US20180282847A1

US20180282847A1 - Aluminum alloy sheet for forming and automobile member

Info

Publication number: US20180282847A1
Application number: US15/928,150
Authority: US
Inventors: Kazunori Kobayashi; Tomoko Abe
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2017-03-29
Filing date: 2018-03-22
Publication date: 2018-10-04

Abstract

Provided is a 5xxx aluminum alloy sheet for forming, which sheet has low elongation anisotropy (in-plane anisotropy) and excellent press formability while having strength maintained at high level. This aluminum alloy sheet is an Al—Mg alloy sheet having a specific chemical composition. In the texture of the sheet, the orientation densities of Brass orientation and S orientation relative to a random orientation are controlled and minimized, where the orientation densities are expressed by orientation distribution functions. This allows the aluminum alloy sheet to have lower elongation anisotropy to thereby have better press formability.

Description

FIELD OF INVENTION

The present invention relates to a 5xxx aluminum alloy sheet (rolled sheet) for forming, which has satisfactorily low elongation anisotropy.

BACKGROUND OF INVENTION

Demands have been made to provide better fuel efficiency of transportation equipment such as automobiles, so as to cope with global environmental issues caused by emission gas from such equipment. To meet these demands, aluminum alloy materials are more and more applied, in particular, to automobile members, instead of conventionally-used steels, because the aluminum alloys have lighter weights as compared with the steels. Representative, but non-limiting examples of automobile members made from aluminum alloy sheets include thin panels (body panels) including outer panels (exterior trim parts) such as hoods, fenders, doors, and roofs; and inner panels (interior parts), which are made from (formed from) material sheets by press forming.
However, such aluminum alloy sheets, although having strength at the same level with steel sheets, have inferior press formability to the steel sheets. Thus, strong demands have been made to allow the aluminum sheets to have better press formability.
Among aluminum alloys, 5xxx aluminum alloys such as Al—Mg JIS 5052 alloy and JIS 5182 alloy have been used as aluminum alloys having relatively excellent press formability. In addition, various techniques have been proposed to allow these 5xxx aluminum alloys to have better formability by metallurgical means such as adjustments of chemical composition, average grain size, and/or texture.
Among these metallurgical means, various proposals have been made on the texture. For example, Japanese Patent No. 4326009, which is a divisional application of Japanese Patent Application No. Hei11(1999)-239550, proposes an Al—Mg alloy sheet having a texture offering excellent deep drawability. This Al—Mg alloy sheet has a specified chemical composition, having a texture including grains with a volume fraction of 30% to 50% in the Cube orientation and a volume fraction of 10% to 20% in the Brass orientation, in which the grain size is from 50 to 100 μm.
Likewise, Japanese Patent No. 4339869, which is a divisional application of Japanese Patent Application No. Hei11(1999)-239550, proposes an Al—Mg alloy sheet having excellent deep drawability. This Al—Mg alloy sheet has a specified chemical composition, includes a texture with a ratio (S/Cube) of the volume fraction in the S orientation to the volume fraction in the Cube orientation of 1 or more and a volume fraction in the GOSS orientation of 5% or less, in which the grain size is from 20 to 100 μm.

SUMMARY OF INVENTION

Some sheets (rolled sheets) are used as forming materials to be subjected particularly to press forming to form the inner panels and other products having complicated shapes with high (deep) side walls. These sheets require low elongation anisotropy (in-plane anisotropy), in addition to good formability evaluated by common evaluations such as evaluation by Erichsen value. The elongation anisotropy is determined as a difference in elongation (total elongation) between a direction (0° direction) parallel to the sheet rolling direction and a direction (90° direction) perpendicular to the sheet rolling direction.
However, the aluminum alloy sheets according to the techniques disclosed in Japanese Patent No. 4326009 and Japanese Patent No. 4339869, which are intended to give better deep drawability by the action of the specific textures, and the conventional 5xxx aluminum alloy sheets regarding their textures are still susceptible to improvements particularly in the elongation anisotropy.
The present invention has been made while focusing on these circumstances and has an object to provide a 5xxx aluminum alloy sheet for forming, which has low elongation anisotropy (in-plane anisotropy) and offers excellent press formability while having strength maintained at high level.

Solution to Problem

To achieve the object, the present invention provides, according to one embodiment, an aluminum alloy sheet for forming as follows. This aluminum alloy sheet is an Al—Mg alloy sheet containing Mg in a content of 3.5 to 5.5 mass %, Mn in a content of 0.03 to 0.60 mass %, Cu in a content of 0.001 to 0.50 mass %, and Zn in a content of 0.001 to 0.50 mass %, with the remainder consisting of Al and unavoidable impurities. In a texture of the aluminum alloy sheet in a plane parallel to the sheet surface, the orientation densities of Brass orientation and S orientation relative to a random orientation of each less than 5, where the plane is positioned at a depth half the thickness of the aluminum alloy sheet, and where the orientation densities are measured by SEM-EBSD analysis and expressed by orientation distribution functions (ODFs).
The inventors of the present invention made investigations anew on the relationship between the texture and the elongation anisotropy (in-plane anisotropy) of a 5xxx aluminum alloy sheet, and found that specific crystal orientations in the texture significantly affect the elongation anisotropy. The specific crystal orientations are two orientations, i.e., the Brass orientation and the S orientation. The inventors further found that the aluminum alloy sheet can have controlled, lower elongation anisotropy while having strength maintained at high level, by controlling and minimizing the orientation densities expressed by ODFs of the two crystal orientations as low as possible, in other words, the elongation anisotropy can be minimized with decreasing orientation densities expressed by ODFs of the two crystal orientations.
This configuration according to the embodiment of the present invention allows the aluminum alloy sheet to have significantly better press formability into, in particular, the inner panels and other products which have complicated shapes and have high (deep) side walls.

