US20150376742A1

US20150376742A1 - Aluminum alloy sheet for structural material

Info

Publication number: US20150376742A1
Application number: US14/767,096
Authority: US
Inventors: Katsushi Matsumoto; Yasuhiro Aruga; Hisao Shishido
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2013-03-14
Filing date: 2014-03-12
Publication date: 2015-12-31
Also published as: JP2014198899A; CN105143484A; JP6273158B2; WO2014142199A1

Abstract

Disclosed is a 7xxx-series aluminum alloy sheet having a specific chemical composition and produced by a common procedure. The aluminum alloy sheet has a good balance between a Zn content and a Mg content while having a lower Zn content so as to have a strength retained at high level. The aluminum alloy sheet has a microstructure having a specific endothermic peak temperature and a specific maximum height of exothermic peak(s) in a differential scanning calorimetric curve, where the curve is plotted after natural aging of the produced sheet. The aluminum alloy sheet can thereby have a high strength, satisfactory formability, and good corrosion resistance which are required for structural components.

Description

TECHNICAL FIELD

The present invention relates to an aluminum alloy sheet for structural components (structural materials), which aluminum alloy sheet has better workability, excellent corrosion resistance, and a high strength. As used herein the term “aluminum alloy sheet” refers to a rolled sheet that is prepared by producing a rolled sheet by rolling, subjecting the rolled sheet sequentially to solution treatment and quenching, thereafter subjecting the rolled sheet to natural aging for two weeks or longer and is before forming into a structural component and before artificial aging treatment.

BACKGROUND ART

A social demand for weight reduction of automobile bodies has been increasingly made in consideration typically of the global environment. To meet the demand, aluminum alloy materials are applied to some of automobile body parts so as to replace part of iron/steel materials such as steel sheets. The target automobile body parts are exemplified by panels such as outer panels and inner panels typically of hoods, doors, and roofs; and reinforcers such as bumper reinforcement (bumper R/F) and door beams.
For further weight reduction of the automobile bodies, however, the aluminum alloy materials should be applied also to automobile structural components such as frames and pillars. This is because these structural components particularly contribute to weight reduction. However, these automobile structural components require higher strength as compared with the automobile panels. For example, the automobile structural components require a 0.2% yield strength of 350 MPa or more. In this regard, JIS or AA 6xxx-series aluminum alloy sheets used as the automobile panels excel in formability, strength, and corrosion resistance and can be satisfactorily recycled because of having low-alloy chemical compositions. The 6xxx-series aluminum alloy sheets, however, are not anywhere near the high strength even when their chemical compositions are controlled and even when they are produced under controlled temper refining (e.g., solution treatment and quenching, and further artificial aging treatment) conditions.
This causes such high-strength automobile structural components to employ JIS or AA 7xxx-series aluminum alloy sheets that are used for the reinforcers requiring high strength at similar level. However, the material 7xxx-series aluminum alloys as being Al—Zn—Mg alloys, have inferior general corrosion resistance. In addition, these alloys achieve a high strength by allowing Zn—Mg precipitates MgZn₂to be distributed in a high density and may thereby cause stress corrosion cracking (hereinafter also simply referred to as “SCC”). To prevent this, the alloys are forced to be subjected to over-aging and are used in actual use at a 0.2% yield strength of about 300 MPa and have diluted features or properties as high-strength alloys.
As possible solutions to this, various proposals have been made on chemical composition control and on microstructure control (e.g., precipitates control) so as to provide 7xxx-series aluminum alloys having a strength and SCC resistance both at satisfactory levels.
The chemical composition control is representatively exemplified by a technique disclosed in Patent Literature (PTL 1). The technique relates to a 7xxx-series aluminum alloy extrusion and utilizes Mg added in excess of the stoichiometric ratio for MgZn₂, where such excess Mg contributes to a higher strength. The stoichiometric ratio refers to the proportions of Zn and Mg to form MgZn₂in just proportion. Specifically, Mg is added in excess of the stoichiometric ratio for MgZn₂so as to reduce the MgZn₂amount and thereby allows the 7xxx-series aluminum alloy extrusion to have a higher strength without deterioration in SCC resistance.
The microstructure control (e.g., precipitates control) is representatively exemplified by a technique disclosed in PTL 2. According to this technique, a 7xxx-series aluminum alloy extrusion after artificial aging treatment is controlled so that precipitates having a particle diameter of 1 to 15 nm are present in grains in a number density of 1000 to 10000 per square micrometer so as to have a smaller potential difference between the inside of a grain and the grain boundary, where the number density is determined by observation with a transmission electron microscope (TEM). Thus, the 7xxx-series aluminum alloy extrusion has better SCC resistance.
Though not all are exemplified, there are many other techniques relating to chemical composition control and microstructure control (e.g., precipitates control) on 7xxx-series aluminum alloy extrusions so as to have a strength and SCC resistance both at satisfactorily levels. This is because a large number of 7xxx-series aluminum alloy extrusions are in actual use. In contrast, there are very few conventional techniques relating to chemical composition control and microstructure control (e.g., precipitates control) on 7xxx-series aluminum alloy sheets, because a small number of the 7xxx-series aluminum alloy sheets are in actual use.
For example, PTL 3 proposes a technique relating to a structural component including a clad sheet prepared by weld-bonding 7xxx-series aluminum alloy sheets with each other. According to the technique, precipitates after artificial aging treatment are controlled to be present in the form of spheres having a diameter of 50 angstroms or less in a predetermined amount so as to offer a higher strength. The literature, however, does not disclose the SCC resistance performance at all and fails to describe corrosion resistance data in working examples.
PTL 4 discloses a technique relating to a 7xxx-series aluminum alloy sheet prepared by rapidly solidifying a molten metal, cold-rolling the workpiece, and subjecting the workpiece to artificial aging treatment. According to this technique, precipitates in grains are controlled to have a size in terms of equivalent circle diameter of 3.0 μm or less and an average area fraction of 4.5% or less so as to have a higher strength and better elongation. The equivalent circle diameter and the average area fraction are measured with an optical microscope at 400-fold magnification, where the size is calculated as the diameter of a circle having an equivalent area.
Although few, there are some proposals relating to metallic texture control in a sheet. For example, PTL 5 and PTL 6 disclose techniques relating to a 7xxx-series sheet for structural components. According to the techniques, an ingot is forged and then repeatedly rolled in a warm working region to have a refined microstructure in order to have a higher strength and better SCC resistance. The techniques are intended to refine the microstructure to thereby restrain large angle grain boundaries having a misorientation of 20° or more so as to have a metallic texture including small angle grain boundaries having a misorientation of 3° to 10° in an amount of 25% or more. This is because such large angle grain boundaries cause the potential difference between the grain boundary and the grain inside, where the potential difference in turn causes SCC resistance deterioration. However, the repeated warm rolling operations are performed because hot rolling and cold rolling by a common procedure fail to give a metallic texture including small angle grain boundaries in an amount of 25% or more. The techniques therefore significantly differ in process from the common procedure and are not considered to be practical techniques to produce sheets.

