dispersoids) upon homogenization as with Mn. These dispersed particles effectively eliminate or minimize grain boundary migration after recrystallization and play the role of refining grains.
Titanium (Ti) forms particles, which act as nuclei of recrystallized grains, and has the function of eliminating or minimizing coarsening of grains to refine the grains.
Zinc (Zn) and silver (Ag) are useful for better artificial age hardenability (bake hardenability). These elements effectively promote precipitation of compound phases such as a Guinier-Preston zone (GP zone) in grains in the sheet microstructure, upon artificial aging under conditions at a relatively low temperature for a relatively short time.
Tin (Sn) captures atomic vacancies, thereby restrains diffusion of Mg and Si at room temperature, and restrains strength increase at mom temperature (natural aging at room temperature). Upon artificial aging, this element releases the captured vacancies, thereby promotes diffusion of Mg and Si, and effectively contributes to better bake hardenability.
However, each of these elements, if present in an excessively high content, may impede the production of the sheet due typically to the formation of coarse compounds and may cause the aluminum alloy sheet to have strength, bend formability and other formability, and corrosion resistance at lower levels. To eliminate or minimize this, the elements, when to be contained, are controlled to be present in contents of equal to or less than the above-specified upper limits.
For higher formability, the present invention also controls the microstructure of the 6xxx-series aluminum alloy sheet, on the precondition that the aluminum alloy sheet has the alloy chemical composition as mentioned above.
Specifically, the microstructure is controlled to be refined (to have a smaller size). This configuration restrains local accumulation of intragranular strain upon forming into an automotive body panel, where the intragranular strain is introduced into the material by tensile deformation so as to accumulate strain in grains uniformly (at relatively high level) over from a low strain region to a high strain region in the tensile deformation.
Average Grain Size
With a decreasing size of recrystallized grains of the 6xxx-series aluminum alloy sheet after heat treatments such as solution treatment and quenching (but before tensile tests and press forming by which strain is imparted to the sheet), intragranular strain upon tensile deformation accumulates more uniformly and more densely. This results in that the aluminum alloy sheet can have better work hardenability.
To give this property, the present invention controls the aluminum alloy sheet to have an average grain size of 40 μm or less at a sheet thickness central position, where the average grain size is measured by SEM/EBSD technique.
In contrast, the aluminum alloy sheet, if having a large average grain size of greater than 40 μm, may fail to have better work hardenability and may have lower elongation and inferior formability, even when the aluminum alloy sheet has a proportion of small angle grain boundaries within the specified range.
Proportion of Small Angle Grain Boundaries
To surely allow the automotive body panel-use material sheet to have high formability, strain should accumulate uniformly and densely in grains when the material sheet is imparted with tensile deformation upon forming into an automotive body panel.
To achieve this, the sheet microstructure refinement alone is insufficient. Thus, the present invention also controls or specifies the amounts (proportions) of small angle grain boundaries in the microstructure of the sheet in a low strain region and a high strain region during the tensile deformation. These configurations (controls) restrain heterogeneous deformation from the high strain region leading to rupture in press forming into an automotive body panel, offer good work hardening properties, and achieve high formability for automotive body panel use.
Specifically, the fine microstructure (small average grain size) of the sheet constitutes a necessary condition; and the predetermined average proportions of small angle grain boundaries constitute a sufficient condition, where the proportions are measured after tensile deformations at a strain of 5% and a strain of 15% are imparted to the material sheet in the sheet rolling direction.
The aluminum alloy sheet, when meeting both the conditions, can surely offer high formability as a material for automotive body panel use. In addition and advantageously, such high formability given by these controls can be achieved without significant changes in conventional aluminum alloy chemical compositions and production conditions.
Specifically, the sufficient condition is specified on the microstructure of the 6xxx-series aluminum alloy sheet at a sheet thickness central position in the following manner. The average proportion of small angle grain boundaries after application of tensile deformation to the sheet is specified at two levels according to the levels of strain applied by the tensile deformation.
The strain at the level from a low strain region to a high strain region as specified in the present invention, where the strain is applied by the tensile deformation, simulates strain applied to (loaded on) the material sheet in forming such as press forming into an automotive body panel.
At the same time, the present invention specifies and controls the amount (proportion) of small angle grain boundaries after application of a strain of 15% up to the high strain region, where the strain of 15% is much higher than strain to be applied to the material sheet in commmon press forming into an automotive body panel.
This is because the proportion (amount) in the above case is used as an index of the severe, high formability required for automotive body panels, so as to really offer the sharp character lines with a beautiful curved plane configuration.
The strain to be applied to the material sheet by common press forming into an automotive body panel corresponds to tensile deformation at a strain of at highest about 5% which corresponds to the low strain region specified in the present invention. However, a strain at a level greater than 5% may be imparted to (loaded on) the material sheet upon press forming so as to have the sharp character lines with a beautiful curved plane configuration.
Advantageously, the present invention employs the conditions and also enables previous (advance) evaluation of the formability of the material sheet even without actual press forming of the material sheet into an automotive body panel so as to actually have the sharp character lines with a beautiful curved plane configuration.
