JP2017179470A - Aluminum alloy material excellent in durability of thermosetting coated film and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy material such as an automobile member, capable of securing major alloy element amount and excellent in durability of a thermosetting coated film without reducing BH property or strength.SOLUTION: There is provided a 6000 series aluminum alloy material having a specific composition and a thermosetting coated film on a surface, and improved coated film durability such as filiform corrosion resistance or coated film adhesiveness by presenting Mg oxide of an aluminum alloy material surface under the thermosetting coated film in a specific range by a heat curing treatment of the thermosetting coated film.SELECTED DRAWING: None

Description

  The present invention relates to an Al—Mg—Si based aluminum alloy material excellent in durability such as yarn rust resistance and resistance to blistering of a thermosetting coating film, and a method for producing the same. The aluminum alloy material referred to in the present invention is a rolled plate such as hot rolling or cold rolling, an aluminum alloy plastic working material such as extruded material, forging material, casting material, etc., which is used as a final application such as an automobile member. It refers to a member that has been molded and applied with a thermosetting paint and a thermosetting treatment to form a thermosetting coating film on the surface. In the following description, aluminum is also referred to as aluminum or Al.

  In recent years, due to consideration for the global environment and the like, social demands for weight reduction of vehicles such as automobiles are increasing. In order to respond to such demands, automobiles such as large body panel structures (outer panels, inner panels), members such as side members, frames, structural members such as pillars, reinforcing materials such as door beams and bumpers, arms, etc. As materials (materials) for automobile parts such as undercarriage parts, the use of lightweight aluminum alloy materials is increasing in place of steel materials such as steel plates.

  These automobile members include Al-Mg-Si-based AA to JIS 6000 series (hereinafter also simply referred to as 6000 series) aluminum alloys as high-strength aluminum alloys having excellent formability, bake coating curability, and corrosion resistance. It is used.

  In many cases, the surface of an aluminum alloy material such as an automobile member is coated to improve corrosion resistance and aesthetics. As this coating method, electro-deposition coating of a thermosetting paint is representative, and a thermosetting treatment by a paint baking hardening process is typical and widely used.

  Thus, in the 6000 series aluminum alloy member which formed the thermosetting type coating film, durability of the coating film for a long-term use becomes one of the important subjects. For example, a so-called panel for external use such as the automobile outer panel is also exposed to a corrosive environment (under-coating corrosive environment) such as seawater or salt water as a driving environment of the automobile. For this reason, thread-like rust, starting from precipitates and inclusions, called thread rust (thread rust) is generated and grows on the surface of the aluminum alloy plate under the coating film. There is a possibility that the adhesion of the coating film may be deteriorated such as peeling. These are all defects and defects that are seriously related to the durability of the coating film, and lead to fatal problems such as a decrease in the strength of the member and poor appearance. Improving film durability is an important issue.

  For the use of the aluminum alloy material, at the stage of the 6000 series (Al-Mg-Si series) aluminum alloy material such as the rolled plate, the coating film durability such as yarn rust resistance and coating film adhesion is improved. Various techniques have been conventionally proposed as is well known. Many of them reduce elements that impair coating film durability, such as Mg, Si, Cu, etc., on the surface of the material or the matrix. For example, Patent Document 1 proposes reducing Mg on the base material surface side. Patent Documents 2 and 3 propose methods for regulating or reducing the surface of the 6000 series aluminum alloy material and the Cu content in the oxide film.

  Furthermore, in Patent Document 4, the Cu content of a 6000 series aluminum alloy material for automobile panels that is coated after being subjected to phosphate treatment is regulated to 0.2% or less, and the thickness of the oxide film on the surface of the aluminum alloy material. It has been proposed to increase the phosphate coverage at the time of phosphate treatment by setting the Mg content in the oxide film measured by X-ray photoelectron spectroscopy (ESCA) to 0%.

Japanese Patent No. 3531616 Japanese Patent No. 3442699 Japanese Patent No. 4171141 Japanese Patent No. 4829412

  The reduction of elements that hinder the durability of the coating film, such as Mg, Si, and Cu, at the stage of the conventional 6000 series aluminum alloy material described above can certainly improve the coating film durability.

  However, according to the knowledge of the present inventors, after forming the material into a 6000 series aluminum alloy material, the surface of the base material of the 6000 series aluminum alloy material is applied by applying a thermosetting paint and a thermosetting treatment. Mg is newly diffused and produced, and the amount of Mg on the surface greatly affects the durability of the coating film such as yarn rust resistance and coating film adhesion.

  For this reason, even in the stage of the material, even if the elements such as Mg, Si, Cu, etc., which inhibit the durability of the coating film are reduced, the thermosetting treatment is performed at a high temperature in the stage of final use such as an automobile member. If the amount of Mg newly diffused and generated from the base material to the surface is not appropriate, the durability of the coating film is greatly reduced.

  Until now, the influence on the coating film durability of elements such as Mg newly diffused and generated from the base material by the thermosetting treatment has not been studied in detail, and as described above, at the material stage. The element reduction measures have become mainstream. For this reason, the actual situation is that it was not possible to completely guarantee the durability of the coating film when used for a long time in an end use such as an automobile member.

  Moreover, if the main alloying elements such as Mg, Si, Cu, etc. of the 6000 series aluminum alloy material are reduced for the durability of the coating film as in the prior art, the basic required characteristics required for the end use such as automobile parts There was also an aspect that BH property (bake hard property) and strength could not be secured, and these were sacrificed.

  The present invention has been made to solve such problems, and can ensure the amount of main alloying elements, and has excellent durability of a thermosetting coating film without deteriorating BH properties and strength. It aims at providing aluminum alloy materials, such as a motor vehicle member, and its manufacturing method.

In order to achieve this object, the gist of the aluminum alloy material excellent in durability of the thermosetting coating film of the present invention is mass%, Mg: 0.2-2.0%, Si: 0.3- Aluminum containing 2.0% and Cu: 0.01 to 3.0%, respectively, the balance being Al and an inevitable impurity Al—Mg—Si based aluminum alloy having a thermosetting coating on its surface An equivalent circle diameter determined from a color map indicating the distribution state of elements of Mg and O when the surface of the aluminum alloy material under the thermosetting coating film is analyzed by an electron beam microanalyzer. Is the average number density of Mg oxide of 1 μm or more in the range of 10 to 1000 pieces / mm ² .