CITATION LIST

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2011-144396
PTL 2: JP-A No. 2010-275611
PTL 3: JP-A No. Hei9(1997)-125184
PTL 4: JP-A No. 2009-144190
PTL 5: JP-A No. 2001-335874
PTL 6: JP-A No. 2002-241882

SUMMARY OF INVENTION

Technical Problem

As described above, various proposals relating to chemical composition control and microstructure control (e.g., precipitates or metallic texture control) of 7xxx-series aluminum alloys have been made in the field of extrusions so as to offer a strength and SCC resistance both at satisfactory levels. However, under present circumstances, there are few proposals relating to rolled sheets produced by a common procedure typically by soaking an ingot and subjecting the soaked ingot to hot rolling and cold rolling, except for the special rolling or production methods such as the use of clad sheet, rapid solidification, and warm rolling.
From the rolled sheets, the extrusions are quite different in production processes such as hot working process and are significantly different in the resulting microstructure such as grains and precipitates. Typically, the extrusions include fibrous grains that are elongated in the extrusion direction, whereas the rolled sheets basically include equiaxial grains as the grains. For these reasons, it is unknown whether the proposals relating to chemical composition control and microstructure control (e.g., precipitates control) in the extrusions are applicable as intact to 7xxx-series aluminum alloy sheets and to automobile structural components including the 7xxx-series aluminum alloy sheets and effectively contribute to both a higher strength and better SCC resistance really. Namely, this still remains in the realm of expectation unless being actually verified.
Under present circumstances, there is still no effective means, are many unknown points, and is room for clarification relating to microstructure control techniques of the 7xxx-series aluminum alloy sheets produced by a common procedure so as to offer a strength and SCC resistance both at satisfactorily levels. In addition, the 7xxx-series aluminum alloy sheets require a lower Zn content from the viewpoints of strength and corrosion resistance, because the presence of Zn causes a less noble potential that is involved in the general corrosion resistance. However, with a decreasing Zn content, the 7xxx-series aluminum alloy sheets have a lower strength, although having better corrosion resistance and having better processability such as bendability which is a property necessary for the structural components. This is inconsistent with the target high strength and constitutes a technical difficulty.
In consideration of the above-described problems, it is an object of the present invention to provide a 7xxx-series aluminum alloy sheet for structural components such as automobile components, which aluminum alloy sheet is provided as a rolled sheet produced according to the common procedure, has a strength and processability both at satisfactory levels, and still has excellent corrosion resistance.

Solution to Problem

The present invention provides, to achieve the object, an aluminum alloy sheet for structural components. The aluminum alloy sheet is an Al—Zn—Mg alloy sheet and contains, in terms of a chemical composition in mass percent, Zn in a content of 3.0% to 6.0%, Mg in a content of 1.5% to 4.5%, and Cu in a content of 0.05% to 0.5%, where the Zn content [Zn] and the Mg content [Mg] meet a condition as specified by: [Zn]≧−0.3[Mg]+4.5, with the remainder consisting of Al and inevitable impurities. When the sheet is subjected sequentially to solution treatment, quenching, and natural aging, and a differential scanning calorimetric curve is plotted after the natural aging, a highest endothermic peak temperature is 130° C. or lower and a maximum height of exothermic peak(s) in a temperature range of 200° C. to 300° C. is 50 μW/mg or more in the differential scanning calorimetric curve, and a work hardening coefficient n (10% to 20%) is 0.22 or more.

