JP2019183264A - High strength aluminum alloy, aluminum alloy sheet and aluminum alloy member using the aluminum alloy - Google Patents

Abstract

To provide an aluminum alloy especially excellent in strength without reducing moldability in T4 conditioning, and an aluminum alloy sheet and an aluminum alloy member using the aluminum alloy.SOLUTION: There is provided a high strength aluminum alloy containing, by mass%, Mg:0.3 to 2.0%, Si:0.3 to 2.0%, Zn:2.2 to 8.0% and the balance Al with impurities.SELECTED DRAWING: None

Description

  The present invention relates to an aluminum alloy particularly excellent in strength.

As is well known, various aluminum alloy plates (hereinafter referred to as aluminum as Al) have been conventionally used for transportation equipment such as automobiles, ships, aircraft or vehicles, machines, electrical products, architecture, structures, optical equipment, and members and parts of equipment. It is also widely used as a high-strength and lightweight material. However, although the mechanical properties of these aluminum alloys have improved dramatically in recent years, they are still not sufficient as compared with high-strength steels and the like.
In recent years, due to consideration for the global environment, social demands for reducing the weight of automobile bodies are increasing. In order to respond to such demands, parts such as panels (outer panels such as hoods, doors and roofs, inner panels), bumper force (bumper R / F) and reinforcing materials such as door beams, etc. An aluminum alloy material is applied instead of a steel material such as a steel plate.

  In order to further reduce the weight of automobile bodies, the application of aluminum alloy materials has been expanded to members such as side members, frames, and automobile structural members such as pillars that contribute to weight reduction among automotive parts. It is necessary to do. However, it is necessary to give these automobile structural members new strength as a new characteristic as compared with automobile panel materials.

As a high-strength reinforcing material in the automobile structural member, an extruded profile produced by hot extrusion of a JIS or AA7000 series aluminum alloy is already widely used as a material.
On the other hand, large structural members such as frames and pillars are preferably made of a rolled plate manufactured by a conventional method such as hot rolling after soaking or further cold rolling the ingot. . However, a 7000 series aluminum alloy is not practically used as a rolled plate because of its low formability in T4 refining.

  For this reason, JIS thru | or AA6000 type | system | group aluminum alloy which is an Al-Mg-Si type | system | group aluminum alloy excellent in a moldability rather than 7000 type | system | group as an alloy for rolling plates manufactured by normal rolling (regular method) attracts attention. Has been.

  This 6000 series aluminum alloy plate is already used as a large body panel (outer panel or inner panel such as a hood, fender, door, roof, trunk lid, etc.) of an automobile. For this reason, many metallurgical improvement measures such as composition and structure have been proposed in order to combine and improve the press formability and BH properties (bake hardness) required for large body panels of these automobiles. ing.

  However, as for the reinforcing material and the like, a 6000 series aluminum alloy extruded shape has been proposed and put into practical use. On the other hand, there are not many proposed examples of automotive structural members for aluminum alloy rolled sheets. For example, as a structure of a rolled aluminum alloy plate, a 6000 series aluminum alloy plate having improved crushing characteristics by controlling the size and aspect ratio of crystal grains and setting the 0.2% proof stress after artificial aging heat treatment (baking hard) to 230 MPa or more. However, this is the extent proposed in Patent Document 1.

JP 2001-294965 A

  As the reason, it is conceivable that the formability and strength of the aluminum alloy plate are in a trade-off relationship. That is, in a 6000 series aluminum alloy, high strength such as 0.2% proof stress after baking hard is 350 MPa or more, which is necessary when applying to automobile structural members while ensuring formability at T4 refining. It was very difficult to get.

  In view of such a situation, an object of the present invention is to provide an aluminum alloy particularly excellent in strength without deteriorating formability in T4 refining.

  As a result of intensive studies by the present inventors in order to solve the above-mentioned problems, it is possible to appropriately control the chemical composition of the aluminum alloy, in particular, to contain Zn in a predetermined amount or more, so that formability in T4 refining can be achieved. It has been found that high strength can be obtained without lowering.