DESCRIPTION OF EMBODIMENTS

The aluminum alloy sheet according to the embodiment of the present invention will be described in detail below on a condition-to-condition basis.
Chemical Composition
Initially, the chemical composition of the Al—Mg 5xxx aluminum alloy sheet according to the embodiment of the present invention will be described below, on the precondition that the aluminum alloy sheet has formability and strength at satisfactory levels necessary for use in forming into the automobile members such as inner panels, and is produced through rolling under conditions without significant changes.
The 5xxx aluminum alloy sheet as above contains, in its chemical composition, Mg in a content of 3.5 to 5.5 mass %, Mn in a content of 0.03 to 0.60 mass %, Cu in a content of 0.001 to 0,50 mass %, and Zn in a content of 0.001 to 0.50 mass %, with the remainder consisting of Al and unavoidable impurities.
Next, the ranges, significance, and permissible levels of the contents of the elements in the 5xxx aluminum alloy sheet will be described below. All % ages as the element contents are in mass %.
Ma Content: 3.5 to 5.5 Mass %
Magnesium (Mg) is dissolved (solid-solutionized) in the matrix, thereby allows the aluminum alloy sheet to have better work hardenability and to surely offer strength and durability necessary as an aluminum alloy material sheet for forming, and is essential. The aluminum alloy sheet, when having a Mg content of 3.5 mass % or more, can enjoy the operation and effects at high levels and can resist reduction in press formability. From the viewpoint of offering the effects at better levels, the Mg content is preferably 4.0 mass % or more, and more preferably 4.5 mass % or more. On the other hand, the aluminum alloy sheet, when having a Mg content of 5.5 mass % or less, can prevent sheet production issues, such as cracking during hot rolling (reduction in deformability at high temperatures), and prevent reduction in resistance to use environment, such as stress corrosion cracking resistance. From the viewpoint of offering the effects at better levels, the Mg content is preferably 5.3 mass % or less, and more preferably 5.0 mass % or less.
Mn Content: 0.03 to 0.60 Mass %
Manganese (Mn) effectively allows the aluminum alloy sheet to have higher strength by solid-solution strengthening and to have better press formability by refinement of grain microstructures, and is essential. The aluminum alloy sheet, when having a Mn content of 0.03 mass % or more, can enjoy the operation and effects at high levels and can meet the condition for the after-mentioned performance of 0.2% proof stress of 100 MPa or more. From the viewpoint of offering the effects at better levels, the Mn content is preferably 0.20 mass % or more, and more preferably 0.30 mass % or more. On the other hand, the aluminum alloy sheet, when having a Mn content of 0.60 mass % or less, may include smaller amounts of coarse particles and precipitates containing this element and less undergoes reduction in press formability. From the viewpoint of offering the effects at better levels, the Mn content is preferably 0.50 mass % or less, and more preferably 0.40 mass % or less.
Cu Content: 0.001 to 0.50 Mass %
Copper (Cu) effectively allows the aluminum alloy sheet to have higher strength by solid-solution strengthening and is essential. The aluminum alloy sheet, when having a Cu content of 0.001 mass % or more, tends to enjoy the effect satisfactorily. From the viewpoint of offering the effect at better level, the Cu content is preferably 0.01 mass % or more, more preferably 0.10 mass % or more, and furthermore preferably 0.20 mass % or more. On the other hand, the aluminum alloy sheet, when having a Cu content of 0.50 mass % or less, is advantageous also in view of production, namely, does not suffer from reduction in slab castability, such as cracking in the slab, and less fails to obtain problem-free slabs to be subjected typically to rolling. From the viewpoint of offering the effects at better levels, the Cu content is preferably 0.40 mass % or less, and more preferably 0.30 mass % or less.
Zn Content: 0.001 to 0.50 Mass %
Zinc (Zn) effectively allows the aluminum alloy sheet to have higher strength by solid-solution strengthening and to offer better press formability, and is essential. The aluminum alloy sheet, when having a Zn content of 0.001 mass % or more, can enjoy these effects sufficiently. From the viewpoint of offering the effects at better levels, the Zn content is preferably 0.01 mass % or more, and more preferably 0.10 mass % or more. On the other hand, the aluminum alloy sheet, when having a Zn content of 0.50 mass % or less, less undergoes a significant age hardening, in which the sheet has increasing strength with the elapse of time after sheet production, and can less suffer from reduction in press formability. From the viewpoint of offering the effects at better levels, the Zn content is preferably 0.40 mass % or less, and more preferably 0.30 mass % or less.
Unavoidable Impurities
The aluminum alloy for use in the embodiment may contain, as unavoidable impurities, elements other than above, as a result of selection of raw materials to be subjected to melting to make ingots. The contents of unavoidable impurity elements other than the above-described elements are controlled within ranges as prescribed for 5xxx alloys typically by Aluminum Association (AA) Standards or Japanese industrial Standards (JIS). Specifically, non-limiting examples of the unavoidable impurity elements include Fe, Si, Cr, Ti, Zr, V, Ni, Sn, In, Ga, B, and Sc. Controls are performed so that, among these elements, the Fe content is 0.7 mass % or less, the Si content is 0.4 mass % or less, the Cr content is 0.3 mass % or less, and the Ti content is 0.3 mass % or less; and so that the contents of elements other than Fe, Si, Cr, and Ti are each 0.05 mass % or less and are in total of 0.15 mass % or less. These elements, when contained in contents within the ranges, do not adversely affect advantageous effects of the embodiment, not only when contained as unavoidable impurities, but also when added actively.
Texture
On the precondition that the 5xxx aluminum alloy sheet has a chemical composition within the range, the embodiment allows the aluminum alloy sheet to have lower elongation anisotropy, by controlling and minimizing the orientation densities of the specific crystal orientations in the texture of the aluminum alloy sheet, where the specific crystal orientations significantly affect the elongation anisotropy, and where the orientation densities are expressed by ODFs. The specific crystal orientations are two crystal orientations, i.e., the Brass orientation and the S orientation.