Advantageous Effects of Invention

As used herein the term “aluminum alloy sheet” refers to a 7xxx-series aluminum alloy sheet that is a sheet produced by rolling and is produced by a common procedure including soaking an ingot, subjecting the soaked ingot sequentially to hot rolling and cold rolling to give a cold-rolled sheet, and further subjecting the cold-rolled sheet to a temper refining treatment (T4 in a temper designation) such as solution treatment and quenching. In other words, the term “aluminum alloy sheet” does not include sheets produced by such a special rolling method as to subjecting an ingot to forging and to repeat warm rolling operations many times, as in the techniques disclosed in PTL 5 and PTL 6.
As the aluminum alloy sheet according to the present invention, a 7xxx-series aluminum alloy sheet produced in the above manner and undergoing natural aging is controlled to have a specific microstructure. In addition, the aluminum alloy sheet is, as a material aluminum alloy sheet, processed or formed into an intended structural component. Accordingly, the term “aluminum alloy sheet” as used herein refers to a sheet that is produced in the above manner and undergoes natural aging (standing at room temperature) after production, but is before being formed into an intended structural component and before being subjected to artificial aging treatment.
The present inventors prepared a 7xxx-series aluminum alloy sheet having a chemical composition with a lower Zn content for better corrosion resistance and a higher Mg content for a certain strength and analyzed the microstructure of the 7xxx-series aluminum alloy sheet after natural aging based on a differential scanning calorimetric curve. As a result, the present inventors have found that dusters formed by natural aging in the 7xxx-series aluminum alloy sheet having the chemical composition have a different chemical composition and act in a different manner from a 7xxx-series aluminum alloy sheet having a high Zn content.
Specifically, the present inventors have found that dusters (atomic dusters) formed by natural aging in the 7xxx-series aluminum alloy sheet having a lower Zn content contributes not only to artificial aging properties (BH response) after forming into an intended structural component, but also to ductility (work hardening properties) necessary upon forming into the structural component. The control of these clusters therefore contributes not only to better corrosion resistance, but also to a better balance between strength and ductility (formability) and can provide a 7xxx-series aluminum alloy sheet that is used for structures (structural components), is produced as a rolled sheet by a common procedure, has a strength and processability both at satisfactory levels, and still excels in corrosion resistance such as SCC resistance.
It is difficult, however, to quantitatively specify the dusters (atomic dusters) with microstructural factors such as chemical composition, size, and/or density, because such microstructural factors of the dusters cannot be directly determined at the present time using a common observation device such as a SEM or TEM.
For this reason, at the present invention, the microstructure of the 7xxx-series aluminum alloy sheet after production and after natural aging is indirectly specified and controlled by analysis based on a differential scanning calorimetric curve. More specifically, the 7xxx-series aluminum alloy sheet has better work hardening properties (as ductility) with a decreasing temperature of an endothermic peak in the differential scanning calorimetric curve analysis, where the temperature of endothermic peak corresponds to re-dissolution of clusters formed by natural aging. In contrast, precipitates after artificial aging are formed in a larger amount and a higher strength is obtained with an increasing maximum height of exothermic peak(s) in the temperature range of 200° C. to 300° C., where the maximum height of exothermic peaks corresponds to the precipitates after artificial aging treatment.
For specifying dusters formed by natural aging, the differential scanning calorimetric curve in the present invention is measured not for a sheet immediately after the temper refining treatment and before undergoing natural aging, but for a sheet after natural aging (standing at room temperature) for approximately 2 weeks or longer as a rough reference, but before forming into a structural component and before artificial aging treatment.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be specifically illustrated for individual factors or conditions below.
Aluminum Alloy Chemical Composition
Initially, the chemical composition of the aluminum alloy sheet according to the present invention, and reasons for specifying the contents of individual elements will be illustrated below. All percentages of individual element contents are by mass.
The chemical composition of the aluminum alloy sheet according to the present invention is specified as such a precondition that a rolled sheet produced by a common procedure can have a strength and processability both at satisfactory levels and can still have satisfactory corrosion resistance, where these properties are required properties as automobile components and other structural components that are intended uses in the present invention. For the reason, the 7xxx-series Al—Zn—Mg—C alloy in the present invention has such a chemical composition as to have a lower Zn content for better corrosion resistance and a higher Mg content for a strength at certain level.
From this viewpoint, the aluminum alloy sheet according to the present invention has a chemical composition including, in mass percent, Zn in a content of 3.0% to 6.0%, Mg in a content of 1.5% to 4.5%, and Cu in a content of 0.05% to 0.5%, where the Zn content [Zn] and the Mg content [Mg] meet a condition specified by formula: [Zn]≧−0.3[Mg]+4.5, with the remainder consisting of Al and inevitable impurities. In addition to the elements, the aluminum alloy sheet may further contain at least one transition element selected from the group consisting of Zr in a content of 0.05% to 0.3%, Mn in a content of 0.1% to 1.5%, Cr in a content of 0.05% to 0.3%, and Sc in a content of 0.05% to 0.3%. The aluminum alloy sheet may further selectively contain Ag in a content of 0.01% to 0.2% in addition to, or instead of the at least one transition element.
Zn: 3.0% to 6.0%
Zinc (Zn) acts as an essential alloy element, forms dusters with Mg upon natural aging of the produced sheet after temper refining, and contributes to better work hardening properties and better processability into the structural component. This element also forms precipitates upon artificial aging after forming into the structural component and contributes to a higher strength. The aluminum alloy sheet, if having a Zn content less than 3.0%, may have an insufficient strength after artificial aging. However, the aluminum alloy sheet, if having an excessively high Zn content greater than 6.0%, may often suffer from grain-boundary corrosion and have inferior corrosion resistance due to an increased amount of grain-boundary precipitate MgZn₂. To prevent these, the Zn content is controlled in the present invention to be relatively low. The Zn content is therefore controlled to 3.0% or more and preferably 3.5% or more in terms of lower limit, and is controlled to 6.0% or less, and preferably 4.5% or less in terms of upper limit.
Mg: 1.5% to 4.5%
Magnesium (Mg) acts as an essential alloy element, forms dusters with Zn upon natural aging of the produced sheet after temper refining, and contributes to better work hardening properties. This element also forms precipitates upon artificial aging after forming into the structural component and contributes to a higher strength. The Mg content is set as relatively high in the present invention, because the Zn content is controlled to be relatively low. The aluminum alloy sheet, if having a Mg content less than 1.5 percent by mass, may have inferior work hardening properties. However, the aluminum alloy sheet, if having a Mg content greater than 4.5 percent by mass, may have inferior rolling properties and higher susceptibility to SCC. To prevent these, the Mg content may be controlled to 1.5% or more, and preferably 2.5% or more in terms of lower limit; and controlled to 4.5% or less in terms of upper limit.
Balance Formula Between Zn and Mg
According to the present invention, it is important to control not only the Zn and Mg contents, but also the balance between the Zn content [Zn] (in mass percent) and the Mg content [Mg] (in mass percent) so as to allow Zn and Mg to surely contribute to a higher strength. The balance control is performed so that [Zn] and [Mg] meet the balance formula: [Zn]≧−0.3[Mg]+4.5, and preferably meet the balance formula: [Zn]≧−0.5[Mg]+5.75.
The aluminum alloy sheet, as meeting the balance formula: [Zn]≧−0.3[Mg]+4.5 and when being produced by an after-mentioned preferred production method, may allow the resulting structural component after artificial aging treatment to have a 0.2% yield strength of 350 MPa or more. The aluminum alloy sheet, when meeting the balance formula: [Zn]≧−0.5[Mg]+5.75 and when being produced by the preferred production method, may allow the structural component after artificial aging treatment to have a 0.2% yield strength of 400 MPa or more.