  That is, the aluminum alloy according to the present invention includes, in mass%, Mg: 0.3 to 2.0%, Si: 0.3 to 2.0%, Zn: 2.2 to 8.0%, and the balance Consists of Al and impurities.

The aluminum alloy which concerns on preferable embodiment of this invention is the mass%, and content of the said Zn is 2.8-8.0%.
The aluminum alloy according to a preferred embodiment of the present invention further contains at least one of Mn: 0.01 to 0.5% and Cu: 0.002 to 1.0% by mass.
The aluminum alloy which concerns on preferable embodiment of this invention has the bake hard property which 0.2% proof stress becomes 350 Mpa or more after implementing the artificial aging treatment for 8 to 24 hours at the temperature of 170 degreeC.

Moreover, the aluminum alloy plate according to the present invention is characterized by using any one of the above aluminum alloys.
The aluminum alloy member according to the present invention is characterized by using the above-described aluminum alloy or the above-described aluminum alloy plate.

  According to the present invention, it is possible to provide an aluminum alloy particularly excellent in strength without deteriorating formability in T4 refining.

FIG. 1 is an optical micrograph of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9 described later (drawing substitute photo). FIG. 2 is a transmission electron microscope (TEM) photograph of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9 described later (drawing substitute photograph).

  The present inventors have conducted intensive studies in order to provide an aluminum alloy that is particularly excellent in strength without degrading the formability in T4 refining. As a result, by containing Zn in a predetermined amount or more as an alloy component (chemical composition) in a conventional 6000 series aluminum alloy, the strength can be greatly improved while ensuring high formability in T4 refining. The present inventors have found that this can be done and have completed the present invention.

Hereinafter, an aluminum alloy, an aluminum alloy plate, and an aluminum alloy member according to an embodiment (this embodiment) of the present invention will be specifically described for each requirement. In addition, the description shown below illustrates the aluminum alloy, the aluminum alloy plate, and the aluminum alloy member for embodying the technical idea of this embodiment, and is not limited to the following embodiment.
In the present embodiment, an aluminum alloy plate (rolled material) is shown as an application example of an aluminum alloy, but it can also be applied to plastic working materials such as extruded materials, cast materials, and forged materials.

<Chemical composition of aluminum alloy>
First, the chemical component composition of the aluminum alloy according to the present embodiment will be described below, including reasons for limiting each element. In addition,% display of content of each element means the mass% altogether. Further, “to” means that the value is not less than the lower limit value and not more than the upper limit value.

(Mg: 0.3-2.0%)
As is well known in conventional 6000 series alloys, Mg, together with Si, is an aging precipitate such as Mg-Si series precipitate that contributes to strength improvement during solid solution strengthening and artificial aging heat treatment such as baking coating treatment. Is an essential element for obtaining age-hardening ability and obtaining strength (proof strength) necessary for an automobile structural member, for example.
If the Mg content is too small, the amount of solid solution Mg before baking coating treatment is reduced, and the amount of Mg-Si-based precipitates is insufficient, so that the BH property is remarkably lowered and the strength is insufficient. Therefore, the Mg content is 0.3% or more, preferably 0.5% or more, and more preferably 0.6% or more.
On the other hand, when there is too much Mg content, it will become easy to form a shear band at the time of cold rolling, and will cause the crack at the time of a raw material plate rolling. The Mg content is 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less.

(Si: 0.3-2.0%)
As is well known in conventional 6000 series alloys, Si and Mg, as well as aging precipitates such as Mg-Si series precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatment such as baking coating treatment. Is an essential element for exhibiting age-hardening ability and obtaining strength necessary for, for example, an automobile structural member.
If the Si content is too small, the amount of solute Si before baking coating treatment (before artificial aging heat treatment) decreases, and the amount of Mg-Si-based precipitates is insufficient. Run short. The Si content is 0.3% or more, preferably 0.5% or more, and more preferably 1.0% or more.
On the other hand, when there is too much Si content, a coarse crystallized substance and a precipitate will be formed, ductility will fall, and it will become the cause of the crack in the case of raw-material plate rolling. The Si content is 2.0% or less, preferably 1.6% or less, and more preferably 1.4% or less.