The aluminum alloy sheet can have lower elongation anisotropy while having strength maintained at high level, by minimizing the orientation densities, as expressed by ODFs, of these two crystal orientations. In other words, the aluminum alloy sheet can have lower elongation anisotropy with decreasing orientation densities, as expressed by ODFs, of the two crystal orientations. The control to have lower elongation anisotropy as above allows the 5xxx aluminum alloy sheet to have significantly better press formability, in particular, into the automobile panels such as inner panels, which have complicated product shapes and have high (deep) side walls, while the aluminum alloy sheet has strength maintained at high level.
Specifically, the orientation densities of the Brass orientation and the S orientation relative to a random orientation are controlled and minimized each to be less than 5, in a texture of the 5xxx aluminum alloy sheet in a plane parallel to the sheet surface, where the plane is positioned at a depth half the thickness of the aluminum alloy sheet, and where the orientation densities are determined by SEM-EBSD analysis and are expressed by orientation distribution functions (ODFs). Preferably, in the texture in the aluminum alloy sheet, the orientation densities of the Brass orientation and the S orientation relative to a random orientation are further minimized each to be less than 2, where the orientation densities are expressed by ODFs. As used herein, the term “orientation density expressed by ODF relative to a random orientation” refers to the distribution density of each crystal orientation, while the orientation density, as expressed by ODF, of a sample having a random texture is defined as 1. Also as used herein, the term “random orientation” is defined as a state where all orientations occur at an equal probability (completely randomized state).
This configuration allows the aluminum alloy sheet to have, as elongation anisotropy, a difference in elongation (total elongation) of 2% or less between a tensile direction (sheetrolling direction) parallel to the sheet rolling direction and a tensile direction perpendicular to the sheet rolling direction, when JIS No. 5 test specimens are sampled from the aluminum alloy sheet and are subjected to tensile tests at room temperature. Specifically, the aluminum alloy sheet can have a lowered elongation anisotropy of 2% or less, where the elongation anisotropy ΔEl is determined by the equation:
ΔEl=(Total elongation(%)in the sheet rolling direction)−(Total elongation(%)in a direction perpendicular to the sheet rolling direction)
As used herein, the term “room temperature” refers to a temperature of from 20° C. to 25° C.
While the Brass orientation and the S orientation significantly affect the elongation anisotropy, other orientations such as four orientations, i.e., the Cube orientation, the ND-rotated Cube orientation, the Cu orientation, and the Goss orientation are considered to affect the elongation anisotropy considerably. Thus, the orientation densities of such other orientations relative to a random orientation are preferably minimized to be less than 5, where the orientation densities are expressed by ODFs.
However, it may be impossible to reduce the orientation densities expressed by ODFs of the Brass orientation and the S orientation to 0 relative to a random orientation, because of production limitations of such 5xxx aluminum alloy sheets. This is also true for the other four crystal orientations, i.e., the Cube orientation, ND-rotated Cube orientation, Cu orientation, and Goss orientation.
In contrast, the aluminum alloy sheet, if having an orientation density expressed by ODF of 5 or more for at least one of the Brass orientation and the S orientation relative to a random orientation, makes little difference in texture from the conventional 5xxx aluminum alloy sheets and fails to have a low elongation anisotropy of 2% or less. This is the reason for which the conventional 5xxx aluminum alloy sheets fail to have low elongation anisotropy.
Texture Measurement Method
The crystal orientations specified in the embodiment as above are measured by a crystal orientation analysis technique using a field emission scanning electron microscope (FE-SEM) equipped with an electron back-scattered diffraction pattern (EBSD) analysis system. A test sample to be subjected to the measurement is sampled from the sheet at any position in a plane parallel to the sheet surface, where the plane is positioned at a depth half the sheet thickness. Of the test sample, a plane parallel to the sheet surface, where the plane is positioned at a depth half the sheet thickness, is prepared so as to be a measurement plane. On the measurement plane, the orientation densities (dimensionless) expressed by ODFs of the Cube orientation, ND-rotated Cube orientation, Brass orientation, Cu orientation, S orientation, and Goss orientation relative to a random orientation are measured by the SEM-EBSD analysis.
The SEM-EBSD analysis is widely used as a technique for measuring crystal orientations and textures. In the analysis, the sample of the aluminum alloy sheet is placed in the lens barrel of the FE-SEM, and electron beams are applied to the sample with scanning the sample surface at 1-μm intervals, while diffraction patterns of the resulting back-scattered electrons are captured into an EBSD analysis system, and crystal orientations are analyzed, where the EBSD analysis system is exemplified typically by EBSD Measurement-Analysis System OIM (Orientation Imaging Microscopy) Data & Analysis, supplied by TSL. This gives an electron back-scattered diffraction pattern (EBSD pattern) at each point, and the obtained EBSD patterns are indexed to determine crystal orientations in the electron beam-applied region. The orientation distribution functions (ODFs) and area % ages are determined by calculation, on the basis of the resulting crystal orientation measurement data resulting from crystal orientation measurement of allover the measurement region by EBSD analysis. In addition, the analysis can also give the average grain size of the microstructure (crystal structure). The details of the crystal orientation analysis technique using an FE-SEM equipped with an EBSD analysis system can be found, in detail, typically in Research and Development, Kobe Steel Engineering Reports Vol. 52, No. 2 (September 2002), pp. 66-70.
Average Grain Size
In the embodiment, the average grain size of the 5xxx aluminum alloy sheet is not limited. However, the average grain size is preferably smaller, and more preferably, specifically, 100 μm or less, so as to allow the aluminum alloy sheet to have better ductility and lower elongation anisotropy. The lower limit of the average grain size is about 10 μm in consideration of sheet production limitations.