Moreover, the summary of the manufacturing method of the aluminum alloy material excellent in durability of the thermosetting coating film of this invention for achieving the said objective is mass%, Mg: 0.2-2.0%, Si : 0.3-2.0%, Cu: 0.01-3.0% respectively, and the rest of the aluminum alloy material consisting of Al and unavoidable impurities, the application of a thermosetting paint and a thermosetting treatment When the thermosetting coating film is formed on the surface of the aluminum alloy material, the elements of Mg and O when the surface of the aluminum alloy material on which the thermosetting coating film is formed are analyzed by an electron beam microanalyzer. By performing the thermosetting treatment so that the average number density of Mg oxides having an equivalent circle diameter of 1 μm or more, determined from a color map indicating the distribution state of, is in the range of 10 to 1000 pieces / mm ^2. is there.

  The present inventors paid attention to the amount of Mg newly diffused and formed from the base material to the surface by the thermosetting treatment of the thermosetting paint, and this Mg is a coating durability such as yarn rust resistance and coating adhesion. It was found that it greatly affects sex.

  As described above, at the stage of the aluminum alloy material, even if elements such as Mg, Si, and Cu that impair the durability of the coating film are reduced, the thermosetting treatment in the aluminum alloy material (the final use) such as an automobile member Thus, Mg is newly diffused and generated from the base material to the surface at a high temperature.

  According to the knowledge of the present inventors, the amount of Mg newly diffused and formed on the surface, if the amount of Mg is too small or too large, the durability of the coating film is greatly reduced. That is, the behavior of Mg on the surface under the thermosetting coating film is completely different from the behavior recognized at the surface of the aluminum alloy material, and if an appropriate amount is actively present, As a result, the coating durability was improved.

  Furthermore, if the amount of Mg on the surface of the aluminum alloy material under the thermosetting coating film is controlled to an appropriate amount, the diffusion rate is significantly slower than Mg, and other elements such as Si, Cu and the like that impair coating film durability. It was also found that diffusion generation on the surface of the aluminum alloy material under the thermosetting coating film can be simultaneously suppressed, which also leads to improvement in coating film durability.

By these mechanisms, in the present invention, it is not necessary to reduce strengthening elements such as Si and Cu from the viewpoint of corrosion resistance, so the amount of alloying elements necessary for strengthening can be secured, and the BH property, strength, or formability is reduced. It is possible to improve the durability of the thermosetting coating film of the aluminum alloy material without causing it.
Further, according to the present invention, it is not necessary to greatly change the manufacturing process of aluminum alloy materials such as automobile members, such as aluminum alloy materials, molding processing of the materials, painting of the thermosetting paint, and thermosetting treatment. is there.

  For this reason, according to the present invention, to an aluminum alloy material such as an automobile member, in which the Al-Mg-Si based aluminum alloy material is required to have various properties such as strength and formability including coating film durability. Can be applied or promoted.

Hereinafter, embodiments of the present invention will be specifically described for each requirement.
First, the Al—Mg—Si-based aluminum alloy material of the present invention (hereinafter also simply referred to as “aluminum alloy material”) is a rolled plate such as hot rolling or cold rolling, an extruded material by hot extrusion, or hot forging. Aluminum alloy plastic working material such as forging material by casting, casting material such as casting or die casting is used as the material. And it refers to a member in which these materials are molded into an end use shape such as an automobile member, and a thermosetting coating film is formed on the surface by applying a thermosetting paint and a thermosetting treatment.

(Chemical composition)
The chemical composition of the aluminum alloy material of the present invention may be selected from the composition range of the 6000 series aluminum alloy specified so as to satisfy each required characteristic according to various uses of the aluminum alloy material such as the automobile member. preferable.
For example, taking a car body outer panel of an automobile member as an example, the required characteristics of the automobile panel are the characteristics of a material plate after T4 tempering such as solution treatment and quenching treatment, and it is 0. BH with a 2% proof stress as low as 110 MPa or less to ensure moldability and a 0.2% proof stress after thermosetting treatment such as baking coating curing treatment as an automobile panel after molding is increased to 200 MPa or higher, for example. It is necessary to have the property (bake hard property). Therefore, it is preferable to enable this as a raw material aluminum alloy plate from the viewpoint of composition. In addition to excellent moldability and BH properties, various characteristics such as rigidity, weldability, and corrosion resistance are also required for automobile panels, so it is preferable to satisfy these requirements from the viewpoint of composition. .

  Also, members such as side members, frames, and automobile structural members such as pillars can be further strengthened compared to the automobile panel material, leading to shock absorption and occupant protection in the event of a vehicle collision. It is necessary to impart crushability (crush resistance, crush characteristics) as new characteristics.

In order to satisfy these required characteristics from the viewpoint of the alloy composition, in the present invention, the composition of 6000 series aluminum alloy is Mg: 0.2-2.0%, Si: 0.3-2.0 by mass%. %, Cu: 0.01 to 3.0% of main elements are included, and the balance is Al and inevitable impurities.
Further, in addition to this basic composition, as a selective additive element, Mn: 0.01 to 1.0%, Cr: 0.001 to 0.3%, Zr: 0.001 to 0.3%, One type or two or more types of Ag: 0.001 to 0.2% and Sn: 0.001 to 0.2% can be included.
Other elements other than these are unavoidable impurities, and the content (allowable amount) at each element level in accordance with AA to JIS standards. In addition,% display of content of each element means the mass% altogether.

In the vehicle body outer panel of an automobile member, the content of the main element and the selective additive element is selected to be low from the alloy composition range. On the other hand, in the automobile structural member, the content of the main element and the selective additive element is selected to be higher from the alloy composition range.
At this time, in the present invention, if the amount of Mg on the surface of the aluminum alloy material under the thermosetting coating is controlled to an appropriate amount, the diffusion rate is slower than Mg, and the durability of other coatings such as Si and Cu It is possible to suppress the diffusion of elements that inhibit the diffusion to the surface of the aluminum alloy material under the thermosetting coating film. For this reason, it is not necessary to reduce strengthening elements, such as Si and Cu, from the viewpoint of corrosion resistance, and sacrifice the BH property and strength as in the prior art, and the amount of alloying elements necessary for strengthening can be included.