(Zn: 2.2 to 8.0%)
Conventionally, in order to improve the strength of a 6000 series aluminum alloy, it has been reported that it is important to precipitate a β ″ phase or β ′ phase, which is a strengthened layer, with high density and fineness by artificial aging treatment (for example, specially No. 2017-179469).
Here, the present inventors have found that the β ″ phase, which is a strengthening layer, can be precipitated with high density and fineness by containing a predetermined amount or more of Zn in the 6000 series aluminum alloy. As can be seen from the optical microscope photograph (see FIG. 1) and the transmission electron microscope (TEM) photograph (see FIG. 2) in the aluminum alloy plates of Example 1 and Comparative Examples 7 to 9, as in Example 1. In an aluminum alloy containing a large amount of Zn (Zn content: 4.54 mass%), it can be seen that the acicular precipitates (Mg ₂ Si) observed in the 6000 series aluminum alloy are finer. .

  The reason why the β ″ phase can be precipitated with high density and fineness by adding a predetermined amount or more of Zn to the 6000 series aluminum alloy is not clear, but contains a lot of Zn based on the 6000 series aluminum alloy. By doing so, the solid solubility of the solute atoms increased, that is, the degree of supersaturation increased, and using this as the driving force, the precipitation of the β ”phase, which is the strengthening layer, was promoted, and it was considered that a highly dense and fine precipitation state was obtained It is done.

If the Zn content is too small, the strength of the β ″ phase, which is a reinforcing layer, cannot be sufficiently finely and finely deposited, resulting in a decrease in strength. The Zn content is 2.2% or more, preferably 2 .4% or more, and more preferably 2.8% or more.
On the other hand, when there is too much Zn content, as a premise for manufacturing the aluminum alloy plate using the aluminum alloy of this embodiment, a soaking process and a solution treatment cannot be performed because the melting point decreases. The Zn content is 8.0% or less, preferably 7.0% or less, and more preferably 6.0% or less.

  For example, in JP 2017-48451A, an aluminum alloy plate containing Zn: 0.44 to 0.60 mass%, or in Patent No. 4060952, Zn: 0.05% or more and less than 0.3% is contained. There exists an aluminum alloy to which a small amount of Zn is added, such as an aluminum alloy plate. However, as described above, it is important for the aluminum alloy according to the present embodiment to contain a large amount of Zn of 2.2% or more. As a result, as found by the present inventors, a specific action of precipitating the β ″ phase, which is a reinforcing layer, with high density and fineness is achieved, and high strength can be obtained without degrading moldability. .

(At least one of Mn and Cu)
Since these elements have the effect of increasing the strength of the plate in common, they can be regarded as synergistic elements. However, the specific mechanism has a common part but a different part.
Mn contributes to strength improvement by refining the crystal grains of the ingot and the final plate product. Further, these elements exist as dispersed particles, contribute to crystal grain refinement, and improve moldability. If the Mn content is too small, the effect of improving strength and formability due to crystal grain refinement is insufficient. On the other hand, when there is too much content of Mn, a coarse compound will be formed and ductility will be degraded.
Cu improves the strength by solid solution strengthening, and can obtain the proof stress necessary for an automobile structural member, for example. If there is too little content of Cu, the effect will be small, and even if there is too much. The effect is saturated and on the contrary, the corrosion resistance is deteriorated.
Therefore, these elements are not essential, but at least one of them can be contained in the content ranges of Mn: 0.01 to 0.5% and Cu: 0.002 to 1.0%. The minimum of preferable content of Mn is 0.03%. A more preferable content of Mn is 0.05% or more. The upper limit of the preferable content of Mn is 0.4%. The more preferable content of Mn is 0.3% or less. The minimum of preferable content of Cu is 0.01%. The more preferable content of Cu is 0.05% or more. The upper limit of the preferable content of Cu is 0.5%. The more preferable content of Cu is 0.2% or less.