The strain levels to be imparted by tensile deformation in the range from the low strain region to the high strain region are as follows. A sheet having a chemical composition within the specified range and after the heat treatments so as to have an average grain size within the specified range is subjected to a tensile test in the sheet rolling direction, where the tensile test simulates the actual press forming into an automotive body panel. In the tensile test, the sheet is imparted with tensile deformation at a strain of 5% corresponding to the low strain region and with tensile deformation at a strain of 15% corresponding to the high strain region.
Specifically, the proportions are measured in the following manner. Initially, JIS No. 13A test specimens (20 mm by 80 mm gauge length (GL) by sheet thickness) as test samples are sampled from a cold-rolled sheet after heat treatments such as solution treatment and quenching, where the test specimens are sampled according to the procedure of the tensile test. The test specimens are subjected to the tensile test at room temperature so that the test specimens are pulled in the sheet rolling direction. This simulates the dislocation density of the sheet in the low strain region in actual forming into an automotive body panel. Thus, the tensile deformation at a strain of 5% corresponding to the low strain region and the tensile deformation at a strain of 15% corresponding to the high strain region are independently imparted to the sheet.
Of the test specimens after the tensile tests as above, average proportions of small angle grain boundaries in crystal orientations at a sheet thickness central position are measured by SEM/EBSD technique, while the sheet thickness central position is taken as an observation plane. The aluminum alloy sheet is controlled so as to have an average proportion of 12% to 30% when the sheet is imparted with tensile deformation at a strain of 5% corresponding to the low strain region; and so as to have an average proportion of 50% to 70% when the sheet is imparted with tensile deformation at a strain of 15% corresponding to the high strain region.
The intergranular strain is uniformly and densely distributed within the specified range in the low strain region at a strain of 5% and in the high strain region at a strain of 15%. This restrains subsequent heterogeneous deformation leading to rupture and allows the aluminum alloy sheet to offer good work hardenability and, as a result, to have higher formability.
Assume that a sample sheet has an average proportion of small angle grain boundaries of less than 12% in the low strain region of tensile deformation at a strain of 5%, and/or has an average proportion of small angle grain boundaries of less than 50% in the high strain region of tensile deformation at a strain of 15%. This indicates that intragranular strain less accumulates and work hardenability in the high strain region is not maintained. This in turn may cause the sheet to have lower breaking elongation and thereby have inferior formability.
On the contrary, assume that a sample sheet has an average proportion of small angle grain boundaries of greater than 30% in the low strain region at a strain of 5%, and/or has an average proportion of small angle grain boundaries of greater than 70% in the high strain region at a strain of 15%. In this case, intragranular strain to be introduced and to accumulate before subsequent rupture is decreased, and this may also cause the sheet to fail to have higher formability.
Assume that a sample has an average grain size of greater than 40 μm without grain size control. This sample, after application of tensile deformation strain, has a proportion of small angle grain boundaries of lower than the specified range both in the low strain region and in the high strain region. The sample having such a large grain size includes grain boundaries in a lower proportion. This causes dislocations to more readily accumulate at grain boundaries and, as a result, decreases the amounts of intragranular strain both in the low strain region and in the high strain region.
Accordingly, the sheet is controlled to have an average proportion of small angle grain boundaries in crystal orientations of 12% to 30%, and preferably from 15% to 27% when tensile deformation at a strain of 5% is imparted to the sheet in the sheet rolling direction. In addition, the sheet is controlled to have an average proportion of small angle grain boundaries of 50% to 70%, and preferably 53% to 67% when tensile deformation at a strain of 15% is imparted to the sheet in the sheet rolling direction.
The technical idea according to the present invention to refine grains is not obtained unless the relationship between the formability into an automotive body panel and the average grain size is found. The technical idea to control the proportions of small angle grain boundaries in the sheet is also not obtained unless the object to provide higher formability into such an automotive body panel is recognized and unless the correlation between elongation and strain or work hardening properties is found, where the strain accumulates in grains by tensile defamation upon forming into the automotive body panel, and where the correlation is grasped as a mechanism for achieving the object. In addition, the present invention is not made unless the proportions of small angle grain boundaries in the low strain region and in the high strain region after application of tensile deformation strain are focused as a measure to achieve the mechanism.
Measurement Methods of Average Grain Size and Average Proportions of Small Angle Grain Boundaries
The average grain size and the average proportions of small angle grain boundaries as specified in the present invention are measured by SEM/EBSD technique. For measurement, test specimens are sampled from the sheet (at any two measurement positions) at a central position (sheet thickness center position) in the sheet transverse direction and are subjected to the measurement The measured values of the test specimens are averaged, and the averages are defined as the average grain size and the average proportion of small angle grain boundaries specified in the present invention.
The observation plane (plane to be analyzed) by the SEM/EBSD technique is a plane at the sheet thickness center of a test specimen sampled from a sheet after heat treatments such as solution treatment and quenching, or samples from the sheet after further subjected to the tensile test by which strain is imparted to the sheet.
The observation plane at a sheet thickness central position is adjusted so that the sheet thickness central position is present in the observation plane in a cross section at the sheet thickness center including both the sheet rolling direction and the sheet thickness direction, where the cross section is perpendicular to the sheet transverse direction. Electron beams at a pitch of 1.0 μm are applied to the observation plane in a region of 300 μm in the sheet thickness direction by 300 μm in the sheet transverse direction.