The aluminum alloy sheet, if having contents of Zn and Mg meeting a condition as specified by: [Zn]<−0.3[Mg]+4.5, may possibly cause the structural component after artificial aging treatment to fail to have a 0.2% yield strength of 350 MPa or more, even when the individual element contents fall within the specific ranges, or even when the sheet is produced by the preferred production method. Likewise, the aluminum alloy sheet, if having contents of Zn and Mg meeting a condition as specified by: [Zn]<−0.5[Mg]+5.75, may possibly cause the structural component after artificial aging treatment to fail to have a 0.2% yield strength of 400 MPa or more.
Cu: 0.05% to 0.5%
Copper (Cu) effectively restrains SCC susceptibility of the Al—Zn—Mg alloy, contributes to better SCC resistance, and still contributes to better general corrosion resistance. Cu, if contained in a content less than 0.05%, may not so effectively contribute to better SCC resistance and better general corrosion resistance. In contrast, Cu, if contained in a content greater than 0.5%, may contrarily adversely affect properties such as rolling properties and weldability. To prevent these, the Cu content may be controlled to 0.05% or more in terms of lower limit and may be controlled to 0.5% or less, and preferably 0.4% or less in terms of upper limit.
At least one selected from Zr in a content of 0.05% to 0.3%, Mn in a content of 0.1% to 1.5%, Cr in a content of 0.05% to 0.3%, and Sc in a content of 0.05% to 0.3%
The transition elements Zr, Mn, Cr, and Sc induce grain refinement of the ingot and of the final product, thereby contribute to a higher strength, and may be selectively added according to necessity. In an embodiment, the aluminum alloy sheet contains at least one of these elements. In this embodiment, the aluminum alloy sheet, if containing at least one of Zr, Mn, Cr, and Sc in a content each lower than the lower limit, may have a lower strength due to the insufficient content. In contrast, the aluminum alloy sheet, if containing at least one of Zr, Mn, Cr, and Sc each in a content greater than the upper limit, may have lower elongation due to the formation of coarse precipitates. To prevent these, the aluminum alloy sheet, when containing at least one of these elements, may contain at least one selected from the group consisting of Zr in a content of 0.05% to 0.3%, Mn in a content of 0.1% to 1.5%, Cr in a content of 0.05% to 0.3%, and Sc in a content of 0.05% to 0.3%; and preferably contains at least one selected from the group consisting of Zr in a content of 0.08% to 0.2%, Mn in a content of 0.2% to 1.0%, Cr in a content of 0.1% to 0.2%, and Sc in a content of 0.1% to 0.2%.
Ag: 0.01% to 0.2%
Silver (Ag) effectively allows precipitates to be formed densely and finely through artificial aging after forming into the structural component and contributes to a higher strength, because the precipitates contribute to such higher strength. The aluminum alloy sheet may selectively contain this element as needed. Ag, if contained in a content less than 0.01%, may not so effectively contribute to a higher strength. In contrast, Ag, if contained in a content greater than 0.2%, may have saturated effects and cause higher cost. To prevent these, the Ag content may be controlled within the range of 0.01% to 0.2%.
Other Elements
Other elements than those mentioned above are basically inevitable impurities. Such inevitable impurity elements may be assumed (accepted) to be contained in the aluminum alloy sheet typically by using aluminum alloy scrap as molten raw materials in addition to pure aluminum ingots. While assuming (accepting) the contamination of the impurity elements, the elements may be contained in contents within ranges as specified by Japanese Industrial Standards for 7xxx-series alloys. Typically, Ti and B are impurities for a rolled sheet, but effectively contribute to grain refinement of the ingot. Ti may be contained in a content of 0.2% or less, and preferably 0.1% or less in terms of upper limit; and B may be contained in a content of 0.05% or less, and preferably 0.03% or less in terms of upper limit. Fe and Si do not affect properties of the aluminum alloy rolled sheet according to the present invention and may be contained therein as long as Fe in a content of 0.5% or less and Si in a content of 0.5% or less.
Microstructure
On the precondition of having the chemical composition, the 7xxx-series aluminum alloy sheet according to the present invention has a microstructure as follows. Assume that the sheet is sequentially subjected to solution treatment and quenching, further subjected to natural aging for 2 weeks or longer as a rough reference, and subjected to differential scanning calorimetry to plot a differential scanning calorimetric curve. In the curve, a highest endothermic peak temperature is 130° C. or lower, and a maximum height of exothermic peak(s) in the temperature range of 200° C. to 300° C. is 50 μW/mg or more.
The highest endothermic peak temperature corresponds to re-dissolution of dusters formed upon natural aging of the sheet. With a decreasing highest endothermic peak temperature, the dusters have decreasing thermal stability (increasing susceptibility to decomposition) and become more susceptible to decomposition not only by heat, but also by cutting of dislocations upon plastic deformation. The dusters, when having decreasing stability as above, may less impede the movement of dislocations and less cause strain concentration upon plastic deformation, such as forming of the sheet into a structural component. An index for the duster stability is a highest endothermic peak temperature of 130° C. or lower. When the microstructure meets the condition, the dusters have low stability (are unstable) and contributes to better work hardening properties and to a work hardening coefficient n (10% to 20%) of 0.22 or more.
In contrast, with increasing stability of the dusters, the highest endothermic peak temperature rises higher than 130° C., the work hardening properties are lowered, and a work hardening coefficient n (10% to 20%) of 0.22 or more is not obtained. This is because the dusters, if having increasing stability, may impede the movement of dislocations upon plastic deformation such as forming of the sheet into a structural component, but, once the dislocations cut the dusters and begin moving, the dislocation movement concentrates on the slip plane. This impedes strain concentration and impairs work hardening properties.
The maximum height of exothermic peak(s) in the temperature range of 200° C. to 300° C. corresponds to the precipitation of precipitates (artificial-aging precipitates) upon artificial aging, where the precipitates contribute to a higher strength. Accordingly, with an increasing maximum height of exothermic peaks, artificial-aging precipitates are formed in a larger amount (in a higher number density) and contribute to a still higher strength. An index for this is a maximum height of exothermic peaks of 50 μW/mg or more in the temperature range of 200° C. to 300° C. If the maximum height of exothermic peaks is less than 50 μW/mg, the structural component after artificial aging treatment may highly possibly fail to have a 0.2% yield strength of 350 MPa or more.
Work Hardening Properties
The 7xxx-series aluminum alloy sheet according to the present invention may have better processability as having the chemical composition and the microstructure and when being produced by the preferred production method. In particular, the work hardening coefficient n (10% to 20%) is specified in the present invention so as to surely have bending formability, where the bending may be used in forming into a structural component. Specifically, assume that a 7xxx-series aluminum alloy sheet having the chemical composition and the microstructure is produced by the preferred production method. Further assume that this sheet is sequentially subjected to solution treatment, quenching, and natural aging for a duration of 2 weeks or longer as a rough reference. In this case, a work hardening coefficient n (10% to 20%) is controlled to 0.22 or more.
The work hardening is a phenomenon in which a workpiece has increased hardness by plastic deformation upon the application of stress typically by forming. The work hardening is also called “strain hardening”. In the work hardening, the workpiece has increasing resistance and increasing hardness with proceeding deformation by forming. A property value for the work hardening acts as an index for processability and called a coefficient “n”. The “coefficient n” refers to a coefficient n when the relation between stress σ and strain ε in a plastic region at a yield point or more are approximated. The approximation may be performed according to the Voce equation that is suitable for aluminum. With an increasing coefficient n, the workpiece may have increasing susceptibility to work hardening and undergo hardening in a portion receiving plastic deformation by forming such as bending. This may allow other portions around the hardened portion to be readily deformable and may contribute to better processability typically in bending. In contrast, with a decreasing coefficient n, the workpiece may more resist work hardening. In this case, a portion receiving plastic deformation at first and receiving maximum stress may not be hardened, but undergo more plastic deformation, and become constricted and susceptible to rupture, resulting in inferior processability typically in bending.
Production Method