(Other elements)
Other than these elements, other elements such as Ti, B, Fe, Cr, V, and Zr are impurities. Ti, together with B, forms a coarse compound and degrades mechanical properties. However, since the inclusion of a small amount also has the effect of refining the crystal grains of the aluminum alloy ingot, each content in the range specified by the JIS standard is allowed as a 6000 series alloy. As an example of this allowable amount, Ti is 0.1% or less, preferably 0.05% or less. Further, B is set to 0.03% or less.

  In addition, other elements such as Fe, Cr, V, and Zr are assumed (allowed) to be mixed with these impurity elements by using aluminum alloy scrap in addition to pure aluminum ingot as a raw material for melting ingots. , 6000 series alloys are allowed to be contained within the range specified by JIS standards. As an example of the allowable amount, Fe is 0.5% or less, Cr is 0.3% or less, V is 0.2% or less, and Zr is 0.2 or less.

The aluminum alloy according to the present invention is applicable to wrought materials (rolled material, extruded material, cast material, forged material), and in order to obtain higher strength, The thickness is preferably smaller. Specifically, the crystal grain size is preferably 100 μm or less. Furthermore, 70 μm or less is more preferable. In addition, the aspect ratio indicating the degree of segregation of crystal grains is preferably a certain value or more. Specifically, the aspect ratio is preferably 0.35 or more. The crystal grain size and aspect ratio should be adjusted according to the amount of components such as Zn, Mn, and Cu, or processing conditions (for example, the rolling rate of hot rolling and cold rolling in the case of plates) and annealing conditions. Is possible.
<Aluminum alloy plate>
The aluminum alloy plate (molding material plate) referred to in this embodiment is a rolled plate such as a hot rolled plate or a cold rolled plate, and is subjected to tempering (T4) such as solution treatment and quenching treatment. This is a material aluminum alloy plate before being formed into an automobile structural member to be used and before being subjected to artificial aging treatment (artificial age hardening treatment) such as paint baking hardening treatment.

(Aluminum alloy plate thickness)
The lower limit of the thickness of the aluminum alloy plate of the present embodiment is not particularly limited, but in order to have necessary strength and rigidity when applied to an automobile structural member, the thickness is, for example, 1. It is 5 mm or more. Also, the upper limit of the plate thickness is not particularly limited, but considering the limit of forming process such as press forming and the range of weight increase that does not impair the lightening effect from the steel plate as a comparative material, for example, 4.0 mm or less is there. From this thickness range, a hot-rolled plate or a cold-rolled plate is appropriately selected.

(Aluminum alloy plate manufacturing method)
The aluminum alloy sheet of the present embodiment is a cold-rolled sheet that is hot-rolled after the soaking heat treatment of the ingot and further cold-rolled, and is further subjected to tempering such as solution treatment, and is manufactured by a conventional method. Is done. That is, an aluminum alloy hot rolled sheet having a thickness of about 2 to 10 mm is manufactured through normal manufacturing processes such as casting, homogenization heat treatment, and hot rolling. Subsequently, it is cold-rolled to obtain a cold-rolled sheet having a thickness of 4 mm or less. Hereinafter, each step will be described in more detail.

[Melting, casting]
First, in the melting and casting process, the molten aluminum alloy adjusted to be dissolved within the above component composition range is cast by appropriately selecting a normal melting casting method such as a continuous casting method or a semi-continuous casting method (DC casting method). .

[Homogenization heat treatment]
Next, the cast aluminum alloy ingot is subjected to a homogenization heat treatment prior to hot rolling. This homogenization heat treatment (soaking) is important for sufficiently dissolving Mg and Si in addition to homogenizing the structure, that is, eliminating segregation in crystal grains in the ingot structure. As long as this condition is achieved, normal one-time soaking may be used, or two-step soaking or two-stage soaking may be employed. In one soaking, the hot rolling is started by cooling to the hot rolling start temperature or holding at or near the hot rolling start temperature.

  The soaking condition for the first soaking in only one time, or the first soaking in the second soaking, or the soaking condition in the second soaking is in the temperature range of 500 ° C. or higher and lower than the melting point, and the holding time range of 2 hours or more. Is appropriately selected.

