JP6810178B2 - High-strength aluminum alloy and its manufacturing method, aluminum alloy plate and aluminum alloy member using the aluminum alloy - Google Patents

Description

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

As is well known, various aluminum alloy plates (hereinafter referred to as "aluminum") have been conventionally used for transport machines such as automobiles, ships, aircraft or vehicles, machinery, electrical products, buildings, structures, optical instruments, and parts and parts of equipment. (Also called), but it is 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 compared to high-strength steels and the like.
In recent years, due to consideration for the global environment, the social demand for weight reduction of automobile bodies has been increasing more and more. In order to meet such demands, the panels (outer panels such as hoods, doors, roofs, inner panels), bumper reinforcements (bumper R / F), and reinforcing materials such as door beams have been added to the parts of the automobile body. Aluminum alloy materials are being applied instead of steel materials such as steel plates.

In order to further reduce the weight of automobile bodies, we will expand the application of aluminum alloy materials to members such as side members, frames, and automobile structural members such as pillars, which contribute to weight reduction in particular. It is necessary to do. However, it is necessary to impart new characteristics to these automobile structural members to further increase the strength of the material as compared with the automobile panel material.

As a high-strength reinforcing material for the automobile structural member, an extruded shape material produced by hot extrusion processing of a JIS to AA7000 series aluminum alloy has already been widely used as a material.
On the other hand, for large structural members such as frames and pillars, it is preferable to use a rolled plate manufactured by a conventional method such that the ingot is hot-rolled after soaking heat treatment or further cold-rolled. .. However, the 7000 series aluminum alloy has low formability in T4 tempering as a rolled plate, and has not been put into practical use very much.

For this reason, as an alloy for rolled plates manufactured by ordinary rolling (conventional method), JIS to AA6000 series aluminum alloys, which are Al-Mg-Si based aluminum alloys having better formability than 7000 series, are attracting attention. Has been done.

This 6000 series aluminum alloy plate has already been used as a large body panel (outer panel and inner panel of 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 press moldability and BH property (bake hard property) required for large body panels of these automobiles. ing.

However, while the 6000 series aluminum alloy extruded profile has been proposed and put into practical use for the above reinforcing materials, there are not many proposed examples of aluminum alloy rolled plates in automobile structural members. For example, as the structure of an aluminum alloy rolled plate, a 6000 series aluminum alloy plate with 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 (bake hard) to 230 MPa or more. However, it is only proposed in Patent Document 1.

Japanese Unexamined Patent Publication No. 2001-294965

The reason may be that there is a trade-off relationship between formability and strength in the aluminum alloy plate. That is, in the 6000 series aluminum alloy, high strength such as 0.2% proof stress after baking hard of 350 MPa or more, which is necessary for application to automobile structural members while ensuring moldability in T4 tempering. It was very difficult to obtain.

In view of such a situation, an object of the present invention is to provide an aluminum alloy having particularly excellent strength without lowering the moldability in T4 tempering.

As a result of diligent studies by the present inventors in order to solve the above problems, formability in T4 tempering is achieved by appropriately controlling the chemical composition of the aluminum alloy, particularly by containing Zn in a predetermined amount or more. It was found that high strength can be obtained without reducing the amount of zinc.

That is, the aluminum alloy according to the present invention contains Mg: 0.3 to 2.0%, Si: 0.3 to 2.0%, Zn: 2.8 to 8.0% in mass%, and the balance. Ri but Do Al and impurities, an average grain size of Ru der less 70μm aluminum alloy, after performing artificial aging for 8 to 24 hours at a temperature of 170 ° C., 0.2% yield strength 350 MPa It is characterized by having the above-mentioned bake-hardness.

The aluminum alloy according to the 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% in mass% .

Further, the aluminum alloy plate according to the present invention is characterized in that any of the above aluminum alloys is used.
Further, the aluminum alloy member according to the present invention is characterized by using the above-mentioned aluminum alloy or the above-mentioned aluminum alloy plate.
Further, the method for producing an aluminum alloy according to the present invention includes a step of casting one of the above-mentioned molten aluminum alloys, a step of homogenizing heat treatment, a step of hot rolling, a step of cold rolling, and a solution. It has a step of quenching, and is characterized in that the cold rolling rate in the step of cold rolling is controlled to 30% or more.