Strength (0.2% Proof Stress): 100 MPa or More
The aluminum alloy sheet according to the embodiment preferably has a 0.2% proof stress of 100 MPa or more. The aluminum alloy sheet, when having a 0.2% proof stress of 100 MPa or more, can surely have strength necessary as an alloy sheet for use in automobile members. Accordingly, the aluminum alloy sheet has a 0.2% proof stress of preferably 100 MPa or more, and particularly preferably 120 MPa or more.
The 0.2% proof stress can be controlled by the alloy element contents, and by the thermal hysteresis and rolling reduction in individual steps, among conditions for the steps of the production method mentioned below.
Aluminum Alloy Sheet Thickness
The thickness of the aluminum alloy sheet according to the embodiment is not limited, but is typically 0.5 mm or more to offer strength and rigidity necessary for automobile member use. On the other hand, the thickness is typically 6.0 mm or less in consideration of limitations in forming such as press forming, and in consideration of such an allowable range of weight increase as not adversely affects the effective weight reduction as compared with comparative (conventionally-used) steel materials.
Production Method
Next, a method for producing the aluminum alloy sheet according to the embodiment will be described below.
Melting and Casting
An aluminum alloy having a chemical composition within the range is melted to give a molten metal, from which an ingot having a predetermined shape is prepared. The way to melt and cast the aluminum alloy is not limited and may employ a common or known procedure.
Homogenization
Next, the aluminum alloy slab is subjected to homogenization (soaking) prior to hot rolling. The homogenization is performed so as to unifoimize or homogenize a heterogenous microstructure formed upon casting. The homogenization of the slab is performed at a soaking temperature (attaining temperature) of from 400° C. to lower than the aluminum alloy melting point. The homogenization, if performed at a soaking temperature lower than 400° C., may offer smaller effects; whereas heating up to a temperature equal to or higher than the melting point is unnecessary.
Hot Rolling
Subsequent to the homogenization, the work is subjected to hot rolling into a sheet having a predetermined thickness. The hot rolling is preferably started at a temperature of 350° C. or higher, because the hot rolling, if started at an excessively low temperature, may cause high resistance to deformation during rolling. The hot rolling is preferably performed so that the sheet during hot rolling passes through the temperature range (temperature region) of from 445° C. down to 400° C. within a short time of preferably 20 minutes or shorter, and more preferably 15 minutes or shorter. This is preferred so as to control the texture of the sheet to be in the range specified in the embodiment. The hot rolling, if performed so that the sheet passes through the temperature range (temperature region) of from 445° C. down to 400° C. over a long time of longer than 20 minutes, may cause at least one of the Brass orientation and S orientation to fail to be present in an orientation density, as expressed by ODF, of less than 5 relative to a random orientation, and may cause the aluminum alloy sheet to have higher elongation anisotropy. For offering preferred orientation densities expressed by ODFs, the time for the sheet to pass through the temperature range (temperature region) is preferably 2 minutes or longer.
Cold Rolling
The work after the hot rolling may be dealt as a product sheet having a predetermined thickness without being subjected to cold rolling. However, the work is preferably subjected to cold rolling into a product sheet having a predetermined thickness, so as to surely have surface quality, such as surface roughness, and flatness at satisfactory levels.
Final Annealing Process
The rolled sheet after the hot rolling or after the cold rolling is subjected to final annealing process. The term “final annealing” refers not to annealing performed selectively according to necessity before cold rolling or during a pass (or between passes) in the cold rolling, but to annealing finally performed on the product sheet. The final annealing may be performed either as batch annealing in a batch furnace, or continuous annealing in a continuous annealing furnace. In the continuous annealing, a heat treatment is performed at a high temperature for a short time. However, when the final annealing is performed as batch annealing, the sheet is preferably held at a sheet attaining temperature of from 300° C. to 450° C. for a time of from 0.5 hour to 24 hours. The final annealing, if performed at an annealing temperature higher than 450° C., tends to cause grains to coarsen, and tends to cause the final product sheet to have an average grain size greater than 100 μm and to have lower ductility. The final annealing, if performed at an attaining temperature lower than 300° C., tends to impede recrystallization from proceeding, tends to cause a deformed microstructure to remain, and may cause the aluminum alloy sheet to have significantly lower elongation, namely, significantly lower ductility and press formability.
When the final annealing is performed as batch annealing, in which the heating rate is relatively low, the annealing is preferably controlled to be performed by heating up to the annealing temperature at a rate of temperature rise of 10° C./h (hour) or more, and more preferably 30° C./h (hour) or more. This control is not always necessary when the final annealing is performed as the continuous annealing, in which the rate of temperature rise is relatively high. The annealing, if performed at a rate of temperature rise of less than 10° C./h, may cause crystal orientations to change, may cause at least one of the Brass orientation and the S orientation to grow excessively and to fail to be present in an orientation density, as expressed by ODF, of less than 5 relative to a random orientation, and may cause the aluminum alloy sheet to have higher elongation anisotropy.
The final annealing, when performed as continuous annealing, is preferably performed though heating up to a sheet attaining temperature of from 400° C. to lower than the aluminum alloy melting point for a short time of from about 0.1 second to about 60 seconds.
The sheet after the final annealing as above is used as a product sheet, which is a forming material for final use such as the automobile panel member. Specifically, the sheet is press-formed to have the shape of the member, then generally subjected to a treatment such as paint baking, and is finally used as the member.