[Hot rolling]
The ingot subjected to the homogenization heat treatment is hot-rolled to obtain a hot-rolled sheet having a thickness of about 2 to 10 mm. Hot rolling is composed of an ingot (slab) rough rolling step and a finish rolling step in accordance with the thickness of the rolled sheet. In these rough rolling process and finish rolling process, a reverse or tandem rolling mill is appropriately used.

  The hot rolling start temperature as the hot rolling start temperature is preferably 350 ° C. or higher and the solidus temperature or lower in the first soaking step, and 350 ° C. or higher and 400 ° C. or lower in the second soaking step. If the starting temperature of hot rough rolling is less than 350 ° C., hot rolling becomes difficult in any soaking process material, and conversely, if it exceeds 400 ° C., transition element-based dispersed particles become coarse in the 2-soaking process material. It may precipitate and deteriorate the properties of the plate.

After such hot rough rolling, hot finish rolling is preferably performed with an end temperature in the range of 250 to 350 ° C. When the finish temperature of this hot finish rolling is too low, such as less than 250 ° C., the rolling load becomes high and the productivity is lowered. On the other hand, in order to make a recrystallized structure without leaving a lot of processed structure, when the finishing temperature of hot finish rolling is increased, if this temperature exceeds 350 ° C., transition element-based dispersed particles may be coarsely precipitated. Get higher.
Annealing (roughening) of the hot-rolled sheet before cold rolling is not necessary, but may be performed.

[Cold rolling]
In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final thickness. However, in order to further refine the crystal grains, the cold rolling rate is desirably 30% or more, and intermediate annealing may be performed between the cold rolling passes for the same purpose as the above roughening. .

[Solution and quenching]
After the cold rolling, solution treatment and subsequent quenching to room temperature are performed. For this solution hardening treatment, a normal continuous heat treatment line may be used. However, in order to obtain a sufficient solid solution amount of each element such as Mg and Si, after the solution treatment at a temperature of 480 ° C. or higher and a melting temperature or lower, the average cooling rate to room temperature is 10 ° C./second or higher. It is preferable to do.

At a temperature lower than 480 ° C., re-solid solution of a compound such as an Mg—Si compound generated before the solution treatment becomes insufficient, and the solid solution Mg amount and the solid solution Si amount decrease.
In addition, when the average cooling rate is less than 10 ° C./second, Mg—Si based precipitates are mainly generated during cooling, and the solid solution Mg amount and the solid solution Si amount are decreased. The possibility that the amount cannot be secured increases. In order to ensure this cooling rate, the quenching treatment is performed by selecting water cooling means and conditions such as air cooling such as a fan, mist, spray, and immersion, respectively.

[Pre-aging treatment: Reheating treatment]
After such a solution treatment, it is preferable to quench the steel sheet and cool it to room temperature, and then subject the cold-rolled sheet to a pre-aging treatment (reheating treatment) within one hour. After the quenching process to room temperature, if the room temperature holding time until the start of pre-aging treatment (heating start) is too long, Si-rich Mg-Si clusters are generated due to room temperature aging, and the balance between Mg and Si is good It becomes difficult to increase Mg-Si clusters. Accordingly, the shorter the room temperature holding time is better, the solution treatment and quenching treatment and the reheating treatment may be continued so that there is almost no time difference, and the lower limit time is not particularly set.

  In this preliminary aging treatment, the holding time at 60 to 120 ° C. is preferably held for 5 hours or more and 40 hours or less. As a result, Mg—Si clusters having a good balance between Mg and Si are formed.

When the preliminary aging temperature is less than 60 ° C. or the holding time is less than 10 hours, the Si-rich Mg—Si cluster is suppressed as in the case where the preliminary aging treatment is not performed, and the balance between the Mg and Si is reduced. However, it is difficult to increase the Mg-Si cluster, and the proof stress after baking coating tends to be low.
On the other hand, if the pre-aging condition exceeds 120 ° C. or exceeds 40 hours, the amount of precipitation nuclei is too much, the strength at the time of molding before baking coating becomes too high, and the moldability tends to deteriorate. .