  The content range and significance of each element in the 6000 series aluminum alloy composition, and the allowable amount will also be described below.

Si: 0.3 to 2.0%
Si, together with Mg, forms an aging precipitate that contributes to strength improvement during solid tempering and artificial aging treatment such as paint baking treatment (thermosetting treatment), and exhibits age hardening ability, such as automotive parts It is an essential element for obtaining the strength required for an aluminum alloy material. If the Si content is too small, the amount of solid solution strengthening and the amount of precipitation after artificial aging are too small, and the amount of increase in strength during baking is too low. On the other hand, if the Si content is too large, coarse crystallized substances and precipitates are formed, which becomes a starting point of yarn rust generation, and the yarn rust resistance is remarkably lowered. In addition, the moldability is significantly reduced. Furthermore, when there is too much Si content, not only the intensity | strength immediately after manufacture of a raw material board but the room temperature aging amount after manufacture will also become high, the intensity | strength before shaping | molding will become high too much, and a moldability will fall. Therefore, the Si content is in the range of 0.3 to 2.0%. However, as described above, in the present invention, if the amount of Mg on the surface of the aluminum alloy material under the thermosetting coating film is controlled to an appropriate amount, other elements such as Si, Cu and the like that impair the coating durability. Since diffusion generation to the surface can be suppressed, it is possible to contain more Si than usual.

Mg: 0.2-2.0%
Mg also forms aging precipitates that contribute to strength improvement together with Si during the above-mentioned artificial aging treatment such as solid solution strengthening and paint baking treatment, exhibits age hardening ability, and obtains the required strength as an aluminum alloy material Is an essential element for. If the Mg content is too small, the amount of precipitation after artificial aging will be too small, and the strength after baking will be too low. On the other hand, when the Mg content is excessively increased, the elution reaction of Mg is promoted, and the rust resistance is significantly lowered. Moreover, coarse crystallized substances and precipitates are formed, which becomes a starting point of yarn rust generation, which also causes a decrease in yarn rust resistance. In addition, the formation of coarse crystals significantly reduces moldability, and further increases not only the strength immediately after the production of the material plate but also the room temperature aging amount after the production, and the strength before molding becomes too high. As a result, the moldability to automobile members and the like is reduced. Therefore, the Mg content is in the range of 0.2 to 2.0%.

Cu: 0.01 to 3.0%
Cu improves strength by solid solution strengthening and improves formability, so it is essential to obtain strength and formability necessary for an aluminum alloy material. If the Cu content is too small, the effect is small, and if it is too much, the effect is saturated, and the corrosion resistance is deteriorated. However, as described above, in the present invention, if the amount of Mg on the surface of the aluminum alloy material under the thermosetting coating film is controlled to an appropriate amount, other elements such as Si, Cu and the like that impair the coating durability. Since diffusion generation to the surface can be suppressed, it is possible to contain more Cu than usual.

Selective additive elements Mn, Cr, Zr, Ag, and Sn can be selectively contained as synergistic elements (strengthening elements) that improve the strength in common.
Mn, Cr, and Zr contribute to improving the strength by refining the crystal grains of the ingot and the final plate product, and exist as dispersed particles to contribute to refinement of the crystal grains and improve the moldability. When there are too many of these elements, a coarse compound will be formed and ductility will deteriorate.
Ag has the effect of accelerating the increase in strength by closely and finely precipitating aging precipitates that contribute to strength improvement by artificial aging treatment.
Sn suppresses cluster formation at room temperature, suppresses room temperature aging of the material, has an effect of maintaining formability for a long time, and further improves strength when subjected to artificial aging heat treatment such as baking coating treatment. Let It is also effective in improving yarn rust resistance.
When these are contained, Mn: 0.01 to 1.0%, Cr: 0.001 to 0.3%, Zr: 0.001 to 0.3%, Ag: 0.001 to 0.2% %, Sn: 0.001 to 0.2%.

Other elements:
Other than these elements, other elements such as Ti, B, Fe, Zn, and V are inevitable impurities. From the viewpoint of resource recycling, not only high-purity Al ingots but also 6000 as a raw material for melting alloys. When a large amount of alloy, other aluminum alloy scrap material, low-purity Al metal, etc. are used, these elements are inevitably mixed in substantially. If these elements are intentionally reduced, refining itself will increase the cost, so it is necessary to allow a certain amount of inclusion. Accordingly, in the present invention, the following elements are allowed to be contained in the range of the upper limit amount or less in accordance with AA to JIS standards defined below. Preferably, the allowable amount is 0.1% or less for Ti, 0.03% or less for B, 1.0% or less for Fe, 1.0% or less for Zn, and 0.3% or less for V.

(Mg amount of aluminum alloy material surface under thermosetting coating)
Based on the above alloy composition, in the present invention, the amount of Mg on the surface of the 6000 series aluminum alloy material under the thermosetting coating film is controlled as the number density of Mg oxide.
That is, when the surface of the aluminum alloy material under the thermosetting coating film is surface-analyzed by an electron beam microanalyzer, the equivalent circle diameter is determined from a color map showing the distribution state of Mg and O (oxygen) elements. The average number density of Mg oxide of 1 μm or more is set to a range of 10 to 1000 pieces / mm ² .

As described above, at the stage of the material of the aluminum alloy material, even if the elements such as Mg, Si, Cu, etc. that impair the coating durability are reduced, the aluminum alloy material (the end use member) such as an automobile member is used. Then, when the thermosetting treatment of the thermosetting paint is performed, Mg is newly diffused and generated from the base material to the surface at a high temperature.
Then, the diffusion-generated Mg reacts with the moisture that has penetrated the thermosetting coating film or originally exists in the coating film, and is mainly used as the Mg oxide defined in the present invention as an aluminum alloy. Present on the material surface.

In this way, the amount of Mg oxide newly present on the surface from the base material due to the thermosetting treatment of the thermosetting paint greatly affects the durability of the coating film such as yarn rust resistance and coating film adhesion. However, if the amount of Mg oxide on the surface of these aluminum alloy materials is too small or too large, the durability of the coating film is greatly reduced.
As described above, the behavior of Mg oxide on the surface under the thermosetting coating film is completely different from the behavior recognized at the stage of the surface of the aluminum alloy material. If an object is actively present, the durability of the coating film is improved.