A method for producing the 7xxx-series aluminum alloy sheet according to the present invention will be specifically illustrated below.
The 7xxx-series aluminum alloy sheet according to the present invention can be produced by a production method including common production processes. Specifically, a material is subjected to common production processes such as casting (direct chill casting (DC casting) or continuous casting), soaking, and hot rolling to give an aluminum alloy hot-rolled sheet having a thickness of 1.5 to 5.0 mm. Next, the aluminum alloy hot-rolled sheet is cold-rolled into a cold-rolled sheet having a thickness of 3 mm or less. In this process, one or more process annealing operations may be selectively performed before or during the cold rolling.
Melting and Casting Cooling Rate
Initially, a molten aluminum alloy adjusted to have a chemical composition within the 7xxx-series chemical composition is prepared by melting, and cast into an ingot by a common casting technique in a melting-casting process. The casting technique may be selected as appropriate typically from continuous casting and semi-continuous casting (DC casting).
Soaking
Next, the cast aluminum alloy ingot is subjected to soaking in advance of hot rolling. The soak ng (homogenization heat treatment) is performed in order to homogenize the microstructure, i.e., to remove or mitigate the segregation in grains in the ingot microstructure.
The soaking in the present invention is preferably performed by a two-stage or double soaking process. The two-stage or double soaking is preferred to offer better work hardening properties when the aluminum alloy sheet is sequentially subjected to the temper refining treatment and natural aging and is then formed into a structural component. The two-stage or double soaking is also preferred to offer a higher strength of the resulting structural component after forming and after artificial aging. In the two-stage soaking, a workpiece after first-stage soaking is cooled not down to 200° C. or lower, but down to a cooling end temperature of higher than 200° C., held at that temperature, and subjected to hot rolling as being held at that temperature or as being reheated to a higher temperature. In contrast in the double soaking, a workpiece after first soaking is once cooled down to a temperature of 200° C. or lower (including room temperature), reheated, held at the reheating temperature for a predetermined time, and subjected to hot rolling.
In the two-stage or double soaking process, the first-stage or first soaking is intended to finely disperse transition element compounds and to perform refinement of such compounds that affect the formability of the aluminum alloy sheet into a structural component; and the second-stage or second soaking is intended to accelerate the solid-solution (dissolution) of Zn, Mg, and Cu and to offer better work hardening properties after natural aging and a higher strength after artificial aging.
To achieve the intentions, the first-stage or first soaking temperature may be controlled to be from 400° C. to 450° C., and preferably from 400° C. to 440° C. The heating and holding of the ingot at a temperature within the range allows zirconium (Zr) compounds and compounds including any of Mn, Cr, and Sc to be finely dispersed. The first-stage or first soaking, if performed at a temperature lower than 400° C., may fail to give sufficiently effective refinement and may fail to contribute to better work hardening properties after natural aging. In contrast, the first-stage or first soaking, if performed at a temperature higher than 450° C., may cause the compounds to coarsen and may also fail to contribute to better work hardening properties after natural aging. Holding in the first-stage or first soaking may be performed for a time (holding time) of about 1 to about 8 hours.
The second-stage or second soaking temperature may be controlled in the range of 450° C. to the solidus temperature, preferably in the range of 470° C. to the solidus temperature. The heating and holding of the ingot at a temperature within the range may accelerate the dissolution of Zn, Mg, and Cu and contribute to a higher strength upon artificial aging after the solution treatment. The second-stage or second soaking, if performed at a temperature lower than 450° C., may fail to give sufficient dissolution of these elements and may fail to contribute to better work hardening properties after natural aging and a higher strength after artificial aging. In contrast, the second-stage or second soaking, if performed at a temperature higher than the solidus temperature, may cause partial melting to impair mechanical properties. To prevent this, the temperature is controlled to be equal to or lower than the solidus temperature in terms of upper limit. The holding in the second-stage or second soaking may be performed for a time (holding time) of from about 1 to about 8 hours.
Hot Rolling
The hot rolling, if started at a temperature higher than the solidus temperature, may cause burning and may be performed with difficulty in itself. In contrast, the hot rolling, if started at a temperature lower than 350° C., may require an excessively large load and may be performed with difficulty in itself. To prevent these, the hot rolling may be performed at a start temperature selected within the range of 350° C. to the solidus temperature and may give a hot-rolled sheet having a thickness of about 2 to about 7 mm. The hot-rolled sheet may be subjected to, but not necessarily, annealing (heat treatment) in advance of cold rolling.
Cold Rolling
In the cold rolling, the hot-rolled sheet is rolled to give a cold-rolled sheet (including a coil) having a desired final thickness of from about 1 to about 3 mm. The workpiece may be subjected to process annealing between passes of cold rolling.
Solution Treatment
The workpiece (cold-rolled sheet) after cold rolling is subjected to solution treatment as temper refining. The solution treatment may be performed as heating and cooling in a common continuous heat treatment line, and the procedure of which is not limited. However, the solution treatment is preferably performed by heating the workpiece to a solution treatment temperature of 450° C. to the solidus temperature, and more preferably 480° C. to 550° C., and holding the workpiece for a duration in the range of from 2 or 3 seconds to 30 minutes after the temperature reaches the predetermined solution treatment temperature. This is preferred for obtaining sufficient amounts of individual elements as solutes and for grain refinement.
Cooling (temperature fall) after the solution treatment may be performed at any average cooling rate not critical. The cooling after the solution treatment may be performed by forced cooling means or by quenching the workpiece in warm water at a temperature of room temperature to 100° C. The forced cooling means may be selected from, alone or in combination, air cooling means such as fan; and water cooling means such as mist, spray, and immersion. In this connection, the solution treatment may be basically performed only once. However, typically when natural age hardening excessively proceeds, solution treatment and/or reversion treatment may be performed again under the preferred conditions so as to temporarily cancel the excessively proceeded natural age hardening. This is preferred for surely offering formability into an automobile component.
In an embodiment, the aluminum alloy sheet according to the present invention is used as a material, formed into an automobile component, and assembled as the automobile component. In another embodiment, the aluminum alloy sheet is formed into an automobile component, then further subjected to artificial aging treatment, and used as an automobile component or automobile body.
Artificial Aging Treatment
The 7xxx-series aluminum alloy sheet according to the present invention, after being formed into a structural component, may be subjected to artificial aging treatment so as to have a desired strength as the structural component such as an automobile component, namely, a strength in terms of a 0.2% yield strength of 350 MPa or more, and preferably 400 MPa or more. The artificial aging treatment is preferably performed after the forming of the material 7xxx-series aluminum alloy sheet into an automobile component. This is because the 7xxx-series aluminum alloy sheet, if subjected to artificial aging treatment before forming, has a higher strength, but offers lower formability, and may resist forming when the target automobile component has a certain complicated shape.
The artificial aging treatment may be performed under conditions (e.g., temperature and time) that can be freely determined within common artificial aging treatment conditions (T6 or T7) depending on the desired strength, the strength of the material 7xxx-series aluminum alloy sheet, and/or the degree of proceeding of natural aging. Exemplary artificial aging treatment conditions are as follows. The artificial aging treatment, when performed as single-stage aging, may be performed as temper aging at 100° C. to 150° C. for 12 to 36 hours (including the over-aging region). The treatment, when performed as two-stage aging, may be performed so that a first heat treatment is performed at a temperature of from 70° C. to 100° C. for 2 hours or longer, and a second heat treatment is performed at a temperature of from 100° C. to 170° C. for 5 hours or longer (including the overaging region).