  Through the above steps, the material plate (aluminum alloy plate) or the aluminum alloy member formed from the material plate is subjected to an artificial aging treatment at a temperature of 170 ° C. for 8 to 24 hours, for example. This can be done.

  Then, the aluminum alloy plate of the present embodiment is subjected to press molding or processing including burring processing, hole expansion processing, etc. as a raw material, for example, structural members such as automobiles, bicycles, railway vehicles, and automobiles Outer panel materials (exterior materials) such as hoods, fenders, doors, and roofs. Further, from the viewpoint of securing formability, after being formed or processed into these structural members, artificial aging heat treatment is separately performed to increase the strength as necessary.

[Artificial aging heat treatment]
This artificial aging heat treatment (artificial age hardening treatment or bake hard treatment) may be performed at the stage of the material plate. As usual, after forming the material plate into a structural member, baking after painting is performed. You may carry out also as a hardening process.
General artificial aging conditions (T6, T7) may be used, and the temperature and time conditions are freely determined from the desired strength, the strength of the material 6000 series aluminum alloy plate, the progress of room temperature aging, and the like. For example, in the case of a one-stage artificial aging heat treatment, an aging treatment is preferably performed in the range of heating temperature: 150 to 225 ° C. × holding time: 20 minutes to 72 hours.

<Aluminum alloy member>
The aluminum alloy member referred to in the present embodiment includes a panel member (outer panel, inner panel, etc.) for a transport device such as an automobile, a member such as a side member, a frame, a pillar, etc. used in a transport device such as an automobile. Aluminum alloy members using the above-mentioned aluminum alloy plate such as members, energy absorbing members such as bumper hose and door beam are widely included.
However, in order to fully demonstrate the performance of the aluminum alloy of the present embodiment, which is particularly excellent in strength, without lowering the formability in T4 refining, it is necessary to further increase the strength of the material compared to panel materials. Application to structural members is desirable.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

An aluminum alloy ingot having each component composition shown in Table 1 below was prepared to produce an aluminum alloy plate. In addition, the obtained aluminum alloy plate is observed for crystal grain structure, the average crystal grain size and aspect ratio are calculated, and the tensile strength after artificial aging treatment (for each condition, see Table 1). The strength and formability of the aluminum alloy sheet were evaluated by measuring the thickness (MPa), 0.2% proof stress (MPa), and elongation at break (%). These results are also shown in Table 1.
In Table 1, “-” in the column of “Chemical composition of aluminum alloy plate” indicates that the content of Cu was less than 0.002%, and that of Zn was 0.01% or less. In Table 1, “-” in the “crystal grain structure” column indicates an unmeasured one.

<Preparation of aluminum alloy plate>
First, manufacturing conditions will be described. In each example, the specific production conditions of the aluminum alloy plate were made common as follows. That is, an aluminum alloy ingot having each chemical composition shown in Table 1 was melted by a die casting method. Subsequently, the ingot was subjected to soaking treatment at a heating rate of 40 ° C./hr and a soaking temperature of 540 ° C. × 4 hours.
Thereafter, hot rough rolling was immediately performed to obtain a hot rolled plate having a thickness of 5.0 mm.
This hot-rolled sheet was cold-rolled with a processing rate of 60% without rough annealing after hot rolling or intermediate annealing in the middle of the cold-rolling pass to obtain a cold-rolled sheet having a thickness of 2.0 mm.

Further, each cold-rolled plate was tempered (T4) with heat treatment equipment. Specifically, the solution treatment is performed in an atmospheric furnace at 550 ° C. for 15 minutes, and after the solution treatment, the solution is cooled to room temperature by water cooling with an average cooling rate of 20 ° C./second or more, and the quenching treatment is performed. went.
After this quenching process was completed, the test piece was held at room temperature for 7 days to obtain a test piece of T4 material (material plate). These T4 materials were processed into JIS13B tensile test pieces, and materials subjected to artificial aging treatment under various conditions (attainment temperature, holding time) shown in Table 1 were used as test pieces.

