JP5918187B2 - Aluminum alloy sheet with excellent bake hardenability - Google Patents

Description

  The present invention relates to an Al—Mg—Si based aluminum alloy plate. The aluminum alloy sheet referred to in the present invention is a rolled sheet such as a hot-rolled sheet or a cold-rolled sheet, and is subjected to tempering such as solution treatment and quenching process, and is baked and coated and cured. An aluminum alloy plate before artificial age hardening treatment. 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 meet such demands, as a material for large-sized body panels (outer panels, inner panels) such as automobile panels, especially hoods, doors, roofs, etc., instead of steel materials such as steel plates, it was excellent in formability and bake coating curability. The application of lighter aluminum alloy materials is increasing.

Among these, panels such as outer panels (outer plates) and inner panels (inner plates) of panel structures such as automobile hoods, fenders, doors, roofs, and trunk lids are thin and high-strength aluminum alloy plates. Al-Mg-Si-based AA to JIS 6000-series (hereinafter also simply referred to as 6000-series) aluminum alloy plates have been studied.

  This 6000 series aluminum alloy plate contains Si and Mg as essential components. Particularly, the excess Si type 6000 series aluminum alloy has a composition in which these Si / Mg is 1 or more in mass ratio, and has excellent age hardening ability. Have. For this reason, during press molding and bending, the moldability is ensured by reducing the yield strength, and the yield strength is improved by age hardening by heating during the artificial aging (hardening) treatment such as paint baking treatment of the panel after molding, There is a bake hardenability (hereinafter referred to as bake hardness = BH property, bake hardenability) that can ensure the required strength as a panel.

  Further, the 6000 series aluminum alloy plate has a relatively small amount of alloy elements as compared with other 5000 series aluminum alloys having a large amount of alloy such as Mg. For this reason, when the scraps of these 6000 series aluminum alloy sheets are reused as the aluminum alloy melting material (melting raw material), the original 6000 series aluminum alloy ingot is easily obtained, and the recyclability is excellent.

  On the other hand, as is well known, an outer panel of an automobile is manufactured by combining an aluminum alloy plate with a forming process such as an extension forming in a press forming or a bending forming. For example, a large outer panel such as a hood or door is formed into a molded product shape as an outer panel by press molding such as overhanging, and then the inner panel and Are joined to form a panel structure.

  Here, the 6000 series aluminum alloy has an advantage of having excellent BH property, but has aging property at room temperature, and after the solution quenching treatment, it is age-hardened by holding at room temperature for several months to increase the strength. As a result, there is a problem that the formability to the panel, particularly the bending workability, is lowered. For example, when a 6000 series aluminum alloy plate is used for an automotive panel application, it is usually about 1 to 4 months after being solution-quenched by an aluminum maker (after manufacture) and before being molded into a panel by an automobile maker. It is left at room temperature (and left at room temperature), and during this time, it is considerably age-hardened (room temperature aging). In particular, the outer panel that undergoes severe bending has problems such as cracking during hem processing after age hardening (room temperature aging), even though it can be molded without any problems immediately after production.

  Further, when such room temperature aging is large, the BH property is lowered, and the necessary strength as a panel is also obtained by heating during the artificial aging (curing) treatment such as the paint baking treatment of the panel after the molding described above. By the time, there arises a problem that the yield strength is not improved.

  Conventionally, various proposals have been made for improving BH properties and suppressing room temperature aging from the viewpoint of the structure of a 6000 series aluminum alloy plate, particularly clusters (aggregates of atoms). However, in many cases, the existence state of clusters (aggregates of atoms) that directly influence the BH property and room temperature aging property of the 6000 series aluminum alloy sheet is merely an indirect analogy of the behavior.

  In contrast, attempts have been made to directly measure and define clusters (aggregates of atoms) that affect the BH properties and room temperature aging properties of 6000 series aluminum alloy plates.

In Patent Document 1, among clusters (aggregates of atoms) observed when a structure of a 6000 series aluminum alloy plate is analyzed by a transmission electron microscope of 1 million times, a cluster having a circle equivalent diameter in the range of 1 to 5 nm. The average number density is specified in the range of 4000 to 30000 pieces / μm ² , and the BH property is excellent and room temperature aging is suppressed.

  Further, in Patent Documents 2 and 3, it is high even in a body paint baking process after aging at room temperature by controlling a specific atomic cluster (cluster) directly measured by a three-dimensional atom probe field ion microscope. It has been proposed to obtain a 6000 series aluminum alloy plate that can exhibit BH properties. In these patent documents, the aggregate of these atoms includes at least 10 or 30 or a total of either Mg atoms or Si atoms, and includes any atom of Mg atoms or Si atoms contained therein. Also as a reference, the distance between the reference atom and any one of the other adjacent atoms is 0.75 nm or less.

In addition, in Patent Document 2, an aggregate of atoms satisfying these conditions is included at an average number density of 1.0 × 10 ⁵ / μm ³ or more.

Further, in Patent Document 3, an aggregate of atoms satisfying these conditions is included at an average number density of 5.0 × 10 ²³ atoms / m ³ or more, and an aggregate of atoms satisfying these conditions is satisfied. Among these, while the average number density of the aggregate of atoms having a maximum equivalent circle radius of less than 1.5 nm is restricted to 10.0 × 10 ²³ atoms / m ³ or less, this maximum equivalent circle The ratio between the average number density a of the aggregate of atoms with a radius of less than 1.5 nm and the average number density b of the aggregate of atoms with a maximum equivalent circle diameter of 1.5 nm or more in size It is assumed that the maximum circle equivalent diameter radius includes a set of atoms having a size of 1.5 nm or more so that a / b is 3.5 or less.