The SEM/EBSD technique is a crystal orientation analysis technique which is widely used as a texture measurement technique and which uses a field emission scanning electron microscope equipped with an electron backscattering (scattered) diffraction pattern (EBSD) system. The measurement technique offers higher resolution and resultantly higher measurement precision as compared with other texture measurement techniques. Advantageously, the technique enables simultaneous, high-precision measurement of the average grain size and the average proportion of grain boundaries in the same measurement portion of the sheet The measurement of the average grain size and the average proportion of grain boundaries of aluminum alloy sheets by the SEM/EBSD technique is conventionally publicly known typically in publications such as JP-A No. 2009-173972, and the present invention also employs this known technique for the measurement.
According to such disclosed SEM/EBSD techniques, an aluminum alloy sheet sample is set in a lens barrel of the FESEM (FE-SEM), to which electron beams are applied so as to project an electron beam scattered diffraction pattern (EBSD) onto a screen. A photograph of this is taken with a highly sensitive camera and captured as an image into a computer. In the computer, the image is analyzed and compared with patterns obtained by simulation on known crystal systems, and on the basis of the comparison, crystal orientations are determined. The determined crystal orientations are recorded as three-dimensional Eulerian angles typically with position coordinates (x, y, z). This process is automatically perfumed on all measurement points, and gives crystal orientation data at several tens of thousands to several hundreds of thousands of points upon the completion of measurement.
Advantageously, the SEM/EBSD technique has a wider observation view field as compared with electron diffractometry using a transmission electron microscope and can obtain average grain size or orientation analysis information on many grains in a number of several hundreds or more within several hours, as described above. In addition and advantageously, the SEM/EBSD technique performs the measurement not on grain-to-grain basis, but by scanning in a specified region at any constant interval, and can obtain the information at the many measurement points covering the entire measurement region. Such crystal orientation analysis techniques using an FESEM equipped with an EBSD system are described in detail typically in Research and Development, Kobe Steel Engineering Reports, Vol. 52, No. 2 (September 2002), pp. 66-70.
In consideration of the above-mentioned points, the average grain size herein is calculated according to the expression:
Average grain size=(Σx)/n
where n represents the number of measured grains; and x represents the equivalent circle diameter of the largest dimension of each grain, which equivalent circle diameter corresponds to the grain size of each grain.
In addition, grain boundaries are identified by the SEM/EBSD technique, on the basis of misorientations between adjacent pixels in the of the sheet which is imparted with tensile deformation at a strain of 5% or 15% in the sheet rolling direction. Grain boundaries with a tilt angle of 2.0° to 15° are defined as small angle grain boundaries, and grain boundaries with a tilt angle of greater than 15° are defined as large angle grain boundaries. The proportion of the small angle grain boundaries can be considered as the amount of very small misorientations in grains, namely, the amount of strain accumulated in the grains.
Specifically, the proportion is measured in the following manner. Initially, JIS No. 13A test specimens (20 mm by 80 mm gauge length by sheet thickness) as test samples are sampled from a old-rolled sheet after heat treatments, where the test specimens are sampled according to the procedure of the tensile test. The test specimens are subjected to the tensile test at loom temperature so that the test specimens are pulled in the sheet rolling direction. This simulates the dislocation density of the sheet in the low strain region imparted in actual forming into an automotive body panel Thus, tensile deformation at a strain of 5% corresponding to the low strain region and tensile deformation at a strain of 15% corresponding to the high strain region are independently imparted to the sheet.
The microstructure of the test specimen at a sheet thickness central position, where the test specimen has been imparted with tensile deformation at a strain of 5% or 15%, is analyzed by the SEM/EBSD technique to define or identify the grain boundaries in a texture of the sheet surface. On the basis of this, the length of large angle grain boundaries and the length of small angle grain boundaries are determined, and the proportion of small angle grain boundaries can be calculated according to the expression:
(Length of grain boundaries with a tilt angle of 2.0° to 15°)/(Length of grain boundaries with a tilt angle of 2.0° to 180°)×100.
The measurement of the average grain size may be performed without imparting the tensile deformation. Specifically, a sample may be prepared by mechanically polishing a cold-rolled sheet after heat treatments (temper) in the transverse direction, and subjecting the same sequentially to buffing and electropolishing so that a sheet thickness central position is exposed as an observation plane from the surface. The sample is then subjected to grain size measurement by EBSD technique using an FESEM.
Independently, the test specimen to which the tensile deformation has been imparted is also subjected to the surface treatment as in the measurement of average grain size, so that a sheet thickness central position is exposed as an observation plane from the surface. The test specimen is subjected to the crystal orientation measurement by the EBSD technique.
These EBSD measurements and analyses may be performed using EBSD System OIM (supplied by EDAX).
These operations are performed on two view fields. This gives the average grain sire and the average proportion of small angle grain boundaries after application of tensile deformation at a strain of 5% or 15%, as specified in the present invention. The average proportions of small angle grain boundaries correspond respectively to the amounts of strain accumulated in grains in the low strain region and in the high strain region. In the present invention, this is referred to as “proportion of small angle grain boundaries as measured by SEM/EBSD technique.”
Indices for Good Work Hardening Properties (High Formability)
The controls on chemical compositions and microstructures as above allow the sheet to have good work hardening properties (high formability). Whether the sheet has such good properties may be indicated (roughly estimated) typically by yield ratio and elongation.