EXAMPLES

A series of 7xxx-series aluminum alloy cold-rolled sheets having Al—Zn—Mg—Cu chemical compositions as given in Tables 1 and 2 below was produced so as to have different microstructures as analyzed by the DSC curve. Each of the produced cold-rolled sheets was sequentially subjected to solution treatment, quenching, and natural aging. A DSC curve of the resulting sample was measured and plotted to measure a highest endothermic peak temperature and a maximum height of exothermic peak(s) in the temperature range of 200° C. to 300° C. A work hardening coefficient n (10% to 20%) was also measured. In addition, each sample was subjected to artificial aging treatment and was examined to evaluate general corrosion resistance and mechanical properties such as strength. The results are indicated in Tables 3 and 4 below.
The microstructures of the cold-rolled sheets were controlled mainly by varying soaking conditions as given in Tables 3 and 4. Specifically, commonly in each sample, a 7xxx-series molten aluminum alloy having the chemical composition given in Tables 1 and 2 was subjected to DC casting and yielded an ingot of a size of 45 mm thick by 220 mm wide by 145 mm long. The ingot was subjected to two-stage soaking or double soaking under conditions given in Tables 3 and 4. In the two-stage soaking, the sample was subjected to first-stage soaking, cooled down to 250° C., the cooling was once stopped at that temperature, and the sample was reheated to and held at a second-stage soaking temperature, and cooled down to a hot rolling start temperature, followed by starting of hot rolling. In the double soaking, the sample was subjected to first soaking, once cooled down to room temperature, reheated to and held at a second soaking temperature, and cooled down to a hot rolling start temperature, followed by starting of hot rolling. In the case in Tables 3 and 4 where soaking was performed only once, the sample was not subjected to reheating (for second soaking) after once cooling, but was held at the soaking temperature for the soaking time by a common procedure, and then cooled down to a hot rolling start temperature, followed by starting of hot rolling.
After the soaking process as above, each sample was subjected to hot rolling at a start temperature given in Tables 3 and 4 and yielded a hot-rolled sheet having a thickness t of 5 mm. The hot-rolled sheet was subjected to heat treatment, in which the hot-rolled sheet was held at 500° C. for 30 seconds and cooled by forced wind cooling. The sample was then subjected to cold rolling to a thickness t of 2 mm to give a cold-rolled sheet. Commonly in each sample, the cold-rolled sheet was subjected to solution treatment at 500° C. for one minute. After the solution treatment, the sample workpiece was cooled by forced wind cooling down to room temperature and yielded a T4 tempered aluminum alloy sheet. Commonly in each sample, the aluminum alloy sheet after the solution treatment was subjected to natural aging for 2 weeks, and sheet-like test specimens were sampled from the resulting workpiece and subjected to DSC measurement and a tensile test. Individual properties were examined in manners as follows.
DSC Measurement (Differential Thermal Analysis
The DSC measurement (differential thermal analysis) was performed under the same conditions as follows commonly in each sample.
Test equipment: DSC220C supplied by Seiko Instruments Inc.,
Reference material: pure aluminum,
Specimen container: pure aluminum,
Temperature rise condition: 15° C./min,
Atmosphere (in the specimen container): argon gas (at a gas flow rate of 50 ml/min), and
Test specimen weight: 24.5 to 26.5 mg.
In the differential thermal analysis, test specimens were sampled from the aluminum alloy sheet after natural aging at ten points essentially including a tip portion, a central portion, and a rear-end portion in the longitudinal direction of the sheet. Measured values of the ten test specimens were each averaged.
The differential thermal analysis profiles (μW) obtained for each sample was divided by the test specimen weight to be standardized (μW/mg). A region where the differential thermal analysis profile leveled off in the section of 0° C. to 100° C. in the differential thermal analysis profile was defined as a reference level of zero (0). Of exothermic peaks in the temperature range of 200° C. to 300° C., the height of a highest exothermic peak was measured as the maximum height of exothermic peaks from the reference level.
The aluminum alloy sheet after natural aging was subjected to artificial aging treatment at 90° C. for 3 hr and at 140° C. for 8 hr as T6 treatment. A sheet-like test specimen was sampled from a central portion of the aluminum alloy sheet after the artificial aging treatment, on which mechanical properties and corrosion resistance were examined in manners as follows. Results of these are also each indicated in Tables 3 and 4.
Mechanical Properties
Commonly in each sample, sheet-like test specimens were each subjected to room temperature tensile test in a direction perpendicular to the rolling direction to measure a 0.2% yield strength (MPa) and a total elongation (%) as mechanical properties. The room temperature tensile test was performed in accordance with JIS Z2241 (1980) at room temperature 20° C. The test was performed at a constant tensile speed of 5 mm/minute until the test specimen was broken.
Work Hardening Coefficient n
The work hardening coefficient n was measured in the following manner. The sheet-like test specimen after the artificial aging treatment was processed into a JIS No. 5 tensile test specimen with a gauge length of 50 mm and subjected to a room temperature tensile test in a direction perpendicular to the rolling direction. A true stress and a true strain were calculated from the endpoint of yield elongation, plotted on a logarithmic scale with the abscissa indicating the strain and the ordinate indicating the stress. The gradient of a straight line indicated by measurement points was calculated between two points of nominal strain of 10% and 20%, and this was defined as the work hardening coefficient n (10% to 20%).
Intergranular Corrosion Susceptibility
To evaluate general corrosion resistance, the sheet-like test specimens after the artificial aging treatment (three test specimens per sample) were each subjected to an grain-boundary corrosion susceptibility test in accordance with former JIS-W1103. The test was performed under conditions as follows. Each test specimen was immersed in an aqueous nitric acid solution (30 percent by mass) at room temperature for one minute, immersed in an aqueous sodium hydroxide solution (5 percent by mass) at 40° C. for 20 seconds, and immersed in an aqueous nitric acid solution (30 percent by mass) at room temperature for one minute so as to clean the surface of the test specimen. The test specimen was applied with a current at a current density of 1 mA/cm²for 24 hours while being immersed in an aqueous sodium chloride solution (5 percent by mass), then raised and retrieved from the solution, cut to give a cross section, and the cross section was polished. The polished cross section was observed with an optical microscope to measure a corrosion depth from the specimen surface. The observation and measurement was performed at 100-fold magnification. A sample having a corrosion depth of 200 μm or less was evaluated as being corroded slightly and indicated with “◯”. A sample having a corrosion depth greater than 200 μm was evaluated as being corroded significantly and indicated with “x”.