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

FIG. 1 is an optical micrograph of each aluminum alloy plate of Example 1 and Comparative Examples 7 to 9 described later (drawing substitute photograph). 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 diligent studies in order to provide an aluminum alloy having particularly excellent strength without deteriorating the moldability in T4 tempering. As a result, by containing Zn in a predetermined amount or more as an alloy component (chemical composition) in the conventional 6000 series aluminum alloy, it is possible to significantly improve the strength while ensuring high moldability in T4 tempering. We have found what we can do and have completed the present invention.

Hereinafter, the aluminum alloy, the aluminum alloy plate, and the aluminum alloy member according to the embodiment of the present invention (the present embodiment) will be specifically described for each requirement. The description shown below exemplifies an aluminum alloy, an aluminum alloy plate, and an aluminum alloy member for embodying the technical idea of the present embodiment, and is not limited to the following embodiments.
Further, in the present embodiment, an aluminum alloy plate (rolled material) is shown as an application example of the aluminum alloy, but it can also be applied to plastic working materials such as extruded materials, casting materials, and forging materials.

<Chemical composition of aluminum alloy>
First, the chemical composition of the aluminum alloy according to the present embodiment will be described below, including the reasons for limiting each element. In addition, the% display of the content of each element means mass%. Further, "~" means that it is equal to or more than the lower limit value and less than or equal to the upper limit value.

(Mg: 0.3-2.0%)
As is also known in conventional 6000 series alloys, Mg, together with Si, is aging precipitates such as Mg-Si precipitates that contribute to strength improvement during solid solution strengthening and artificial aging heat treatment such as baking finish. It is an indispensable element for forming, aging hardening ability, and obtaining the strength (bearing capacity) required for, for example, an automobile structural member.
If the Mg content is too small, the amount of solid solution Mg before the baking finish treatment decreases, and the amount of Mg—Si-based precipitates produced is insufficient, so that the BH property is remarkably lowered and the strength is insufficient. Therefore, the content of Mg is 0.3% or more, preferably 0.5% or more, and more preferably 0.6% or more.
On the other hand, if the Mg content is too high, a shear band is likely to be formed during cold rolling, which causes cracks during rolling of the material plate. 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 also known in conventional 6000 series alloys, Si and Mg are also aging precipitates such as Mg-Si precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatments such as baking finish. It is an indispensable element for forming, aging hardening ability, and obtaining the strength required for, for example, an automobile structural member.
If the Si content is too low, the amount of solid solution Si before the baking coating process (before the artificial aging heat treatment) decreases, and the amount of Mg-Si-based precipitates produced is insufficient, so that the BH property is significantly reduced and the strength is increased. 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, if the Si content is too high, coarse crystallization and precipitates are formed, the ductility is lowered, and it causes cracking during rolling of the material plate. The Si content is 2.0% or less, preferably 1.6% or less, and more preferably 1.4% or less.

(Zn: 2.2-8.0%)
Conventionally, in order to improve the strength of a 6000 series aluminum alloy, it has been reported that it is important to deposit the β "phase or β'phase, which is a reinforcing layer, with high density and fineness by artificial aging treatment (for example, special treatment). See Kai 2017-179469).
Here, the present inventors have found that the β "phase, which is a reinforcing layer, can be deposited in a highly dense and fine manner by containing Zn in a predetermined amount or more in a 6000 series aluminum alloy, which will be described later. As can be seen from the optical micrograph (see FIG. 1) and the transmissive electron microscope (TEM) photograph (see FIG. 2) of each of the aluminum alloy plates of Examples 1 and Comparative Examples 7 to 9, as in Example 1. It can be seen that in the aluminum alloy containing a large amount of Zn (Zn content: 4.54% by mass), the acicular precipitate (Mg ₂ Si) observed in the 6000 series aluminum alloy becomes finer. ..

Although it is not clear why the β "phase can be precipitated with high density and fineness by containing Zn in a predetermined amount or more with respect to the 6000 series aluminum alloy, it contains a large amount of Zn based on the 6000 series aluminum alloy. It is considered that the solid solubility of the solute atom becomes high, that is, the supersaturation degree becomes high, and the precipitation of the β "phase, which is the reinforcing layer, is promoted by using this as a driving force, and a highly dense and fine precipitation state is obtained. Be done.

If the Zn content is too low, the β "phase, which is the reinforcing layer, cannot be sufficiently densely and finely precipitated, so that the strength is lowered. The Zn content is 2.2% or more, preferably 2. It is 0.4% or more, more preferably 2.8% or more.
On the other hand, if the Zn content is too large, the soaking heat treatment and the solution treatment cannot be performed because the melting point is lowered as a precondition for producing the aluminum alloy plate using the aluminum alloy of the present embodiment. The Zn content is 8.0% or less, preferably 7.0% or less, and more preferably 6.0% or less.