A low yield ratio and a high elongation, when the sheet has both simultaneously, support such high formability for automotive body panel use, without performing a forming test on a small test specimen sampled from the sheet, or without performing a test of actually forming the sheet into an automotive body panel.
Specifically, as indices (rough estimates) for achieving such high formability, the sheet preferably has a yield ratio of 0.56 or less and a total elongation of 26% or more, as supported by the after-mentioned examples (experimental examples). The yield ratio herein is defined as the ratio of the 0.2% yield strength to the tensile strength of the aluminum alloy (sheet).
The aluminum alloy sheet, if having an excessively high yield ratio of greater than 0.56 and/or if having an excessively low total elongation of less than 26%, may fail to have good work hardening properties and high formability for automotive body panel use.
Production Method
Next, a method for producing the aluminum alloy sheet according to the present invention will be illustrated below. The aluminum alloy sheet according to the present invention may be produced by a production process, by itself according to a common procedure or known procedure. Specifically, the aluminum alloy sheet may be produced by preparing an aluminum alloy ingot having the 6xxx-series chemical composition by casting, subjecting the ingot sequentially to homogenization (soaking), hot rolling, and cold rolling into a sheet having a predetermined sheet thickness, and further subjecting the sheet to heat treatment or treatments such as solution treatment and quenching.
However, the production process is preferably performed under conditions all within preferred ranges as mentioned below, where the conditions include homogenization conditions, hot rolling conditions, cold rolling conditions, and solution treatment and quenching conditions. This is preferred for surely and reproducibly offering the microstructure (average grain size and proportion of small angle grain boundaries upon tensile deformation) as specified in the present invention. The aluminum alloy sheet, if produced under any of these conditions out of the ranges, may highly possibly fail to have the microstructure specified in the present invention.
Melting and Casting—Cooling Rate
Initially, in the melting and casting process, a molten aluminum alloy melted and adjusted so as to have a chemical composition within the 6xxx-series chemical composition range is cast. The casting technique is selected as appropriate flow common melting-casting techniques such as continuous casting and semicontinuous casting (direct chill (DC) casting). In this process, cooling in casting in the range of from the liquidus temperature to the solidus temperature is preferably performed at an average cooling rate as high as possible of WC/min or more (the cooling in this temperature range is preferably performed as rapidly as possible). This is preferred to control the (average grain sin. and proportion of small angle grain boundaries upon tensile deformation) within the ranges specified in the present invention.
Homogenization
Next, the aluminum alloy ingot after casting is subjected to homogenization prior to hot rolling. The homogenization is important so as not only to offer the common object, homogenization of the microstructure (elimination or minimization of segregation in grains in the ingot microstructure), but allow Si and Mg to be dissolved sufficiently.
Hot rolling is performed after the homogenization. Herein, the ingot is preferably maintained in temperature not to be 500° C. or lower after the homogenization and before the start of hot rough rolling, so as to ensure the amounts (dissolved amounts) of solute Mg and solute Si.
The ingot, if having a lower temperature of 500° C. or lower before the start of hot rough rolling, may become susceptible to precipitation of coarse Al—Fe and Mg—Si compounds and may fail to have uniform grain size distribution and/or uniform accumulated strain distribution. This may cause the resulting sheet to have inferior formability with higher possibility. In addition, Si and Mg, when contained in such compounds, may be dissolved as solutes in smaller concentrations. This may cause the sheet to have inferior formability because of insufficient effects of the solute Si and solute Mg.
Hot Rolling
The hot rolling includes a rough rolling process and a finish rolling process of the ingot, according to the target sheet thickness after rolling. These rough rolling process and finish rolling process may be performed using a rolling mill such as reverse mill or tandem mill selected as appropriate.
Rolling of the work from the start to the end of hot rough preferably performed so that the temperature of the work is not lowered to 450° C. or lower. This is preferred to ensure the amounts of solute Si and solute Mg. The rough rolled sheet between passes, if having a minimum temperature of 450° C. or lower due typically to a long rolling time, may often suffer from precipitation of Mg—Si compounds and may fail to have uniform grain size distribution and uniform accumulated strain distribution. This may highly possibly cause the sheet to have inferior formability. In addition, Si and Mg, when contained in such compounds, may be dissolved as solutes in smaller concentrations. This may cause insufficient effects of the solute Si and solute Mg.
After the hot rough rolling, the work is subjected to hot finish rolling preferably at an end temperature of 300° C. to 360° C. The hot finish rolling, if performed at an excessively low end temperature of lower than 300° C., may cause inferior productivity because of high rolling load. In contrast, assume that the hot finish rolling is performed at a high set end temperature of higher than 360° C. so as to allow the microstructure to be a recrystallized structure with deformed microstructure remained in a small amount. In this case, grains at this stage may coarsen.
In a temperature range from the material (sheet) temperature immediately after the completion of hot finish rolling down to 100° C., cooling is preferably performed at an average cooling rate of 5° C./hr or more. The cooling, if performed at an average cooling rate of less than 5° C./hr, may cause Mg—Si precipitates to form in a larger amount during the cooling and may cause the sheet to fail to have uniform grain sig distribution and uniform accumulated strain distribution. This may more possibly cause the sheet to have inferior formability. In addition, Si and Mg, when contained in such compounds, may be dissolved as solutes in smaller concentrations. This may cause the sheet to have inferior formability because of insufficient effects of the solute Si and solute Mg.
Cold Rolling