As apparent from Tables 1 and 3, each of examples (samples according to the present invention) had aluminum alloy chemical compositions within the range as specified in the present invention and were produced under conditions within the preferred production conditions.
The resulting sheet was sequentially subjected to solution treatment, quenching, natural aging, and differential scanning calorimetry to plot a differential scanning calorimetric (DSC). In the DSC curve, a highest endothermic peak temperature was 130° C. or lower, and a maximum height of exothermic peak(s) in the temperature range of 200° C. to 300° C. was 50 μW/mg or more. Thus, the examples met the conditions for microstructure as analyzed based on the DSC curve.
The examples therefore had a high work hardening coefficient n (10% to 20%) of 0.22 or more even after natural aging, had excellent ductility, and offered excellent processability (formability) into a structural component. In addition, the examples had excellent bake hardening response and a high strength even after natural aging and offered satisfactory corrosion resistance.
Of these examples, examples having a chemical composition meeting the condition: [Zn]≧−0.3[Mg]+4.5 had a 0.2% yield strength of 350 MPa or more after the artificial aging treatment; and Examples 2, 4, 6, 8 to 12, 16 to 19, and 21 having a chemical composition further meeting the condition: [Zn]≧−0.5[Mg]+5.75 in addition to the former condition had a 0.2% yield strength of 400 MPa or more after the artificial aging treatment.
In contrast, each of comparative examples had an alloy chemical composition out of the range specified in the present invention and/or was produced under conditions out of the preferred ranges. The comparative examples did not have workability and a strength both at satisfactory levels, as in Tables 2 and 4.
Comparative Example 22 to 25 in Table 4 had Zn and Mg contents each within the specified range, but met neither of the conditions: [Zn]≧−0.3[Mg]+4.5 and [Zn]≧−0.5[Mg]+5.75 (see Alloy Nos. 22 to 25 in Table 2). These samples did not meet the conditions for microstructure as analyzed by the DSC curve although they were produced under the preferred production conditions including soaking conditions. The samples had a 0.2% yield strength of less than 350 MPa after the artificial aging treatment, although having a work hardening coefficient n (10% to 20%) of 0.22 or more after natural aging. Thus, the samples did not have workability and a strength both at satisfactory levels.
Comparative Example 26 in Table 4 had a Zn content less than the lower limit (see Alloy No. 26 in Table 2). Comparative Example 27 had a Zn content greater than the upper limit (see Alloy No. 27 in Table 2). Comparative Example 28 had a Cu content less than the lower limit (see Alloy No. 28 in Table 2). Comparative Example 29 had a Cu content greater than the upper limit (see Alloy No. 29 in Table 2). These comparative examples, although produced under the preferred production conditions including soaking conditions, did not meet the conditions for microstructure as analyzed by the DSC curve, had a work hardening coefficient n (10% to 20%) of less than 0.22 after natural aging, and did not have workability and a strength both at satisfactory levels. In addition, Comparative Example 27 had an excessively high Zn content, and Comparative Example 28 had an excessively low Cu content. Both the comparative examples had poor corrosion resistance.
In Table 4, Comparative Examples 30 to 32 employed Alloy No. 2 in Table 1, namely, the aluminum alloy for use in the present invention, but were produced under conditions out of the preferred production condition ranges. Comparative Example 30 underwent a single soaking (corresponding to second soaking). Comparative Example 31 underwent first soaking at an excessively low soaking temperature. Comparative Example 32 underwent second soaking at an excessively low soaking temperature. Accordingly, these comparative examples underwent soaking under conditions out of the preferred range and did not meet the conditions for microstructure as analyzed by the DSC curve, had a work hardening coefficient n (10% to 20%) of less than 0.22 after natural aging, or had a 0.2% yield strength of less than 350 MPa after the artificial aging treatment. They did not have workability and a strength both at satisfactory levels.
Comparative Example 33 in Table 4 had Zn and Mg contents each within the specified range, but met neither of the conditions: [Zn]≧−0.3[Mg]+4.5 and [Zn]≧−0.5[Mg]+5.75 (see Alloy No. 33 in Table 2). This sample did not meet the conditions for microstructure as analyzed by the DSC curve, although produced under the preferred production conditions including soaking conditions. The sample thereby had a work hardening coefficient n (10% to 20%) on the order of 0.22 after natural aging, but a 0.2% yield strength of less than 350 MPa after the artificial aging treatment, and did not have workability and a strength both at satisfactory levels.
Comparative Examples 34, 35, and 36 in Table 4 each had a Mg content lower than the lower limit (see Alloy Nos. 31, 32, and 33 in Table 2). These samples therefore failed to meet the conditions for microstructure as analyzed by the DSC curve even though they met both the conditions: [Zn]≧−0.3[Mg]+4.5 and [Zn]≧−0.5[Mg]+5.75, and/or even though they were produced under the preferred production conditions including soaking conditions. As a result, the samples had a work hardening coefficient n (10% to 20%) on the order of 0.21 to 0.22 after natural aging, but a 0.2% yield strength of less than 350 MPa after the artificial aging treatment, and did not have workability and a strength both at satisfactory levels.
Comparative Examples 37, 38, and 39 in Table 4 had a Zn content greater than the upper limit (see Alloy Nos. 34, 35, and 36 in Table 2). The samples therefore failed to meet the conditions for microstructure as analyzed by the DSC curve even though they met both the conditions: [Zn]≧−0.3[Mg]+4.5 and [Zn]≧−0.5[Mg]+5.75 and/or even though they were produced under the preferred production conditions including soaking conditions. As a result, the samples had a work hardening coefficient n (10% to 20%) on the order of 0.21 after natural aging and did not have workability and a strength both at satisfactory levels. In addition, these comparative examples also had poor corrosion resistance due to the excessively high Zn content.
Comparative Examples 40 to 43 in Table 4 had a Mg content greater than the upper limit (see Alloy Nos. 37 to 40 in Table 2). The samples failed to meet the conditions for microstructure as analyzed by the DSC curve even though they met both the conditions [Zn]≧−0.3[Mg]+4.5 and [Zn]≧−0.5[Mg]+5.75 and/or even though they were produced under the preferred production conditions including soaking conditions. As a result, Comparative Examples 40 and 41 having a relatively high Zn content had a low work hardening coefficient n (10% to 20%) on the order of 0.21 after natural aging and did not have workability and a strength both at satisfactory levels. Comparative Examples 42 and 43 having a relatively low Zn content had a work hardening coefficient n (10% to 20%) on the order of 0.22 after natural aging, but a 0.2% yield strength of less than 350 MPa after the artificial aging treatment, and did not have workability and a strength both at satisfactory levels. These comparative examples also had poor corrosion resistance due to the excessively high Mg content.
These results support critical significance of the individual conditions specified in the present invention so as to allow the aluminum alloy sheets according to the present invention to have all of high strength, good ductility (formability), and satisfactory SCC resistance.