For example, the aluminum alloy plate containing Zn: 0.44 to 0.60 mass% in Japanese Patent Application Laid-Open No. 2017-48451 and Zn: 0.05% or more and less than 0.3% in Patent No. 40060952. There are aluminum alloys to which a small amount of Zn is added, such as aluminum alloy plates. However, as described above, it is important that the aluminum alloy according to the present embodiment contains a large amount of Zn of 2.2% or more. As a result, as found by the present inventors, a peculiar action of precipitating the β "phase, which is a reinforcing layer, with high density and fineness is exerted, and high strength can be obtained without lowering the 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 the same effect elements, but as a specific mechanism thereof, there are some common parts but some different parts.
Mn contributes to the improvement of strength by refining the crystal grains of the ingot and the final plate product. In addition, these elements exist as dispersed particles, contribute to grain refinement, and improve moldability. If the Mn content is too small, the effect of improving strength and moldability due to grain refinement is insufficient. On the other hand, if the Mn content is too high, a coarse compound is formed and the ductility is deteriorated.
The strength of Cu can be improved by strengthening the solid solution, and for example, the proof stress required for an automobile structural member can be obtained. If the Cu content is too low, the effect will be small, and if it is too high. The effect is saturated, and on the contrary, the corrosion resistance and the like are deteriorated.
Therefore, although these elements are not essential, at least one can be contained in the range of Mn: 0.01 to 0.5% and Cu: 0.002 to 1.0%. The lower limit of the 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%. A more preferable content of Mn is 0.3% or less. The lower limit of the preferable content of Cu is 0.01%. A more preferable content of Cu is 0.05% or more. The upper limit of the preferable content of Cu is 0.5%. A more preferable content of Cu is 0.2% or less.

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

In addition to pure aluminum base metal, other elements such as Fe, Cr, V, and Zr are also assumed (allowed) to be mixed with these impurity elements due to the use of aluminum alloy scrap as a melting raw material for ingots. , 6000 series alloys are allowed to be contained within the range specified by JIS standard. As an example of this 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 can be applied to wrought materials (rolled materials, extruded materials, cast materials, forged materials), and in order to obtain higher strength, the size of crystal grains in the structure of the wrought materials is large. The size is preferably smaller. Specifically, the crystal particle size is preferably 100 μm or less. Furthermore, 70 μm or less is more preferable. Further, the aspect ratio indicating the degree of deviation and weight of the 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 added, the processing conditions (for example, the rolling ratio of hot rolling and cold rolling in the case of a plate), and the annealing conditions. Is possible.
<Aluminum alloy plate>
The aluminum alloy plate (molding material plate) referred to in the present embodiment is a rolled plate such as a hot rolled plate or a cold rolled plate, and the rolled plate is subjected to tempering (T4) such as solution heat treatment and quenching treatment. A material aluminum alloy plate that has been rolled and has not been molded into an automobile structural member to be used, and has not been subjected to artificial aging treatment (artificial aging hardening treatment) such as coating quench hardening treatment.

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

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

[Melting, casting]
First, in the melting and casting steps, the molten aluminum alloy melt-adjusted 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]
The cast aluminum alloy ingot is then subjected to a homogenizing heat treatment prior to hot rolling. This homogenization heat treatment (uniform heat treatment) is important not only for homogenizing the structure, that is, eliminating segregation in the crystal grains in the ingot structure, but also for sufficiently solid-solving Mg and Si. As long as the conditions for achieving this purpose are met, the usual one-time heat equalization may be used, or two-time heat equalization or two-stage heat equalization may be used. In one soaking process, the heat spreading is started by cooling to the hot spreading start temperature or holding at or near the hot spreading start temperature.

The first-stage heat equalization condition in the one-time heat equalization, the second heat equalization, or the second-stage heat equalization is a temperature range of 500 ° C. or higher and lower than the melting point, and a holding time range of 2 hours or longer. Is appropriately selected from.

[Hot rolling]
The ingot that has undergone homogenization heat treatment is hot-rolled to obtain a hot-rolled plate having a plate thickness of about 2 to 10 mm. Hot rolling is composed of a rough rolling process of ingots (slabs) and a finish rolling process according to the plate thickness to be rolled. In these rough rolling steps and finish rolling steps, a reverse type or tandem type rolling mill is appropriately used.