EXAMPLES

Next, the aluminum alloy sheet according to the embodiment will be illustrated in further detail with reference to several examples below. Aluminum alloy sheets for forming, having chemical compositions given in Tables 1A, 1B and 2, were produced and, after final annealing, were investigated for texture, elongation anisotropy, strength (0.2% proof stress), and formability. Results of these are also given in Tables 1A, 1B and 2. So as to have different textures, the samples according to the examples and the comparative examples in Table 2 were produced under different production conditions, by appropriately adjusting the time for a sample sheet to pass through the temperature range of from 445° C. down to 400° C. in hot rolling, and the rate of temperature rise in final annealing according to batch system.
Specifically, common production conditions for the sample aluminum alloy sheets will be described below. Aluminum alloy slabs having a thickness of 500 mm and having the chemical compositions given in Tables 1A, 1B and 2 were made by direct chill casting. The slabs were subjected to surface scalping, then to homogenization at 500° C. for 4 hours, and to hot rolling started at that temperature (500° C.), where the hot rolling was finished at 300° C. or lower. The resulting hot-rolled sheets each had a final thickness of 3.3 mm. In the samples in Tables 1A and 1B, the time (passing time) for the sheet to pass through the temperature range of from 445° C. down to 400° C. in the hot rolling was set to 15 minutes in common with each sample. In contrast, the samples in Table 2 were produced at different passing times.
The hot-rolled sheets were subjected to cold rolling to a thickness of 1.0 mm in common and process annealing. In the samples in Tables 1A and 1B, the cold rolling was performed to a cold rolling reduction of 20% or 70%, and in the samples in Table 2, the cold rolling was performed to a cold rolling reduction of 20% in common. The cold-rolled sheets were subjected to final annealing, where the final annealing was performed according to batch system or continuous system at the rate of temperature rise (° C./h) and at the annealing temperature (° C.) for the holding time (in hour or second) given in Tables 1A, 1B and 2. A batch annealing furnace was employed in Examples 1 to 20, Examples 28 to 35, Comparative Examples 1 to 7, and Comparative Examples 11 to 14; and a continuous annealing furnace was employed in Examples 21 to 27.
Test samples (sheets) were sampled from the aluminum alloy sheets after the final annealing and were each investigated and evaluated for mechanical properties, texture, and formability according to measurement procedures as follows.
Mechanical Properties
From the test samples after the final annealing, tensile test specimens to be subjected to mechanical properties measurements were sampled and prepared as JIS No. 5 test specimens (having a width in the parallel portion of 25 mm, a gauge length (GL) of 50 mm, and a thickness of 1 mm) so that the tensile directions be a direction parallel to, and a direction perpendicular to, the sheet rolling direction. The tensile test specimens were subjected to tensile tests at room temperature (20° C.) and a tensile speed of 5 mm/min. In the tensile tests, 0.2% proof stress and total elongation were measured. Measurements in the tensile tests were perfoimed two times per test sample (N=2), the two measurements were averaged, and the average was defined as a value for each of the properties. The elongation anisotropy was determined in the tensile tests as a difference (%) in elongation between a tensile direction parallel to the sheet rolling direction and a tensile direction perpendicular to the sheet rolling direction, namely, was determined according to the equation:
Elongation anisotropy ΔEl=(Total elongation(%)in the sheet rolling direction)−(Total elongation(%)in a direction perpendicular to the sheet rolling direction)
The 0.2% proof stress was evaluated so that a sample having a 0.2% proof stress of 100 MPa or more was evaluated as being acceptable, which strength level is necessary as the automobile inner panel member. The elongation anisotropy was evaluated by calculating the difference in elongation between the parallel direction and the perpendicular direction (to the sheet rolling direction), and making an evaluation according to criteria as follows: A sample having a difference in elongation of 2% or less was evaluated as being very good and indicated by “VG”; a sample having a difference in elongation from greater than 2% to 4% was evaluated as being good and indicated by “Good”; and a sample having a difference in elongation greater than 4% was evaluated as being rejected and indicated by “Poor”.
Texture
The texture was determined in the following procedure. Specimens were sampled from the test samples after the final annealing so that an observation plane was defined as a plane parallel to the sheet surface, where the plane was located at a position of a depth half the sheet thickness in the through-thickness direction The texture of the observation plane of each sample was measured on the observation plane in a region of 600 μm in the sheet rolling direction by 600 μm in the sheet transverse direction at step intervals of 1 μm, using an FE-SEM JSM-7000F (supplied by JEOL Ltd.) at an acceleration voltage of 20 kV and using an EBSD measurement software TSL-OIM (Orientation Imaging Microscopy)-Data Collection Ver. 5 (supplied by TSL Solutions KK). As used herein the term “through-thickness direction” refers to a direction parallel to the thickness of the sheet to be measured.
The resulting measurement data were analyzed using an analysis software TSL-OIM (Orientation Imaging Microscopy) Analysis Ver. 7 (supplied by TSL Solutions KK), which is an EBSD analysis software, and the orientation densities expressed by ODFs of the Cube orientation, ND-rotated Cube orientation, Brass orientation, Cu orientation, S orientation, and Goss orientation relative to a random orientation were deteimined in a plane of the sheet, where the plane was parallel to the sheet surface and positioned at a depth half the thickness in the through-thickness direction.
The specimens to be subjected to the measurement were sampled in a number of 5 per test sample (N=5) at any positions in a plane of the sheet, where the plane was parallel to the sheet surface and positioned at a depth half the sheet thickness. The samples were subjected to mechanical polishing and buffing, and further to ion sputtering using an electron spectroscopy for chemical analysis (ESCA) system, to give an observation plane, as a measurement plane, where the observation plane was parallel to the sheet surface and positioned at a depth half the sheet thickness. The measurement results of the five test specimens measured by the SEM-EBSD analysis were averaged, and the averages were defined as the orientation densities (dimensionless) expressed by ODFs specified in the embodiment.