	TABLE 1

	Aluminum alloy chemical composition in mass percent (the remainder: Al)

[Zn] ≧ −0.3

[Zn] ≧ −0.5

Claims

1. An aluminum alloy sheet for structural components, the aluminum alloy sheet being an Al—Zn—Mg alloy sheet and comprising:

in terms of a chemical composition in mass percent,

Zn in a content of 3.0% to 6.0%;

Mg in a content of 1.5% to 4.5%; and

Cu in a content of 0.05% to 0.5%,

the Zn content [Zn] and the Mg content [Mg] meet a condition as specified by:

[Zn]≧−0.3[Mg]+4.5,

with the remainder consisting of Al and inevitable impurities,

wherein, when the sheet is subjected sequentially to solution treatment, quenching, and natural aging, and a differential scanning calorimetric curve is plotted after the natural aging, a highest endothermic peak temperature is 130° C. or lower, and a maximum height of exothermic peak(s) in a temperature range of 200° C. to 300° C. is 50 μW/mg or more in the differential scanning calorimetric curve, and a work hardening coefficient n (10% to 20%) is 0.22 or more.

2. The aluminum alloy sheet for structural components according to claim 1, wherein the aluminum alloy sheet further comprises at least one element selected from the group consisting of:

in mass percent,

Zr in a content of 0.05% to 0.3%;

Mn in a content of 0.1% to 1.5%;

Cr in a content of 0.05% to 0.3%; and

Sc in a content of 0.05% to 0.3%.

3. The aluminum alloy sheet for structural components according to claim 1,

wherein the aluminum alloy sheet further comprises in mass percent, Ag in a content of 0.01% to 0.2%.

4. The aluminum alloy sheet for structural components according to claim 1,

wherein the Zn content [Zn] and the Mg content [Mg] in the aluminum alloy sheet meet a condition as specified by formula:

[Zn]≧−0.5[Mg]+5.75, and

wherein a 0.2% yield strength after artificial aging treatment is 400 MPa or more.