Such a coating durability improvement mechanism is based on the fact that a certain amount of Mg oxide defined in the present invention is present on the surface of an aluminum alloy material, which causes thread rust growth. It is thought that it suppresses the pH fall on the material surface.
That is, during use of the aluminum alloy material, moisture that has penetrated the thermosetting coating film or originally exists in the coating film reacts, and the reaction formula of MgO + H ₂ O = Mg ²⁺ + 2OH ^{− represents} an OH group ( Hydroxyl group). This OH group is alkaline and suppresses a decrease in pH on the surface of the thermosetting coating film and the aluminum alloy material, which is the cause of yarn rust growth, and maintains neutrality.
Incidentally, this action is the same for hydroxides such as MgOH, and an OH group (hydroxyl group) is generated by the reaction formula of Mg (OH) ₂ = Mg ²⁺ + 2OH ⁻ . Naturally, this OH group is also alkaline and suppresses a decrease in pH on the surface of the thermosetting coating film and the aluminum alloy material, which causes yarn rust growth, and is kept neutral. However, as will be described later, this hydroxide cannot be distinguished from Mg oxide by surface analysis using an electron beam microanalyzer. Therefore, in surface analysis using an electron beam microanalyzer, the Mg hydroxide is also regarded as Mg oxide, and the average number density is quantified.

Therefore, when the average number density of the Mg oxide having an equivalent circle diameter of 1 μm or more on the surface of the aluminum alloy material under the thermosetting coating film is less than 10 / mm ² , the Mg oxidation defined in the present invention The average number density of the product is too small, and the effect and mechanism for improving the durability of the coating film due to the Mg oxide does not appear. For this reason, it becomes impossible to suppress the acidification by the pH fall in the said thermosetting type coating film and aluminum alloy material surface which is the cause of thread rust growth.

On the other hand, the average number density of the Mg oxide having an equivalent circle diameter of 1 μm or more on the surface of the aluminum alloy material under the thermosetting coating film exceeds 1000 / mm ² , If the average number density is excessive, the OH groups are excessive. For this reason, conversely, alkalinization of the thermosetting coating film and the surface of the aluminum alloy material proceeds (the pH increases), and the adhesion of the coating film is lowered, resulting in coating film swelling and coating film peeling.

Further, when the surface of the aluminum alloy material under the thermosetting coating film is not in the form of Mg oxide defined in the present invention but in the form of Mg alone, Mg + 2H ₂ O = Mg (OH) ₂ + 2H ^{According to} the reaction formula of ++ 2e ⁻ , an acidic H group (hydrogen group) is generated instead of an OH group. This H group is acidic and promotes a decrease in pH on the thermosetting coating film and the aluminum alloy material surface, which is the cause of yarn rust growth, and causes yarn rust growth.
However, if the amount of the Mg oxide defined in the present invention on the surface of the aluminum alloy material under the thermosetting coating film is controlled within the above-described appropriate range by controlling the thermosetting treatment, the Mg alone or In the form of hydroxide such as Mg (OH) ₂ , it hardly exists, and even if it exists, it becomes an allowable range that does not affect the durability of the coating film such as yarn rust.

  Furthermore, if the amount of Mg oxide defined in the present invention on the surface of the aluminum alloy material under the thermosetting coating film is controlled within the above-mentioned range by controlling the thermosetting treatment, the diffusion rate is higher than that of Mg. Diffusion generation on the surface of the aluminum alloy material under the thermosetting coating film, which is slower than 1/100, which inhibits coating film durability such as other Si and Cu, can also be suppressed. It leads to improvement.

  By controlling the Mg oxide defined in the present invention on the surface of the aluminum alloy material under the thermosetting coating film, the durability of the thermosetting coating film withstanding long-term use can be improved. Therefore, according to the present invention, the application of the 6000 series aluminum alloy material to the aluminum alloy material such as the automobile member described above can be enabled or promoted.

(Controlling the number density of Mg oxide on the surface of aluminum alloy material)
The control of the average number density of Mg oxide on the surface of the aluminum alloy material defined in the present invention is difficult to control in the alloy composition such as Mg content of the aluminum alloy material and the production conditions of the material, and the thermosetting of the thermosetting paint It is reasonable to control mainly depending on the processing conditions.

  As described above, the present invention eliminates the need to reduce reinforcing elements such as Si and Cu from the viewpoint of coating film durability, so that the amount of alloying elements necessary for strengthening can be secured, and a normal aluminum alloy material The advantage that you can use is great. In addition, it is not necessary to greatly change the production process of aluminum alloy materials such as automobile members, such as aluminum alloy material, molding of this material, coating of the thermosetting paint, and thermosetting treatment.

However, for the control in such a thermosetting treatment, it is preferable to reduce in advance the average number density of the Mg oxide on the surface of the aluminum alloy material before coating.
That is, in order to make the average number density of the Mg oxide on the surface of the aluminum alloy material under the thermosetting coating film the optimum range, in the present invention, the surface of the aluminum alloy material before coating with the thermosetting paint is used. It is preferable that the average number density of the Mg oxide to be defined is previously reduced to less than 10 pieces / mm ² including 0 pieces / mm ^2, and then the thermosetting paint is applied and heat-cured. .
The surface of the aluminum alloy material before painting refers to the surface of the aluminum alloy material manufactured by plastic working, the aluminum alloy material before molding, the aluminum alloy material after molding and before painting, etc. Contains.

The average number density of Mg oxides with an equivalent circle diameter of 1 μm or more on these surfaces exceeds 10 / mm ^{2 in} advance, such as Mg being diffused or concentrated on the surface by tempering (heat treatment) or standing at room temperature. In the case of increasing the number, the average number density of Mg oxide defined in the present invention may exceed the optimum range described above depending on the temperature and time conditions of the thermosetting treatment.
The temperature and time conditions of the thermosetting treatment are usually determined from the curing (crosslinking reaction) of the thermosetting paint and the BH property for ensuring the strength of the aluminum alloy material, and the thermosetting treatment thus selected. This is because the average number density of the Mg oxide having an equivalent circle diameter of 1 μm or more inevitably increases under the above temperature and time conditions.
The average number density of the Mg oxide having an equivalent circle diameter of 1 μm or more on the surface of the aluminum alloy material before the thermosetting coating is applied is less than 10 / mm ² including 0 / mm ^{2 in} advance. In addition, it is preferable to apply the thermosetting paint and a thermosetting treatment.