On the basis of these measurement results, a sample having orientation densities expressed by ODFs of the Brass orientation and the S orientation of each less than 5 relative to a random orientation was evaluated as having low elongation anisotropy and as being good. A sample having orientation densities expressed by ODFs of the Brass orientation and the S orientation of each less than 2 relative to a random orientation was evaluated as having still lower elongation anisotropy and as being very good (VG). In contrast, a sample having an orientation density as expressed by ODF of at least one of the Brass orientation and the S orientation of more than 5 relative to a random orientation was evaluated as having high elongation anisotropy and as being poor.
In this connection, all the examples and the comparative examples had average grain sizes of from 10 to 30 μm in the sheet rolling direction, where the average grain sizes were measured by the SEM-EBSD analysis on the plane being parallel to the sheet surface and positioned at a depth half the sheet thickness.
Erichsen Cupping Test
The test samples after the final annealing were subjected to Erichsen cupping tests so as to evaluate formability. The formability was evaluated in accordance with Method B of Erichsen cupping test prescribed in JIS Z 2247:1977. A sample having an Erichsen value of 9.0 mm or more was evaluated as having good formability (as being acceptable); and a sample having an Erichsen value less than 9.0 mm was evaluated as having poor formability (as being rejected).
The examples in Tables 1A and 1B had chemical compositions within the range specified for the aluminum alloy sheet according to the embodiment, and were produced under production conditions for the hot rolling and final annealing within preferred ranges. The examples in Tables 1A and 1B therefore had textures as specified in the embodiment, had strength maintained at a level required of 5xxx aluminum alloy sheets, still had low elongation anisotropy between a direction parallel to, and a direction perpendicular to, the sheet rolling direction, and offered good formability as evaluated by the Erichsen value.
In contrast, Comparative Examples 1 to 10 in Tables 1A and 1B had chemical compositions out of the range specified in the present invention, although these samples were produced under conditions for hot rolling and final annealing within the preferred ranges. The samples therefore had an excessively low strength or offered poor formability evaluated by the Erichsen value, because of having chemical compositions out of the range specified in the embodiment, although these samples had relatively low elongation anisotropy.
Comparative Example 8 had a Mg content out of the chemical composition range specified in the embodiment, thereby occurred cracking in hot rolling, and failed to be produced through subsequent steps, by which the textures and the properties could not be evaluated. Comparative Examples 9 and 10 had a content of at least one of Cu and Zn out of the chemical composition range specified in the embodiment, thereby occurred cracking in slab casting, and failed to be produced through subsequent steps, by which the textures and the properties could not be evaluated. The symbol “-” in Tables 1A and 1B means that the corresponding production condition was not applied (corresponding production step was not performed).
The examples in Table 2 had aluminum alloy sheet chemical compositions within the range specified in the embodiment, and were produced under production conditions within preferred ranges, for the time for the aluminum alloy sheet to pass through the temperature range of from 445° C. down to 400° C. in the hot rolling step and for the rate of temperature rise in the final annealing. The examples in Table 2 therefore also had textures as specified in the embodiment, had strength maintained at a level required of 5xxx aluminum alloy sheets, still had low elongation anisotropy between a direction parallel to, and a direction perpendicular to, the sheet rolling direction, and offered good formability as evaluated by the Erichsen value.
In contrast, the comparative examples in Table 2 had aluminum alloy sheet chemical compositions within the range specified in the embodiment, but were produced under a production condition(s) out of the preferred range(s), for at least one of the time for the aluminum alloy sheet to pass through the temperature range from 445° C. down to 400° C. in the hot rolling step, and the rate of temperature rise in the final annealing. These comparative examples therefore tended to have textures out of the range specified in the embodiment and, in particular, had an orientation density expressed by ODF of at least one of the Brass orientation and the S orientation of 5 or more relative to a random orientation. As a result, the comparative examples had higher elongation anisotropy, lower Erichsen indices, and offered lower press formability, as compared with the examples.
Comparative Examples 11 to 13 in Table 2 underwent a hot rolling step in which the aluminum alloy sheet passed through the temperature range from 445° C. down to 400° C. over an excessively long time of longer than 20 minutes. Comparative Example 14 in Table 2 underwent final annealing performed at an excessively low rate of temperature rise of less than 10° C./h.
Samples, when having a texture out of the range specified in the embodiment and having high elongation anisotropy as with the comparative examples in Table 2, have lower Erichsen values as with the comparative examples in Table 2, because the values are affected by deformability in a direction with lower elongation. However, the Erichsen value itself is significantly affected not only by elongation anisotropy, but also by other metallurgical factors than the texture, such as alloy chemical composition, grain size, and precipitate state, and acts as an index of overall formability. Accordingly, a high Erichsen value does not always mean low elongation anisotropy. Specifically, a known sample having a high Erichsen value is not always considered to have low elongation anisotropy.
These results support the significance of the chemical composition and the texture specified in the embodiment and the significance of preferred production conditions to give these properties, so as to provide lower elongation anisotropy and better press formability while having strength maintained at high level.