Next, the hot rolled sheet is subjected sequentially to primary cold rolling, process annealing (intermediate annealing), and secondary cold rolling to give a cold-rolled sheet (including one in the form of a coil) having a desired final sheet thickness. The cold rolling process is preferably performed so that the ratio of B to A is 0.7 or more, where A represents the rolling reduction of the primary cold rolling before the process annealing; and B represents the rolling reduction of the secondary cold rolling after the process annealing. This is preferred for refinement of the microstructure in final recrystallization (recrystallization by solution treatment after the cold rolling). The cold rolling process, if performed at a ratio of B to A of less than 0.7, may cause the recrystallized structure formed by the solution treatment to coarsen and to fail to give a fine microstructure. This may cause the sheet to have inferior formability.
The secondary cold rolling after the process annealing is preferably performed at a rolling reduction (B) of 30% or more. This is preferred for stabilizing the crystal microstructure during recrystallization. The secondary cold rolling, if performed at a of less than 30%, may cause recrystallized grains to coarsen or fail to be in the solution treatment. This may cause the sheet to have inferior formability.
The secondary cold rolling herein is preferably performed at a minimum rolling reduction per one rolling (one pass) of 20% or more. The secondary cold rolling, if performed at a minimum rolling reduction per one rolling of less than 20%, may cause strain to penetrate the sheet to a smaller depth and may give smaller strain to the sheet thickness central part This may cause strain introduced by the grains to differ in amount from a region to another, thereby impede the sheet from having a uniform microstructure, and cause the sheet to have inferior formability.
The cold rolling in the present invention preferably includes a process annealing between the primary cold rolling and the secondary cold rolling. The process annealing is the step of holding the work at 350° C. to 450° C. for 1 to 24 hours. The process annealing is performed so as to reduce heterogeneous working strain in the rolled material.
Assume that the process annealing is performed using a batch furnace. In this cam, the process annealing, if performed at a temperature of lower than 350° C. and/or for a holding time of shorter than one hour may fail to offer the effects sufficiently. The process annealing, if performed at a temperature of higher than 450° C., may cause the sheet to have a heterogeneous microstructure with a larger variation in grain size after the solution treatment The process annealing, if performed for an annealing time (holding time) of longer than 24 hours, may cause lower productivity and economical inefficiency.
Assume that the process annealing is performed using a continuous annealing furnace. In this case, the process annealing is preferably performed by holding the work at a temperature in the range of 450° C. to 550° C. for 5 minutes or shorter. The process annealing in this case, if performed at an annealing temperature of lower than 450° C., may fail to give sufficient annealing effects. The process annealing, if performed at an annealing temperature of higher than 550° C., may cause recrystallized grains to coarsen and may cause the sheet to have inferior formability. The process annealing, if performed by holding the work for a time longer than 5 minutes, may cause recrystallized grains to coarsen and may cause the sheet to have inferior formability.
Solution Treatment and Quenching
After the cold rolling, the work is subjected to solution treatment and subsequent quenching down to room temperature. The solution treatment and quenching may be performed using a common continuous heat treatment line.
However, the solution treatment and quenching are preferably performed by holding the work at a solution treatment temperature of from 500° C. to the melting temperature for 10 seconds longer, and cooling the work at an average cooling rate of 30° C./sec or more in the temperature range from the holding temperature down to 100° C. This is preferred so as to allow elements such as Mg and Si to be dissolved as solutes in sufficient amounts. The solution treatment, if performed at a temperature lower than 500° C. and/or for a holding time shorter than 10 seconds, may cause reversion of Al—Fe compounds and Mg—Si compounds, which have been formed before the solution treatment. This may cause the sheet to receive insufficient effects by the solute Si and solute Mg and to have inferior formability, because of a smaller amount of the solute Si.
The quenching (cooling), if performed at an average cooling rate of less than 30° C./sec, may cause recrystallized grains to coarsen in size during cooling and simultaneously cause the solute Si and solute Mg to be dissolved in smaller amounts. This may cause the sheet to receive insufficient effects by the solute Si and solute Mg and to have inferior formability more possibly. To ensure the cooling rate as above, the quenching may be performed while selecting a cooling process or device typically from air cooling such as fan cooling, and water cooling such as mist cooling, spraying, and immersion; and selecting cooling conditions.
Pre-Aging: Reheat Treatment
After the solution treatment and quenching as above, the weak may be selectively subjected to pre-aging, where necessary typically for better bake hardenability.
The pre-aging (reheat treatment), when to be performed, is preferably performed within one hour after the quenching down to room temperature. The holding of the work between the completion of the quenching down to mom temperature and the pre-aging start (heating start), if performed for an excessively long time, may cause Mg—Si clusters that do not contribute to bake hardenability to form as a result of natural aging at room temperature and may impede increase in amount of Mg—Si clusters that contain Mg and Si in good balance and contribute to bake hardenability. Accordingly, the holding time at room temperature is preferably minimized, and the lower limit of the holding time is not specified. For example, the reheat treatment may be performed approximately subsequently to the solution treatment and quenching with approximately no delay.
The pre-aging is preferably performed by holding the work at a temperature of 60° C. to 120° C. for a holding time of 10 hours to 40 hours. This allows the Mg—Si clusters containing Mg and Si in good balance to form.
The present invention will be illustrated in further detail with reference to several examples (experimental examples) below. It should be noted, however, that the examples are by no means intended to limit the scope of the present invention; that various changes and modifications can naturally be made therein without deviating from the spirit and save of the present invention as described herein; and that all such changes and modifications should be considered to be within the scope of the present invention.