In order to reduce the average number density of Mg oxides defined in the present invention on the surface of the aluminum alloy material before coating to less than 10 / mm ² including 0 / mm ^{2 in} advance, It is preferable to perform cleaning with etching on the aluminum alloy material.
This cleaning operation is performed after the existing cleaning process, such as after the aluminum alloy material manufactured by plastic working, after the aluminum alloy material before being formed, and after being formed and before painting. Can be used. Then, the average number density of the Mg oxide on the surface of the aluminum alloy material before coating is adjusted to 10 by adjusting the etching level or cleaning strength (liquid concentration, liquid temperature, processing time, etc.) of the cleaning (liquid). Control to less than / mm ² .

(Mg oxide number density control by thermosetting)
As described above, the Mg oxide defined in the present invention is governed by and controlled by the temperature and time conditions of the thermosetting treatment.
In this respect, the thermosetting coating film (thermosetting paint) is cured (crosslinking reaction), satisfies the BH property for ensuring strength, and further has an average number density of Mg oxides defined in the present invention. In order to make the above-described optimum range, the heating temperature of the thermosetting treatment, the time for holding at the heating temperature, the heating temperature 160 ~ while considering the relationship with each content of Mg, Si, Cu of the aluminum alloy material It is preferable to select from a range of 180 ° C. and a heating and holding time of 10 to 30 minutes.
Within such a range, for example, if the main alloy element amount is relatively small, the heating temperature is selected to be low and the heating holding time is selected to be short. If the main alloy element amount is relatively large, the heating temperature is increased. The heating and holding time is selected from a longer one.

  However, this is merely a guideline for optimum conditions, and the thermosetting coating film is considered while considering the aluminum alloy composition selected according to the required characteristics of the end use as a member and the optimum curing characteristics of the thermosetting paint. In order to ensure that the amount of Mg present on the surface of the lower aluminum alloy material is within the specified range of the present invention, the optimum temperature and time conditions for the thermosetting treatment are actually tested, and trial and error (Trial It is preferable to determine (by and error).

(Measurement of Mg oxide number density on the surface of aluminum alloy material)
The average number density of Mg oxide defined in the present invention on the surface of the aluminum alloy material before coating or after the thermosetting treatment is an electron beam microanalyzer (also known as an electron probe microanalyzer, EPMA: Electron Probe MicroAnalyzer). Is used.
This EPMA is an apparatus suitable for analyzing the wavelength of characteristic X-rays emitted from a sample when irradiated with electrons and identifying the elements present in the irradiated region and analyzing the concentration.
The scanning function of the electron probe can perform surface analysis of a large number of elements called a color map. Using this function, only Mg and O elements exist at the same position in the color map. Alternatively, when most of them exist as Mg and O, they can be identified as Mg oxide.
However, since this EPMA cannot identify H, the possibility of including Mg hydroxide: Mg (OH) _{2, which} is also composed of elements of Mg and O, is denied while being identified as Mg oxide. Can not. However, when Mg and oxygen diffused and generated from the base material react on the surface of the aluminum alloy material under the thermosetting coating film by the thermosetting treatment, most of them become Mg oxide, and Mg hydroxide The production of things is rare. Even if it is included, as described above, it has a good effect on the durability of the coating film, so it can be ignored even if it cannot be measured accurately.
Therefore, in the present invention, when it is identified by the color map of this EPMA that only elements of Mg and O are present, they are all regarded as Mg oxide.

Samples including the surface of the aluminum alloy material before the coating and after the thermosetting treatment are sampled from any 10 locations, and the oxide film is formed from the outermost surface of the aluminum alloy material to a depth of 0.25 mm. Consider and scrape off by mechanical polishing.
The surface of the aluminum alloy material after the thermosetting treatment has a chemical conversion treatment film and a paint film in this order on an aluminum matrix. Therefore, before removing the oxide film, the upper paint film is a mechanical film. It is removed by polishing or chemical polishing, and the underlying chemical conversion film is removed by pickling. And this polished surface is made into the surface of an aluminum alloy material, and it measures by EPMA (For example, JXA-8000 series made from JEOL, measurement conditions are acceleration voltage 20 kV). The measurement area is about 0.1 to 0.2 mm ² for surface analysis, and the magnification during measurement is 600 times.
The elements to be detected are at least four elements of Mg, Si, Cu and O which are main alloy elements, and analysis of other elements contained therein is also added if necessary.

Then, the element identified by the computer is semi-quantitatively analyzed from the characteristic X-rays of each element, and a color map indicating the surface distribution position and the surface distribution state of the element in the same plane of the test material is created. When only Mg and O elements are present at the same position in the color map or most of them are present as Mg and O, the aggregate of these elements is identified as Mg oxide, The maximum length of the element aggregate is measured as the equivalent circle diameter of a circle having the maximum length as the diameter, that is, the equivalent circle diameter of the Mg oxide.
Then, from the surface analysis results of these EPMA on the surface of 10 aluminum alloy materials, the number density (pieces / mm ² ) of Mg oxide having an equivalent circle diameter of 1 μm or more is calculated, and the number density of each of these samples is calculated. It averages and it is set as the average number density (pieces / mm < ² >) of Mg oxide prescribed | regulated by this invention.

(Aluminum alloy material manufacturing method)
The material of the aluminum alloy material of the present invention is a hot rolled plate, a cold rolled plate, a hot extruded profile (hollow, solid) manufactured by a conventional method other than the cleaning step described above or including this cleaning step. ), Near-net shape hot forging, castings such as casting and die casting. These aluminum alloy plastically processed materials are subjected to plastic processing or are subjected to tempering (heat treatment) and surface treatment after plastic processing to obtain a material (molding material) of the aluminum alloy material of the present invention.