TABLE 1A

	Passing time	Final-

	Aluminum alloy	(min)				annealed
	sheet chemical	through	Cold			sheet

	composition (mass	temperature	rolling	Final	(annealed
	%, the remainder being	range from	reduction	annealing conditions	material)

	Al and unavoidable	445° C. down	(%) before	Rate of		strength
	impurities)	to 400° C. in	final	temperature	Holding	0.2% Proof

Category	Mg	Mn	Cu	Zn	hot rolling	annealing	rise	condition	stress (MPa)

Ex. 1	3.7	0.25	0.02	0.01	15	20	50° C./h	at	104
Ex. 2	4.2	0.25	0.02	0.01				380° C.	108
Ex. 3	4.7	0.25	0.02	0.01				for 4 h	116
Ex. 4	5.3	0.25	0.02	0.01					125
Ex. 5	4.7	0.05	0.02	0.01					107
Ex. 6	4.7	0.35	0.02	0.01					123
Ex. 7	4.7	0.55	0.02	0.01					128
Ex. 8	4.7	0.25	0.40	0.01					128
Ex. 9	4.7	0.25	0.02	0.30					121
Ex. 10	4.7	0.25	0.02	0.45					124
Ex. 11	3.7	0.40	0.05	0.05					103
Ex. 12	4.2	0.10	0.15	0.15					109
Ex. 13	4.7	0.50	0.20	0.15					124
Ex. 14	4.5	0.30	0.30	0.30					118
Ex. 15	3.7	0.25	0.02	0.01		70			116
Ex. 16	4.2	0.25	0.02	0.01					129
Ex. 17	4.7	0.25	0.02	0.01					138
Ex. 18	5.3	0.25	0.02	0.01					146
Ex. 19	4.7	0.12	0.02	0.01					133
Ex. 20	4.7	0.25	0.02	0.30					143

		Texture (orientation
		density expressed by ODF)		Form-

		ND-						ability
		rotated						(Erich-
	Cube	Cube	Brass	Cu	S	Goss	Elonga-	sen
	orien-	orien-	orien-	orien-	orien-	orien-	tion	value)
Category	tation	tation	tation	tation	tation	tation	anisotropy	(mm)

Ex. 1	1	1	3	2	3	1	Good	9.2
Ex. 2	1	1	3	2	3	1	Good	9.5
Ex. 3	1	1	3	2	3	1	Good	9.5
Ex. 4	1	1	3	2	3	1	Good	9.6
Ex. 5	1	1	3	2	3	1	Good	9.5
Ex. 6	1	1	3	2	3	1	Good	9.6
Ex. 7	1	1	3	2	3	1	Good	9.7
Ex. 8	1	1	4	3	2	0	Good	9.4
Ex. 9	1	0	3	2	3	1	Good	9.8
Ex. 10	1	0	3	2	3	1	Good	9.9
Ex. 11	1	0	3	2	3	1	Good	9.5
Ex. 12	1	0	4	3	2	0	Good	9.6
Ex. 13	1	0	4	3	2	0	Good	9.6
Ex. 14	1	0	4	3	2	0	Good	9.7
Ex. 15	1	1	3	4	3	2	Good	9.1
Ex. 16	1	1	4	2	3	2	Good	9.4
Ex. 17	1	1	4	2	3	2	Good	9.4
Ex. 18	1	1	3	4	3	2	Good	9.5
Ex. 19	1	1	3	2	3	2	Good	9.3
Ex. 20	1	0	4	3	3	1	Good	9.6

TABLE 1B

	Passing time	Final-

	Aluminum alloy	(min)				annealed
	sheet chemical	through	Cold			sheet

	composition (mass %,	temperature	rolling	Final	(annealed
	the remainder being	range from	reduction	annealing conditions	material)

Category	Mg	Mn	Cu	Zn	hot rolling	annealing	rise	condition	stress (MPa)

Ex. 21	3.7	0.25	0.02	0.01	15	70	5000° C./h	at	102
Ex. 22	4.2	0.25	0.02	0.01				500° C.	115
Ex. 23	4.7	0.25	0.02	0.01				for	124
Ex. 24	5.3	0.25	0.02	0.01				30 sec	130
Ex. 25	4.7	0.12	0.02	0.01					119
Ex. 26	4.7	0.25	0.10	0.01					127
Ex. 27	4.7	0.25	0.02	0.30					128
Comp.	3.3	0.25	0.02	0.01		20	50° C./h	at	93
Ex. 1								380° C.
Comp.	4.7	0.01	0.02	0.01				for 4 h	102
Ex. 2
Comp.	4.7	0.70	0.02	0.01					135
Ex. 3
Comp.	3.6	0.25	0.02	0.0005					98
Ex. 4
Comp.	4.7	0.25	0.02	0.60					151
Ex. 5
Comp.	3.6	0.25	0.0005	0.01					97
Ex. 6
Comp.	3.3	0.60	0.40	0.40					117
Ex. 7
Comp.	5.7	0.25	0.02	0.01	—	—	—	—	Cracked in hot rolling
Ex. 8									and could not be produced
									through subsequent steps
Comp.	4.7	0.25	0.60	0.01	—	—	—	—	Cracked in slab casting
Ex. 9									and could not be produced
									through subsequent steps
Comp.	4.7	0.30	0.60	0.60	—	—	—	—	Cracked in slab casting
Ex. 10									and could not be produced
									through subsequent steps

		Texture (orientation
		density expressed by ODF)