EXAMPLES

Next, the present invention will be illustrated with reference to the examples below. Under different production conditions, 6xxx-series aluminum alloy sheets having different chemical compositions as given in Table 1 and having different microstructures as presented in Table 3 were independently produced. Specifically, the 6xxx-series aluminum alloy sheets differ sum each other in chemical compositions and/or in microstructure conditions including average grain size and proportion of small angle grain boundaries after application of tensile deformation at a strain of 5% or 15%.
The produced sheets were held at room temperature for 10 days after production (underwent natural aging at room temperature) and were then subjected to measurements and evaluations on average grain size, proportion of small angle grain boundaries after application of tensile deformation at a strain of 5% or 15%, 0.2% yield strength, tensile strength, yield ratio (ratio of the 0.2% yield strength to the tensile strength), and total elongation. Results of the measurements and evaluations are presented in Table 3. Tables 2 and 3 are continued from Table 1; and the alloy numbers in Table 1 respectively correspond to and are identical to the numbers in Tables 2 and 3.
Specifically, the different aluminum alloy sheets were produced by producing 6xxx-series aluminum alloy sheets having chemical compositions given in Table 1 under different production conditions as given in Table 2. The production conditions include the minimum temperature of a rough rolled sheet between passes of hot rough rolling (presented as “minimum temperature” in Table 2), the end temperature of hot finish rolling, the rolling reduction in cold rolling the minimum rolling reduction per one pass of cold rolling, the process annealing temperature, the solution treatment holding temperature, and the average cooling rate.
A blank in an element content in Table 1 indicates that the content of the element in question is equal to or lower than the detection limit.
Specifically, the aluminum alloy sheets were produced under conditions as follows. Aluminum alloy ingots having the chemical compositions given in Table 1 were made through melting and direct chill (DC) casting. Cooling in casting was performed at an average cooling rate of 50° C./min in a temperature range from the liquidus temperature to the solidus temperature, in common in each sample.
Subsequently, the ingots were subjected to homogenization at 550° C. for six hours in common in each sample, at which temperature hot rough rolling was then started while the ingots were held so that they did not have a temperature of 500° C. or lower during between the homogenization and the hot rough rolling start. The minimum temperatures (lowest pass temperatures) in the hot rough rolling are presented in Table 2. The works were subsequently subjected to hot finish rolling to a thickness of 3.5 mm in common in each sample at the end temperatures given in Table 2 and yielded hot-rolled sheets.
The aluminum alloy sheets after hot rolling were subjected to a heat treatment at 500° C. for one minute and subjected sequentially to primary cold annealing and secondary cold rolling under the conditions given in Table 2, and yielded cold-rolled sheets having a thickness of 10 mm in common in each sample. The process annealing was performed using a batch furnace in each sample.
The cold-rolled sheets were each subjected to heat treatments (temper treatments; T4) continuously while being recoiled and coiled in a continuous heat treatment system in common in each sample. Specifically, the works were each subjected to a solution treatment, in which the works were heated up to the target temperatures (holding temperatures) given in Table 2 at an average heating rate in a temperature range of up to 500° C. of 50° C./sec in common in each sample, and held at the holding temperatures for 20 seconds in common in each sample. The works were then cooled down to room temperature by water cooling at the average cooling rates (° C./sec) given in Table 2.
After these heat treatments, the works were left stand at mom temperature for 10 days and yielded final product sheets. Test samples (blanks) were cut out of the final product sheets and each subjected to measurements and evaluations on the microstructure as specified by the average grain size and the proportion of small angle grain boundaries after application of tensile deformation at a strain of 5% and 15% and on the mechanical properties. Results of them are presented in Table 3.
Measurements of Average Grain Size and Average Proportion of Small Angle Grain Boundaries
The average grain size and the average proportion of grain boundaries of a test specimen after the solution treatment were measured on a microstructure of the sheet in the transverse direction by the above-mentioned measurement method. The average grain size (μm) and the average proportion (%) of grain boundaries of the microstructure were measured using the SEM JEOL 7100 (supplied by JEOL Ltd.) equipped with the EBSD Measurement/Analysis System OIM (supplied by EDAX).
In each sample, two test specimens were sampled from any points in the sheet transverse direction and independently subjected to the measurement, as described above, and the two measurements were averaged. The measurement in each test specimen was performed in a measurement region of 300 by 300 μm at the center of a cross section in parallel with the sheet thickness direction and with the sheet transverse direction at measurement step intervals of 1 μm in common.
Tensile Test
The tensile test of each test sample was performed in the following manner. Tensile test specimens (20 mm by 80 mm gauge length by sheet thickness) of JIS No. 13A were sampled from the test sample and subjected to a tensile test at room temperature. In the tensile test, the test specimens were pulled in the sheet rolling direction at a tensile speed of 5 mm/min, the mechanical properties were measured on two test specimens (N=2), and the two measurements were averaged. Thus, the 0.2% yield strength, tensile strength, yield ratio (0.2% yield strength to tensile strength ratio), and total elongation were calculated on each sample.
As presented in Tables 1 and 2, Examples 1 to 12 have chemical compositions within the ranges specified in the present invention and are produced under conditions all within the preferred production condition ranges.
Accordingly, these examples each have an average grain size of 40 μm or less and have proportions of small angle grain boundaries with a tilt angle of 2.0° to 15.0° of 12% to 30% after application of tensile deformation at a strain of 5% in the sheet rolling direction and of 50% to 70% after application of tensile deformation at a strain of 15% in the sheet rolling direction, as presented in Table 3, where these factors are measured by the SEM/EBSD technique and fall within the ranges as specified in the present invention.