Manufacturing method of raw material board The outline of the manufacturing method of a typical hot-rolled board and a cold-rolled board among the said aluminum alloy plate raw materials is illustrated below.
First, in the melting and casting process, an aluminum alloy melt adjusted to be dissolved within the above-mentioned 6000-based component composition range is appropriately selected by a normal melting casting method such as a continuous casting method or a semi-continuous casting method (DC casting method). Cast.
Next, prior to hot rolling, the cast aluminum alloy ingot is subjected to a homogenizing heat treatment by a conventional method. The conditions for the soaking are appropriately selected from a range of holding time of 2 hours or more in a temperature range of 500 ° C. or higher and lower than the melting point.
In hot rolling, a hot rolling start temperature is selected from a range of 350 ° C. to a solidus temperature, an end temperature is selected from a range of 300 to 360 ° C., and hot rolling is performed. To do. When this hot-rolled sheet is further cold-rolled, annealing (roughening) before cold rolling may be performed as necessary.
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. .

After cold rolling, in order to satisfy the various properties as materials for the automobile material, it is preferable to temper the plate to make a T4 material. The tempering of T4 is usually performed by solution treatment and subsequent quenching to room temperature. For this solution hardening treatment, a normal continuous heat treatment line may be used. At this time, it is preferable to set the average cooling rate to room temperature to 20 ° C./second or higher after solution treatment at a temperature of 550 ° C. or higher and below the melting temperature.
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. In this preliminary aging treatment, the holding time at 60 to 120 ° C. is preferably held for 2 hours or more and 40 hours or less. As a result, Mg—Si clusters having a good balance between Mg and Si are formed.

In order to reduce the average number density of Mg oxide on the surface of the aluminum alloy material before painting in advance to less than 10 / mm ² including 0 / mm ² The tempered material plate is preferably washed with etching. As the cleaning condition for this, the stronger etching or the stronger cleaning condition is selected as the Mg content is higher according to the composition of the material plate, particularly the Mg content. The cleaning with etching generally includes electrolytic degreasing, an aqueous alkali solution such as sodium hydroxide and sodium carbonate, an aqueous acid solution such as sulfuric acid and nitric acid, a commercially available cleaning solution, or a cleaning step combining these. Also in the present invention, the specific cleaning method and conditions are not selected, but the conditions satisfying the oxide film conditions of the present invention are appropriately selected from these known cleaning conditions. Further, after the washing and drying, the plate may be subjected to a surface treatment such as oil coating for preventing the surface from being oxidized.

(Manufacture of aluminum alloy materials)
The aluminum alloy material is subjected to molding such as press molding, bending, and drilling in the shape of an end use such as an automobile member. And thermosetting paints that are widely used for phosphate treatment and electrostatic coating at the stage after they are attached and assembled to end-use products such as automobile bodies, or at the parts and parts stage before attachment and assembly. A thermosetting coating film is formed on the surface of the coating and a thermosetting treatment (baking curing treatment) that also serves as an artificial aging treatment.

Among these steps, cleaning with etching for reducing the average number density of Mg oxide on the surface of the aluminum alloy material before coating, including 0 / mm ² and less than 10 / mm ^{2 in} advance, You may perform to the aluminum alloy material in the suitable stage before a painting instead of the stage of the above-mentioned aluminum alloy material, or combining with the said washing | cleaning in the stage of the said aluminum alloy material.
The chemical conversion treatment liquid for cleaning uses industrially common alkaline cleaning such as potassium hydroxide, sodium hydroxide, calcium hydroxide, alkali metal salts, and the like, which involves etching that can remove the base material surface and oxide film. be able to.
However, the cleaning conditions involving the etching of the surface of the aluminum alloy material before charging, including the cleaning accompanied by the etching of the tempered material plate, differ depending on the alloy composition of the aluminum alloy material and the material manufacturing history. It is preferable to find an optimum cleaning condition according to each aluminum alloy material by error.

  For the application of the thermosetting coating film (thermosetting paint), a known paint or method can be employed. For example, the electrodeposition coating can be performed using a cationic electrodeposition paint containing an amine-added epoxy resin and a blocked polyisocyanate curing agent, which are widely used.

In the thermosetting treatment, the conditions are selected from the range of the heating temperature of 160 to 200 ° C. and the heating and holding time of 10 to 30 minutes in relation to the contents of Mg, Si and Cu of the aluminum alloy material, The amount of Mg present on the surface of the aluminum alloy material under the thermosetting coating film is preferably in the range of 10 to 1000 / mm ² as the Mg oxide having an equivalent circle diameter of 1 μm or more.
However, even within the preferable thermosetting treatment condition range, depending on the alloy composition and material production history of the aluminum alloy material, the amount of Mg present on the surface of the aluminum alloy material under the thermosetting coating film is within the range. In some cases, it may be impossible to do so, and it is preferable to find an optimum condition according to each aluminum alloy material by trial and error.

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.
The present invention is not only an invention of an aluminum alloy material excellent in the durability of a thermosetting coating film, but also an invention of a method for producing an aluminum alloy material excellent in the durability of a thermosetting coating film. It is also an invention of a method for improving the durability of a thermosetting coating film.

  Next, examples of the present invention will be described. The amount of the Mg oxide on the surface of the 6000 series aluminum alloy material was made to evaluate the coating properties such as mechanical properties, yarn rust resistance and coating adhesion. These results are shown in Tables 1 and 2.

(Manufacturing conditions of material plate)
A rolled plate was selected as the aluminum alloy material. First, a 6000 series aluminum alloy ingot having each composition shown in Table 1 was melted by a DC casting method. In addition, in the display of the content of each element in Table 1, the display in which the numerical value in each element is blank indicates that the content is not more than the detection limit and is 0% not including these elements.

In common with each example, these ingots were subjected to soaking treatment at 540 ° C. for 6 hours, and then hot rough rolling was started at that temperature. And it hot-rolled to thickness 2.5mm by the subsequent finish rolling, and was set as the hot rolled sheet. After subjecting this hot-rolled sheet to rough annealing at 500 ° C. for 1 minute, cold rolling at a processing rate of 60% is performed without intermediate annealing in the middle of the cold-rolling pass, did.
Furthermore, each cold-rolled sheet was subjected to a tempering treatment (T4) with a heat treatment facility under the same conditions as each example. Specifically, the solution treatment is performed by holding at 550 ° C. for 30 seconds. At this time, the average heating rate up to the solution treatment temperature is 50 ° C./second, and after the solution treatment, the average cooling rate is 30 ° C./second. It cooled to room temperature by performing water cooling for 2 seconds. Immediately after this cooling, in common with each example, the preliminary aging treatment was immediately performed at 100 ° C. for 5 hours, and after the preliminary aging treatment, it was gradually cooled (cooled) to obtain a T4 material plate.