The start temperature of hot rough rolling as the hot rolling start temperature is preferably 350 ° C. or higher in the one-time heat soaking step and 350 ° C. or higher and 400 ° C. or lower in the double heat soaking step. If the start temperature of hot rough rolling is less than 350 ° C, it becomes difficult to spread the heat with any heat equalizing process material, and conversely, if it exceeds 400 ° C, the transition element-based dispersed particles become coarse in the double heat equalizing process material. It may precipitate and reduce the properties of the plate.

After such hot rough rolling, hot finish rolling is preferably performed with the end temperature in the range of 250 to 350 ° C. If the end temperature of this hot finish rolling is less than 250 ° C., which is too low, the rolling load becomes high and the productivity decreases. On the other hand, when the end temperature of hot finish rolling is raised in order to obtain a recrystallized structure without leaving a large amount of processed structure, if this temperature exceeds 350 ° C., transition element-based dispersed particles may be coarsely precipitated. It gets higher.
Annealing (roughing) of this hot-rolled sheet before cold rolling is not necessary, but it may be carried out.

[Cold rolling]
In cold rolling, the hot-rolled plate is rolled to produce a cold-rolled plate (including a coil) having a desired final plate thickness. However, in order to make the crystal grains finer, it is desirable that the cold rolling ratio is 30% or more, and intermediate annealing may be performed between the cold rolling passes for the same purpose as the above rough blunting. ..

[Solution and quenching]
After cold rolling, solution treatment and subsequent quenching treatment to room temperature are performed. For this solution quenching 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, the average cooling rate to room temperature is set to 10 ° C./sec or more after solution treatment at a temperature of 480 ° C. or higher and melting temperature or lower. It is preferable to do so.

At a temperature lower than 480 ° C., the resolidification of the Mg-Si-based compound produced before the solution treatment becomes insufficient, and the amount of solid solution Mg and the amount of solid solution Si decrease.
Further, when the average cooling rate is less than 10 ° C./sec, Mg-Si-based precipitates are mainly generated during cooling, and the amount of solid-dissolved Mg and the amount of solid-dissolved Si decrease, and the solid-solution of Si and Mg also decreases. There is a high possibility that the amount cannot be secured. In order to secure this cooling rate, the quenching process uses air cooling such as a fan, water cooling means such as mist, spray, and immersion, and conditions are selected.

[Preliminary aging treatment: reheating treatment]
After such solution treatment, quenching treatment is performed to cool to room temperature, and then the cold rolled plate is preferably pre-aged (reheated) within 1 hour. If the room temperature holding time from the completion of quenching to room temperature to the start of pre-aging treatment (heating start) is too long, Si-rich Mg-Si clusters will be generated due to room temperature aging, and the balance between Mg and Si is good. It becomes difficult to increase the Mg-Si cluster. Therefore, the shorter the room temperature holding time is, the better, and the solution heat treatment and the quenching treatment and the reheating treatment may be continuous so that there is almost no time difference, and the lower limit time is not particularly set.

In this preliminary aging treatment, it is preferable to hold the holding time at 60 to 120 ° C. for 5 hours or more and 40 hours or less. As a result, an Mg—Si cluster with a good balance of Mg and Si is formed.

When the pre-aging temperature is less than 60 ° C. or the holding time is less than 10 hours, the Si-rich Mg-Si cluster is suppressed and the balance between Mg and Si is suppressed in the same manner as in the case where this pre-aging treatment is not performed. It becomes difficult to increase the number of Mg-Si clusters, and the proof stress after baking finish tends to decrease.
On the other hand, if the pre-aging condition exceeds 120 ° C. or more than 40 hours, the amount of precipitated nuclei generated is too large, 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 this material plate is subjected to artificial aging treatment for 8 to 24 hours at a temperature of, for example, 170 ° C., and the 0.2% resistance is 350 MPa. It can be the above.

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 material, and then, for example, structural members such as automobiles, bicycles, railroad vehicles, and automobiles. It is used as an outer panel material (exterior material) for hoods, fenders, doors, roofs, etc. Further, in terms of ensuring moldability, after the structural members are molded or processed, they are separately subjected to artificial aging heat treatment as necessary to increase their strength.