As a result, the examples each have a yield ratio of 0.56 or less and a total elongation of 26% or more, as presented in Table 3, even after natural aging at room temperature and have such high formability as to be acceptable for automotive body panel use, where the yield ratio is defined as the ratio of the 0.2% yield strength to the tensile strength
In contrast, Comparative Examples 13 to 17 are produced under conditions within the preferred condition ranges as presented in Table 2, but employ Alloys Nos. 13 to 17 in Table 1 and have Si, Mg, Cu, Mn, and Fe contents, one or more of which contents are out of the range specified in the present invention.
Accordingly, these comparative examples each have an average grain size and proportions of small angle grain boundaries after application of tensile deformation strain, either one or both of which parameter are out of the range specified in the present invention, and have a yield ratio of greater than 0.56 and/or a total elongation of less than 26%, as presented in Table 3. This demonstrates that the comparative examples have inferior formability as compared with the examples.
The comparative examples are therefore rejected for automotive body panel use.
Comparative Example 13 is derived from Alloy No. 13 in Table 1 and has an excessively low Mg content.
Comparative Example 14 is derived from Alloy No. 14 in Table 1 and has an excessively law Si content.
Comparative Example 15 is derived from Alloy No. 15 in Table 1 and has an excessively low Cu content.
Comparative Example 16 is derived from Alloy No. 16 in Table 1 and has an excessively high Mn content.
Comparative Example 17 is derived from Alloy No. 17 in Table 1 and has an excessively high Fe content.
Comparative Examples 18 to 26 employ alloys having chemical compositions within the ranges specified in the present invention, as presented in Table 1. These comparative examples, however, are produced under conditions, any one or more of which are out of the ranges specified in the present invention, as presented in Table 2. The production conditions include the minimum temperature in hot rough rolling, the end temperature of hot finish rolling, the rolling reductions in primary cold rolling and secondary cold rolling, the minimum rolling reduction per one rolling of the secondary cold rolling, the holding temperature and time in process annealing, and the holding temperature and average ling rate (° C./sec) in solution treatment.
As a result, the comparative examples have properties such as average grain sin. and average proportion of small angle grain boundaries in the low strain region or in the high strain region, any one or more of which properties are out of the range specified in the present invention. Thus, the comparative examples have a yield ratio of greater than 0.56 and/or a total elongation of less than 26% as presented in Table 3 and are inferior as compared with the examples. The comparative examples are therefore rejected for automotive body panel use.
Among them, Comparative Example 18 underwent hot rough rolling at an excessively low minimum temperature. This sample has an average grain size within the specified range, but has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions, due to a heterogeneous microstructure and smaller amounts of solute Mg and solute Si. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 19 underwent hot rough rolling at an excessively low minimum temperature and hot finish rolling at an excessively high end temperature. The sample has an average grain size of greater than the specified level and has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 20 underwent process annealing at an excessively high temperature, where the process annealing is performed after primary cold rolling. The sample has an average grain size meeting the specified condition, but has an excessively low average proportion of small angle grain boundaries in the low strain region. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 21 underwent process annealing for an excessively short time, where the process annealing is performed after primary cold rolling. The process annealing therefore fails to fully remove working strain, and this causes the sample to have an excessively high average proportion of small angle grain boundaries in the high strain region, although the sample has an average grain size meeting the specified condition. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 22 underwent secondary cold rolling at an excessively low rolling reduction. The sample has an average grain size of greater than the specified level and has excessively low proportions of small angle grain boundaries both in the low strain and high strain regions. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 23 underwent secondary cold rolling at an excessively low minimum rolling reduction per one rolling. The sample has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions, although the sample has an average grain size meeting the specified condition. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 24 underwent cold rolling at an excessively low ratio (B/A) of the secondary cold rolling reduction (B) to the primary cold rolling reduction (A). The sample has an average grain size of greater than the specified level, and has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and of inferior formability.
Comparative Example 25 is produced under conditions within the preferred production condition ranges, except for an excessively low solution treatment temperature. The sample has an average grain size meeting the specified condition, but has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions, due to smaller amounts of solute Si and solute Mg. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
Comparative Example 26 is produced under conditions within the preferred production condition ranges, except for an excessively low cooling rate after solution treatment. The sample has an average grain size of greater than the specified level, and has excessively low average proportions of small angle grain boundaries both in the low strain and high strain regions. The sample therefore has a yield ratio of greater than 0.56 and a total elongation of less than 26% and offers inferior formability.
The results of the experimental examples support the significance of meeting all the conditions on chemical compositions and microstructures specified in the present invention, so as to give 6xxx-series aluminum alloy sheets having high formability for automotive body panel use, without significant changes in conventional chemical mm positions and production conditions.