  As an example of the aluminum alloy material of the present invention, the material plate thus manufactured simulates that it is aged at room temperature until it is formed into an outer panel of an automobile. A test piece having a length of 100 mm and a width of 25 mm was collected from each test plate after being allowed to stand (room temperature aging). Then, each test piece aged at room temperature was simulated to be press-molded on an outer panel of an automobile, and a pre-strain of 2% was given in the rolling direction by a tensile test.

Tests caused by the raw material plate (manufacture of raw material plate) by subjecting this pre-strained test piece to chemical conversion treatment under various conditions of liquid temperature (° C.) and immersion time (minute) shown in Table 2. Various amounts of Mg on one surface were adjusted.
In this chemical conversion treatment, the test piece is immersed and washed (with stirring by a stirrer) in a sodium carbonate-based degreasing bath in which the liquid temperature (° C.) and the immersion time (minutes) are different, and the surface of the test material is tetched. Washing process with.

After this cleaning treatment, in common with each of the examples, the outer panel of the automobile was simulated to be coated, immersed in a zinc-based surface conditioning bath at room temperature for 1 minute (with stirring by a stirrer), and then rinsed at 35 ° C. It was immersed in a zinc acid bath for 2 minutes to give a zinc phosphate treatment.
Furthermore, after that, the above-mentioned cationic electrodeposition coating (thickness 20 μm) was performed in common with each example, simulating a normal automobile member coating process, and the heating temperature (° C.) and heating shown in Table 2 were performed. The coating film curing process (baking curing process) was performed under various conditions of holding time (minutes).

(Average number density of Mg oxide)
The test specimens after coating were subjected to surface analysis by EPMA in the specific manner described above, with the surface of the aluminum alloy material under the thermosetting coating film as an aluminum alloy material having a thermosetting coating film on the surface. Then, the average number density (pieces / mm ² ) of Mg oxide having an equivalent circle diameter of 1 μm or more, determined from the obtained color map showing the distribution state of Mg and O elements, was measured and calculated.

(Film durability test)
In the yarn rust resistance test, the test piece after coating is common to each example, and a 50 mm long crosscut wrinkle is put in the coating film, and then sprayed with salt water for 24 hours → wet (humidity 85%, 40 ° C.) 120 Eight cycles of time → natural drying (room temperature) 24 hours were performed, and the rust width on one side of the crosscut portion was measured.
Evaluation of yarn rust resistance is based on the maximum yarn rust length. The maximum yarn rust length is less than 1 mm. ◎ 1 mm or more and less than 2 mm ○, 2 mm or more and less than 3 mm △, 3 mm or more Those with a length of 評 価 were evaluated as x, and those with ◎ and ○ were judged as materials (accepted) having excellent yarn rust resistance.

(Coating film adhesion)
The coating film adhesion was subjected to a cross-cut test based on JIS K5600-5-6G330. Specifically, in order to make 100 squares at 1 mm intervals on the surface of the test piece after painting, a base-like cut is applied, and a tape is applied to the part, and the tape is peeled off after peeling. The remaining rate of the coating film was observed.
When the peeling area of the coating film is less than 10%, it is evaluated as “◯”, and when the peeling area is more than 10% and within 20%, it is evaluated as “△”, and the peeling area exceeds 20%. Was evaluated as “×”, and the evaluations up to ◯ and △ were regarded as acceptable.
And from the above results, the yarn rust property is ◯ or more, the coating film adhesion is △ or more, and the coating film durability is comprehensively evaluated as being acceptable for long-term use as an aluminum alloy material. In the case, it was evaluated as a failure and was evaluated as x.

(Tensile properties)
In order to investigate the mechanical properties of the aluminum alloy material after aging at room temperature and the aluminum alloy material after BH, respectively, a test piece pre-strained after aging at room temperature and a test piece after BH, The specimens were processed into JISZ2201 No. 5 test pieces (25 mm × 50 mmGL × plate thickness) and subjected to tensile tests at room temperature.
The tensile direction of the test piece at this time 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 proof stress. The N number for the measurement of mechanical properties was 5, and each was calculated as an average value.

Each invention example has an alloy composition within the specified range as shown in Table 1, and as shown in Table 2, the cleaning conditions involving etching with the chemical conversion solution on the surface of the aluminum alloy material before coating are appropriate, and thermosetting The heating temperature and heating time (heating holding time) of the treatment are also appropriate.
For this reason, as shown in Table 2, the average number density of Mg oxide defined in the present invention is less than 10 pieces / mm ^{2 in} advance on the surface of the aluminum alloy material before coating, and is below the thermosetting coating film. On the surface of the aluminum alloy material, the range is 10 to 1000 pieces / mm ² .
As a result, each invention example is excellent in coating film durability as shown in Table 2 even when the amount of alloying elements is high. Moreover, each invention example has the moldability required as an outer panel of an automobile and the strength after BH, and achieves these coating film durability improving effects without reducing the strength.

On the other hand, as shown in Table 2, Comparative Examples 17 to 19 have unsuitable cleaning conditions such as no cleaning with etching with the chemical conversion solution on the surface of the aluminum alloy material before coating, or too light. For this reason, the average number density of Mg oxide on the surface of the aluminum alloy material before the thermosetting treatment is increased, and the aluminum alloy material under the thermosetting coating film even if the thermosetting treatment is performed under normal conditions at 180 ° C. The average number density of Mg oxide on the surface exceeds the upper limit of 1000 / mm ² and is too large.
Therefore, as shown in Table 2, these comparative examples are inferior in coating film durability even if they have the necessary strength after BH, and cannot be used as an outer panel of an automobile.