<Observation of grain structure>
The crystal grain structure of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9 was observed using an optical microscope. The sample used for observation was a solution-treated plate, resin-filled so that the observation surface had a longitudinal section parallel to the plate rolling direction, and finished to a mirror surface by mechanical polishing. Thereafter, electrolytic etching was performed using a 2% HBF ₄ aqueous solution. For each of the obtained samples, the crystal grain structure in the central portion of the plate thickness was observed using an optical microscope (manufactured by Olympus Corporation, trade name: PMG3) at an observation magnification of 100 times. FIG. 1 shows an optical micrograph of each test piece (aluminum alloy plate).
Moreover, from the obtained optical micrograph, an average crystal grain size (μm) was calculated using a section method. Specifically, the slice length was measured by N10 in the vertical direction (plate thickness direction) and the horizontal direction (rolling direction) of the optical micrograph, and the average slice length was calculated. Then, in the measurement results, the total average in the vertical direction and the horizontal direction is defined as the average grain size (μm), and the ratio of the average intercept length in the longitudinal direction to the transverse direction (that is, the average intercept length in the longitudinal direction / the transverse direction The average section length) was taken as the aspect ratio.

<Observation of precipitate structure>
The precipitate structure in the center part of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9 is converted into the (001) direction by the transmission electron microscope (TEM) with a magnification of 200,000 times. Observed by tilting. The sample used for observation was a sample taken from a cross-sectional structure parallel to the surface of the plate as a plate after solution treatment. The sample was taken as a cross section parallel to the surface of the aluminum alloy plate by cutting an arbitrary part of the aluminum alloy plate so that the plate surface becomes a polished surface. And after carrying out mechanical polishing and electrolytic polishing so that an observation part may become a plate | board thickness center part, after manufacturing the thin film sample for TEM, the precipitation structure | tissue was image | photographed by TEM of 200,000 times magnification. FIG. 2 shows a TEM photograph of each test piece (aluminum alloy plate).

As described above, as can be seen from the optical microscope photograph (FIG. 1) and the transmission electron microscope (TEM) photograph (FIG. 2) of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9, as in Example 1. In an aluminum alloy containing a large amount of Zn (Zn content: 4.54 mass%), it can be seen that the acicular precipitates (Mg ₂ Si) observed in the 6000 series aluminum alloy are finer. .

<Strength Evaluation: Measurement of Tensile Strength and 0.2% Yield Strength>
The tensile test is performed on the T4 material JIS13B test piece and the aging material JIS13B test piece obtained by artificial aging heat treatment without applying strain. Tensile strength and 0.2% yield strength (MPa) of each material (after artificial aging treatment) were measured.
About the test method, based on JIS2241 (1980), No. 13 B test piece (20 mm x 200 mm x board thickness (2.0 mm)) was extract | collected, and the test was done at room temperature (20 degreeC). At this time, the tensile direction of the test piece was the direction perpendicular to the rolling direction. The tensile speed was 5 mm / min up to 0.2% proof stress, and 20 mm / min after 0.2% proof stress.
In this example, for the aluminum alloy structural member, the 0.2% proof stress in the test piece of aging material (that is, after artificial aging treatment) passed 350 MPa or more (“Aging characteristics” in Table 1 “ 0.2% yield strength ").

<Evaluation of formability: measurement of elongation at break>
JIS13B tensile test pieces (12.5 mm × 50 mmGL × 2.0 mm) were sampled from the test plates (T4 material and aging material), respectively, and subjected to a tensile test at room temperature under the following conditions. In the tensile test, a tensile tester was used to pull the test piece at a speed of 5 mm / min, and after 0.2% proof stress at a speed of 20 mm / min, and measure the elongation when the test piece was cut (broken).
The tensile direction of the test piece was the direction perpendicular to the rolling direction, and the value calculated by the following formula was the elongation at break. In the following formula, Lo is the distance between the gauge points (GL) before the tensile test, and L is the distance between the gauge points at the time of fracture.
Elongation at break (%) = 100 × (L-Lo) / Lo
In this example, if the elongation at break in the test piece of T4 material (that is, before the artificial aging treatment) is 20% or more, it is determined that the material has sufficient formability, and passed (Table 1). (See “Elongation at Break” in “T4 Mechanical Properties”).