		ND-						Form-
		rotated						ability
	Cube	Cube	Brass	Cu	S	Goss	Elonga-	(Erichsen
	orien-	orien-	orien-	orien-	orien-	orien-	tion	value)
Category	tation	tation	tation	tation	tation	tation	anisotropy	(mm)

Ex. 21	2	3	0	0	0	1	VG	9.2
Ex. 22	2	3	0	0	0	1	VG	9.5
Ex. 23	2	3	0	0	0	1	VG	9.6
Ex. 24	2	3	0	0	0	1	VG	9.7
Ex. 25	2	3	0	0	0	1	VG	9.4
Ex. 26	2	3	0	0	0	1	VG	9.6
Ex. 27	2	3	0	0	0	1	VG	9.7
Comp.	1	1	3	2	3	1	Good	9.0
Ex. 1
Comp.	1	1	3	2	3	1	Good	8.8
Ex. 2
Comp.	1	1	3	3	3	1	Good	8.9
Ex. 3
Comp.	1	1	3	2	3	1	Good	9.3
Ex. 4
Comp.	1	0	3	2	3	1	Good	8.9
Ex. 5
Comp.	1	1	3	2	3	1	Good	9.3
Ex. 6
Comp.	1	0	3	2	3	1	Good	8.7
Ex. 7

	Comp.	Cracked in hot rolling and could not be produced through subsequent steps
	Ex. 8
	Comp.	Cracked in slab casting and could not be produced through subsequent steps
	Ex. 9
	Comp.	Cracked in slab casting and could not be produced through subsequent steps
	Ex. 10

TABLE 2

	Passing time	Final-
	(min)	annealed

Aluminum alloy sheet

through

Cold

sheet

	chemical composition	temperature	rolling	Final	(annealed
	(mass %, the remainder being	range from	reduction	annealing conditions	material)

Category	Mg	Mn	Cu	Zn	hot rolling	annealing	rise	condition	stress (MPa)

Ex. 28	4.7	0.25	0.02	0.01	11	20	50° C./h	at	118
Ex. 29	4.7	0.25	0.02	0.01	15			380° C.	116
Ex. 30	4.7	0.25	0.02	0.01	18			for 4 h	115
Ex. 31	3.7	0.25	0.02	0.01	18				102
Ex. 32	4.2	0.25	0.02	0.01	18				107
Ex. 33	3.7	0.40	0.05	0.05	15				103
Ex. 34	4.5	0.30	0.30	0.30	12				120
Ex. 35	4.7	0.25	0.02	0.01	15		15° C./h		116
Comp.	4.7	0.25	0.02	0.01	23		50° C./h		116
Ex. 11
Comp.	3.7	0.40	0.05	0.05	22				100
Ex. 12
Comp.	4.5	0.30	0.30	0.30	25				119
Ex. 13
Comp.	4.7	0.25	0.02	0.01	15				115
Ex. 14							6° C./h

Texture (orientation density expressed by ODF)

		ND-rotated						Formability
	Cube	Cube	Brass	Cu	S	Goss	Elongation	(Erichsen value)
Category	orientation	orientation	orientation	orientation	orientation	orientation	anisotropy	(mm)

Ex. 28	1	1	3	2	2	1	Good	9.4
Ex. 29	1	1	3	2	3	1	Good	9.5
Ex. 30	1	1	3	2	3	1	Good	9.5
Ex. 31	1	1	3	2	3	1	Good	9.2
Ex. 32	1	1	3	2	3	1	Good	9.5
Ex. 33	1	0	3	2	3	1	Good	9.5
Ex. 34	1	0	3	3	2	1	Good	9.7
Ex. 35	1	1	4	2	3	0	Good	9.4
Comp.	1	1	6	3	5	2	Poor	8.7
Ex. 11
Comp.	1	0	6	3	4	2	Poor	8.8
Ex. 12
Comp.	1	0	6	3	5	2	Poor	8.7
Ex. 13
Comp.	1	1	6	2	5	2	Poor	8.8
Ex. 14

INDUSTRIAL APPLICABILITY

The embodiment of the present invention can provide 5xxx aluminum alloy sheets for forming, which aluminum alloy sheets have low elongation anisotropy (in-plane anisotropy) and excellent press formability while having strength maintained at high level. This can expand the applications of 5xxx aluminum alloy sheets to forming typically into the automobile members.

Claims

1. An aluminum alloy sheet comprising:

Mg in a content of 3.5 to 5.5 mass %;

Mn in a content of 0.03 to 0.60 mass %;

Cu in a content of 0.001 to 0.50 mass %;

Zn in a content of 0.001 to 0.50 mass %,

Al and unavoidable impurities,

wherein orientation densities of Brass orientation and S orientation relative to a random orientation are each less than 5 in a texture of the aluminum alloy sheet, in a plane parallel to a sheet surface, where the plane is positioned at a depth half the thickness of the aluminum alloy sheet, and where the orientation densities are measured by SEM-EB SD analysis and are expressed by orientation distribution functions (ODFs).

2. The aluminum alloy sheet according to claim 1,

wherein the orientation densities of the Brass orientation and the S orientation are each less than 2 in the texture of the aluminum alloy sheet.

3. The aluminum alloy sheet according to claim 1,

wherein the aluminum alloy sheet has such elongation anisotropy that a JIS No. 5 test specimen sampled from the aluminum alloy sheet, when subjected to a tensile test at room temperature, has a difference in elongation of 2% or less between a tensile direction parallel to a sheet rolling direction and a tensile direction perpendicular to the sheet rolling direction.

4. An automobile member formed from the aluminum alloy sheet according to claim 1.

5. The aluminum alloy sheet according to claim 2,