Comparative examples 20, 21, and 25 are examples in which normal thermosetting treatment conditions were intentionally removed for comparison. As shown in Table 2, the heating temperature of the thermosetting treatment is too low or the heating time is too short. For this reason, as shown in Table 2, in these comparative examples, the average number density of Mg oxides on the surface of the aluminum alloy material under the thermosetting coating film is too small below the lower limit of 10 pieces / mm ² . Therefore, as shown in Table 2, these comparative examples are inferior in coating film durability even if they have the necessary strength after BH, and cannot be used as an outer panel of an automobile.

For comparison, Comparative Examples 22, 23, and 24 are also examples in which normal thermosetting treatment conditions are intentionally removed. As shown in Table 2, the heating temperature of the thermosetting treatment is too high. For this reason, as shown in Table 2, the average number density of Mg oxides on the surface of the aluminum alloy material under the thermosetting coating film exceeds the upper limit of 1000 / mm ² and is too much. Therefore, as shown in Table 2, these comparative examples are inferior in coating film durability even if they have the necessary strength after BH, and cannot be used as an outer panel of an automobile.

In Comparative Examples 26 to 29, alloy numbers 10 to 13 in Table 1 are used, and the alloy composition deviates from the specified range.
In Comparative Example 26, the Mg content is too low (alloy number 10 in Table 1). For this reason, as shown in Table 2, the cleaning conditions involving the etching with the chemical conversion solution on the surface of the aluminum alloy material before coating are appropriate, and even if the heating temperature and heating time of the thermosetting treatment are appropriate, the present invention defines The average number density of Mg oxides on the surface of the aluminum alloy material under the thermosetting coating film is less than the lower limit of 10 pieces / mm ² and is too small. Therefore, as shown in Table 2, Comparative Example 26 is too low in strength after BH, is inferior in coating film durability, and cannot be used as an outer panel of an automobile.

In Comparative Example 27, as shown in Alloy No. 11 in Table 1, the Mg content is too large. For this reason, as shown in Table 2, the cleaning conditions involving the etching with the chemical conversion solution on the surface of the aluminum alloy material before coating are appropriate, and even if the heating temperature and heating time of the thermosetting treatment are appropriate, the present invention defines The average number density of Mg oxides on the surface of the aluminum alloy material under the thermosetting coating film is too ^high , exceeding the upper limit of 1000 / mm ² . Therefore, as shown in Table 2, Comparative Example 27 is inferior in coating film durability even when the strength after BH is high, and cannot be used as an outer panel of an automobile.

In Comparative Examples 28 and 29, as shown in Alloy Nos. 12 and 13 in Table 1, the contents of Si and Cu are too large.
Therefore, in these comparative examples, as shown in Table 2, the cleaning conditions involving etching with the chemical conversion solution on the surface of the aluminum alloy material before coating are appropriate, the heating temperature and heating time of the thermosetting treatment are appropriate, Even when the average number density of Mg oxide under the thermosetting coating film defined in the invention is satisfied, the coating film durability is poor.

  From the results of the above examples, the significance of improving the coating film durability without impairing the BH property after aging at room temperature, which is a requirement defined in the present invention, is supported.

  ADVANTAGE OF THE INVENTION According to this invention, the 6000 series aluminum alloy material which can improve coating-film durability, and its manufacturing method can be provided, without inhibiting BH property after room temperature aging, or a moldability. As a result, it is suitable for 6000 series aluminum alloy materials where the durability and appearance of the coating film are particularly problematic, such as an outer panel of an automobile.

Claims (7)

In mass%, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0%, Cu: 0.01 to 3.0%, respectively, the balance being Al and inevitable impurities Al -An aluminum alloy material comprising a Mg-Si-based aluminum alloy and having a thermosetting coating film on its surface, and when the surface of the aluminum alloy material under the thermosetting coating film is analyzed by an electron beam microanalyzer The average number density of Mg oxide having an equivalent circle diameter of 1 μm or more, which is discriminated from a color map showing the distribution state of Mg and O elements, is in the range of 10 to 1000 / mm ^2. Aluminum alloy material with excellent durability of thermosetting coating film.   The Al—Mg—Si-based aluminum alloy further comprises Mn: 0.01 to 1.0%, Cr: 0.001 to 0.3%, Zr: 0.001 to 0.3%, Ag: 0.00. The aluminum alloy material excellent in durability of the thermosetting type coating film of Claim 1 containing 1 type or 2 types or more of 001-0.2% and Sn: 0.001-0.2%.   The aluminum alloy material excellent in durability of the thermosetting coating film according to claim 1 or 2, wherein the Al-Mg-Si-based aluminum alloy is an automobile member. Aluminum containing Mg: 0.2-2.0%, Si: 0.3-2.0%, Cu: 0.01-3.0%, with the balance being Al and inevitable impurities. When the thermosetting coating film is formed on the surface of the aluminum alloy material by applying a thermosetting coating and thermosetting treatment to the alloy material, the surface of the aluminum alloy material on which the thermosetting coating film is formed The average number density of Mg oxide having an equivalent circle diameter of 1 μm or more determined from a color map showing the distribution state of Mg and O elements when an area analysis is performed by an electron beam microanalyzer is 10 to 1000 pieces / mm. ^The method for producing an aluminum alloy material excellent in durability of a thermosetting coating film, wherein the thermosetting treatment is performed so as to be in the range of ² .   The Al—Mg—Si based aluminum alloy material is further provided with Mn: 0.01 to 1.0%, Cr: 0.001 to 0.3%, Zr: 0.001 to 0.3%, Ag: 0. The manufacturing method of the aluminum alloy material excellent in durability of the thermosetting type coating film of Claim 4 containing 1 type or 2 types or more of 0.001-0.2% and Sn: 0.001-0.2% . The average number density of Mg oxides having an equivalent circle diameter of 1 μm or more on the surface of the aluminum alloy material before coating with the thermosetting paint is set to less than 10 pieces / mm ^{2 in} advance. The manufacturing method of the aluminum alloy material excellent in durability of the thermosetting type coating film of Claim 4 or 5 which performs a coating and a thermosetting process. The durability of the thermosetting coating film according to any one of claims 4 to 6, wherein the Al-Mg-Si aluminum alloy material is an automobile member, and the coating of the thermosetting paint is electrodeposition coating. A method for producing an aluminum alloy material excellent in quality.