	TABLE 1

	Aluminum alloy sheet chemical composition (in mass percent, the remainder being Al)

Claims

1. An aluminum alloy sheet having high formability, the aluminum alloy sheet being an Al—Mg—Si aluminum alloy sheet comprising, in mass percent:

Si in a content of 0.30% to 2.0%;

Mg in a content of 0.20% to 1.5%;

Cu in a content of 0.05% to 1.0%;

Mn in a content of greater than 0% to 1.0%; and

Fe in a content of greater than 0% to 1.0%,

with the remainder consisting of Al and inevitable impurities,

the aluminum alloy sheet having a microstructure at a sheet thickness central position,

the microstructure having an average grain size of 40 μm or less,

the microstructure comprising small angle grain boundaries with a tilt angle of 2.0° to 15.0° in an average proportion of 12% to 30% after tensile deformation at a strain of 5% being imparted in a rolling direction of the sheet; and

the microstructure comprising small angle grain boundaries with a tilt angle of 2.0° to 15.0° in an average proportion of 50% to 70% after tensile deformation at a strain of 15% being imparted in the rolling direction of the sheet.

s measured by SEM/EBSD technique.

2. The aluminum alloy sheet according to claim 1, further comprising at least one element selected from the group consisting of:

Cr in a content of greater than 0% to 0.3%;

Zr in a content of greater than 0% to 0.3%;

V in a content of greater than 0% to 0.3%;

Ti in a content of greater than 0% to 0.1%;

Zn in a content of greater than 0% to 1.0%;

Ag in a content of greater than 0% to 0.2%; and

Sn in a content of greater than 0% to 0.15%.

3. The aluminum alloy sheet according to claim 1,

wherein the aluminum alloy sheet has a 0.2% yield strength, a tensile strength and has a yield ratio of 0.56 or less, where the yield ratio is defined as a ratio of the 0.2% yield strength to the tensile strength, and

wherein the aluminum alloy sheet has a total elongation of 26% or more.