[Artificial aging heat treatment]
This artificial aging heat treatment (artificial aging hardening treatment or bake hard treatment) may be performed at the stage of the material plate, and as usual, after the material plate is formed into a structural member, it is baked after painting. It may also be used for hardening treatment.
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 6000 series aluminum alloy plate of the material, the progress of room temperature aging, and the like. For example, in the case of a one-stage artificial aging heat treatment, the 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 material (outer panel, inner panel, etc.) of a transport machine such as an automobile, a member such as a side member, a frame, a pillar, and the like used in the transport machine such as an automobile. A wide range of aluminum alloy members using the above aluminum alloy plates, such as members, energy absorbing members such as bumper line hoses and door beams, are included.
However, in order to fully exhibit the performance of the aluminum alloy of the present embodiment, which is particularly excellent in strength, without deteriorating the moldability in T4 tempering, it is required to further increase the strength of the material as compared with the panel material or the like. It is desirable to apply it to structural members.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited by the following examples as well as the present invention, and appropriate modifications are made to the extent that it can be adapted to the gist of the above and the following. It is also possible to carry out, all of which are within the technical scope of the invention.

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

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

Further, each of the cold rolled plates was tempered (T4) with a heat treatment facility. Specifically, the solution treatment is performed in an air furnace at 550 ° C. for 15 minutes, and after the solution treatment, water cooling is performed at an average cooling rate of 20 ° C./sec or more to cool to room temperature, and then quenching treatment is performed. went.
After the completion of this quenching treatment, the material was kept 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 the materials subjected to artificial aging treatment under various conditions (reaching temperature, holding time) shown in Table 1 were used as test pieces.

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

<Observation of precipitate structure>
The deposit structure in the central portion of each of the aluminum alloy plates of Examples 1 and Comparative Examples 7 to 9 is set to the (001) direction by a transmission electron microscope (TEM) having a magnification of 200,000 times. It was tilted and observed. The sample to be observed was a plate after solution treatment, and was taken from a cross-sectional structure parallel to the surface of this plate. The sample was taken by cutting an arbitrary part of the aluminum alloy plate so that the plate surface became a polished surface, with a cross section parallel to the surface of the aluminum alloy plate. Then, mechanical polishing and electrolytic polishing were performed so that the observation portion was at the center of the plate thickness to prepare a thin film sample for TEM, and then the precipitated structure was photographed by TEM at a magnification of 200,000 times. FIG. 2 shows a TEM photograph of each test piece (aluminum alloy plate).

As described above, as can be seen from the optical micrograph (FIG. 1) and the transmission electron microscope (TEM) photograph (FIG. 2) of the aluminum alloy plates of Example 1 and Comparative Examples 7 to 9, as in Example 1. It can be seen that in the aluminum alloy containing a large amount of zinc (Zn content: 4.54% by mass), the needle-like precipitate (Mg ₂ Si) observed in the 6000 series aluminum alloy becomes finer. ..

<Evaluation of strength: Measurement of tensile strength and 0.2% proof stress>
In the tensile test, the JIS13B test piece of the T4 material and the JIS13B test piece of the aging material obtained by artificially aging the test piece were subjected to a tensile test without applying strain to the T4 material (before artificial aging treatment) and aging. Tensile strength and 0.2% proof stress (MPa) of the material (after artificial aging treatment) were measured.
As for the test method, a No. 13 B test piece (20 mm × 200 mm × plate thickness (2.0 mm)) was collected based on JIS2241 (1980) and tested at room temperature (20 ° C.). At this time, the pulling direction of the test piece was set to 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.
Then, in this embodiment, the 0.2% proof stress of the test piece of the aging material (that is, after the artificial aging treatment) of 350 MPa or more was passed for the aluminum alloy structural member ("Aging characteristics" in Table 1 ". 0.2% proof stress ”).