As shown in the results of Table 1, in Examples 1 to 4, the chemical composition of the aluminum alloy is within the scope of the present invention. For this reason, the breaking elongation at T4 tempering satisfied 20% or more, and the 0.2% proof stress at T6 tempering satisfied 350 MPa or more, and the strength and formability were excellent.
On the other hand, in Comparative Examples 1-9, since Zn content was less than 2.2% prescribed | regulated by this invention, 0.2% yield strength in T6 refining becomes less than 350 Mpa, and obtains sufficient intensity | strength. I could not.
Furthermore, in Comparative Example 10, although the content of Zn satisfies 2.2 to 8.0% defined in the present invention, the content of Mg and Si was less than the lower limit specified in the present invention. The 0.2% proof stress in tempering was less than 350 MPa, and sufficient strength could not be obtained.

<Evaluation when the conditions of artificial aging treatment are changed>
Each aging material in which the conditions of the artificial aging treatment were changed as shown in Table 2 on the basis of the JIS13B test piece of T4 material having the chemical composition of the aluminum alloy plate used in Example 1. Prepared. Then, about each prepared aging material, the tensile strength, the 0.2% yield strength, and elongation at break were measured similarly to the above.
Similarly, based on the JIS13B test piece of T4 material having the chemical composition of the aluminum alloy plate used in Comparative Example 7, the conditions of the artificial aging treatment were changed as shown in Table 2 for this test piece. Each aging material was prepared. Then, about each prepared aging material, the tensile strength, the 0.2% yield strength, and elongation at break were measured similarly to the above.
In Table 2, “-” indicates that the chemical composition was below the detection limit.

As shown in the results of Table 2, when the chemical composition of the aluminum alloy is within the scope of the present invention (Examples in Table 2), the retention time in the artificial aging treatment is in a wide range of 8 to 24 hours, and the T6 tone is adjusted. It was found that 0.2% proof stress in quality has a high strength of 350 MPa or more.
On the other hand, in Table 2, by comparison with Examples and Comparative Examples in which the retention time of the artificial aging treatment is the same, the effect of improving the strength by adding Zn can be obtained even when the artificial aging treatment time is less than 8 hours and more than 24 hours. It was confirmed that it was obtained.
That is, in order to obtain a high strength with a 0.2% proof stress at T6 tempering of 350 MPa or more, it is limited to a specific artificial aging treatment time. By containing more than a predetermined amount, the strength can be greatly improved.
From the above, it has been experimentally shown that the aluminum alloy according to the present invention can ensure a very high strength within the range of the usually assumed artificial aging treatment conditions.

  According to the present invention, it is possible to ensure high strength, which is a characteristic particularly required for structural member applications, without reducing the formability at T4 tempering in a 6000 series aluminum alloy produced by ordinary rolling. it can. For this reason, application of a 6000 series aluminum alloy plate can be expanded over aluminum alloy members in general including structural members, such as an automobile structural member.

Claims (6)

  It is characterized by containing Mg: 0.3-2.0%, Si: 0.3-2.0%, Zn: 2.2-8.0%, and the balance consisting of Al and impurities. Aluminum alloy.   The aluminum alloy according to claim 1, wherein the Zn content is 2.8 to 8.0% by mass%.   Furthermore, the aluminum alloy of Claim 1 or 2 which contains at least 1 type in Mn: 0.01-0.5% and Cu: 0.002-1.0% by mass%.   It has the bake hard property which 0.2% proof stress becomes 350 Mpa or more after implementing the artificial aging treatment for 8 to 24 hours at the temperature of 170 degreeC, The any one of Claims 1-3 characterized by the above-mentioned. Aluminum alloy.   The aluminum alloy plate using the aluminum alloy of any one of Claims 1-4.   The aluminum alloy member using the aluminum alloy of any one of Claims 1-4, or the aluminum alloy plate of Claim 5.