<Evaluation of moldability: Measurement of elongation at break>
Tensile test pieces (12.5 mm × 50 mm GL × 2.0 mm) of JIS13B were collected from each of the above test plates (T4 material and aging material), and a tensile test was conducted at room temperature under the following conditions. In the tensile test, the test piece was pulled at a speed of 5 mm / min using a tensile tester and at a speed of 20 mm / min after 0.2% proof stress, and the elongation when the test piece was cut (broken) was measured.
The tensile direction of the test piece was perpendicular to the rolling direction, and the value calculated by the following formula was defined as the elongation at break. In the following formula, Lo is the distance between the gauge points before the tensile test (GL), and L is the distance between the gauge points at the time of breaking.
Elongation at break (%) = 100 × (L-Lo) / Lo
In this example, if the breaking elongation of the T4 material test piece (that is, before the artificial aging treatment) was 20% or more, it was judged that the test piece had sufficient moldability, and the test piece was accepted (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. Therefore, the breaking elongation in T4 tempering was 20% or more, and the 0.2% proof stress in T6 tempering was 350 MPa or more, and the strength and moldability were excellent.
On the other hand, in Comparative Examples 1 to 9, since the Zn content was less than 2.2% specified in the present invention, the 0.2% proof stress in T6 tempering was less than 350 MPa, and sufficient strength was obtained. I couldn't.
Further, in Comparative Example 10, although the Zn content satisfied 2.2 to 8.0% specified in the present invention, the Mg and Si contents were not more than the lower limit specified in the present invention, and therefore T6. 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>
Based on the JIS13B test piece of T4 material having the chemical composition of the aluminum alloy plate used in Example 1, each aging material in which the conditions of artificial aging treatment were changed for this test piece as shown in Table 2. Prepared. Subsequently, the tensile strength, 0.2% proof stress and breaking elongation were measured for each of the prepared aging materials in the same manner as described 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 for this test piece as shown in Table 2. Each aging material was prepared. Subsequently, the tensile strength, 0.2% proof stress and breaking elongation were measured for each of the prepared aging materials in the same manner as described above.
In addition, "-" 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 range of the present invention (Examples in Table 2), the holding time in the artificial aging treatment is in a wide range of 8 to 24 hr, and the T6 tone is adjusted. It was found that the 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 holding times of the artificial aging treatment are the same, the strength improving effect by adding Zn can be obtained even when the artificial aging treatment time is less than 8 hr and more than 24 hr. It was confirmed that it could be obtained.
That is, in order to obtain a high strength with a 0.2% proof stress of 350 MPa or more in T6 tempering, it is limited to a specific artificial aging treatment time, but even at other times, Zn is used as an alloy component in the 6000 series aluminum alloy. By containing the above amount in a predetermined amount or more, the strength can be significantly improved.
From the above, it has been experimentally shown that the aluminum alloy according to the present invention can secure extremely high strength within the range of conditions of artificial aging treatment normally assumed.

According to the present invention, it is possible to secure high strength, which is a characteristic particularly required for structural member applications, in a 6000 series aluminum alloy produced by ordinary rolling without deteriorating the formability in T4 tempering. it can. Therefore, the application of the 6000 series aluminum alloy plate can be expanded to all aluminum alloy members including structural members such as automobile structural members.

Claims (6)

By mass%, Mg: 0.3~2.0%, Si : 0.3~2.0%, Zn: 2.8 comprises 8.0%, Ri Do the balance of Al and impurities, the average crystal particle size is an der Ru aluminum alloy below 70 [mu] m,
An aluminum alloy having a bake-hard property having a 0.2% proof stress of 350 MPa or more after being subjected to an artificial aging treatment at a temperature of 170 ° C. for 8 to 24 hours. The aluminum alloy according to claim 1, further containing at least one of Mn: 0.01 to 0.5% and Cu: 0.002 to 1.0% in mass%. An aluminum alloy plate using the aluminum alloy according to claim 1 or 2 . The aluminum alloy member according to claim 1 or 2 , or the aluminum alloy member using the aluminum alloy plate according to claim 3 . The method for producing an aluminum alloy according to claim 1.
A molten aluminum alloy containing Mg: 0.3 to 2.0%, Si: 0.3 to 2.0%, Zn: 2.8 to 8.0% in mass%, and the balance being Al and impurities. It has a casting process, a homogenizing heat treatment process, a hot rolling process, a cold rolling process, and a solution solution and quenching process.
A method for producing an aluminum alloy, which comprises controlling the cold rolling ratio in the cold rolling step to 30% or more. The method for producing an aluminum alloy according to claim 2.
By mass%, Mg: 0.3 to 2.0%, Si: 0.3 to 2.0%, Zn: 2.8 to 8.0%, Mn: 0.01 to 0.5% and Cu: A step of casting an aluminum alloy molten metal containing at least one of 0.002 to 1.0% and the balance being Al and impurities, a step of homogenizing heat treatment, a step of hot rolling, and a step of cold rolling. And has a process of solution and quenching,
A method for producing an aluminum alloy, which comprises controlling the cold rolling ratio in the cold rolling step to 30% or more.