  On the other hand, as prior patents related to the addition of Sn in the present invention, there are many methods other than Patent Documents 4 and 5, in which Sn is positively added to a 6000 series aluminum alloy sheet to suppress room temperature aging and improve bake coating hardening. Proposed. For example, Patent Document 4 limits the component relationship between Mg and Si to -2.0> 4Mg-7Si, adds an appropriate amount of Sn having a temporal change suppressing effect, and performs pre-aging after solution treatment, thereby suppressing room temperature aging. And a method that combines baking and curing. Patent Document 5 limits the component relationship between Mg and Si to −2.0 ≦ 4Mg−7Si ≦ 1.0, adds Sn having a temporal change suppressing effect and Cu for improving formability, and performs zinc-based plating. On the other hand, methods for improving formability, baking paintability and corrosion resistance have been proposed.

JP 2009-242904 A JP 2012-193399 A JP 2013-60627 A JP 09-249950 A JP-A-10-226894

  However, the demand for improving the fuel efficiency of automobiles is still high, and further weight reduction is being promoted. This tends to require a thinner aluminum alloy sheet. On the other hand, the size and number density of the relatively large atomic aggregates evaluated by TEM observation or the prior art that analogizes the behavior of the atomic aggregates (clusters) by indirect measurement. In Patent Document 1, which is only controlled, an atomic assembly cannot be evaluated accurately or in detail. For this reason, precise control of the atomic assembly has not been achieved, and the BH property after aging at room temperature has been insufficient. Further, Patent Documents 2 and 3 which control specific atomic aggregates (clusters) directly measured by a transmission electron microscope of 1 million times or a three-dimensional atom probe field ion microscope also have long-term room temperature. There is still room for improvement in order to combine high BH properties after aging and good workability. This also applies to Patent Documents 4 and 5 in which Sn is positively added to a 6000 series aluminum alloy plate.

  In view of such problems, the object of the present invention is to evaluate the aggregates of atoms in the structure in more detail, so that high BH properties and good workability can be achieved even in car body paint baking treatment after room temperature aging. It is to provide an Al—Si—Mg based aluminum alloy plate that can exhibit the above.

In order to achieve this object, the gist of the aluminum alloy plate excellent in bake coating curability of the present invention is, by mass, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0%. , Sn: 0.005 to 0.3% each, and the balance is an Al—Mg—Si based aluminum alloy plate made of Al and unavoidable impurities, all measured by a three-dimensional atom probe field ion microscope While the _total of the number of Mg atoms and Si atoms is N _total , as an aggregate of atoms measured by this three-dimensional atom probe field ion microscope, either or both of Mg atoms and Si atoms are 10 in total. In addition, the distance between the reference atom and any one of the other atoms adjacent to each other is 0.75 nm or less. is there It is contained in all of the atomic aggregate meeting the matter, when the N _cluster sum of the number of all Mg and Si atoms, the ratio for the N _total of _{N cluster (N cluster / N total} ) × 100 Is 1% or more and 15% or less, and the average radius of the equivalent circle diameter of the aggregate of atoms is 1.20 nm or more and 1.50 nm or less.

  In the present invention, for an Al—Mg—Si-based aluminum alloy plate containing Sn, among clusters (clusters) of atoms measured by 3DAP, fine clusters having a mutual distance between the atoms of 0.75 nm or less are used. It is assumed that there are many. Further, the total amount (total amount) of Mg and Si present in these clusters is balanced with the total amount of Mg and Si dissolved in the matrix, and the distribution state of the cluster size is also determined by the circle of the cluster. It controls as an average radius of an equivalent diameter, and improves BH property.

  If the total amount of Mg atoms and Si atoms present in the clusters defined as described above is balanced with the total amount of Mg and Si dissolved in the matrix, the BH property can be increased. In addition, Mg and Si contained in the 6000 series aluminum alloy plate can be converted into a coarser cluster than the standard, a coarser precipitate, or an intermetallic compound as a mode other than the solid solution in the cluster and matrix defined in the present invention. May be included. On the other hand, if the total amount of Mg and Si present in the cluster is controlled on the balance with the total amount of Mg and Si dissolved in the matrix, the coarseness caused by Mg and Si. Leading to a reduction in the number of large clusters, coarser precipitates, or intermetallic compounds themselves.

  Further, the distribution state of the size of the cluster greatly influences the BH property. In order to improve the BH property of the Al—Mg—Si based aluminum alloy plate containing Sn, the average radius of the equivalent circle diameter of the aggregate of atoms is described. Must be 1.20 nm or more and 1.50 nm or less. By controlling the total amount of Mg atoms and Si atoms present in the cluster and the distribution state of the cluster size, the moldability is good even when it is aged for 100 days at room temperature for a long time. An Al—Si—Mg based aluminum alloy plate that can also exhibit higher BH properties can be provided.

  Hereinafter, embodiments of the present invention will be specifically described for each requirement.

Cluster (a collection of atoms):
First, the meaning of the cluster in the present invention will be described. The cluster in the present invention refers to an aggregate of atoms (cluster) measured by 3DAP, which will be described later, as in Patent Documents 2 and 3, and is mainly expressed as a cluster in the following description. In a 6000 series aluminum alloy, it is known that Mg and Si form an aggregate of atoms called clusters during a room temperature hold or a heat treatment at 50 to 150 ° C. after solution treatment and quenching treatment. However, the behavior of the clusters generated at room temperature and during the heat treatment at 50 to 150 ° C. is completely different.

  The cluster formed by holding at room temperature suppresses the precipitation of the GP zone or β ′ phase that increases the strength in the subsequent artificial aging or baking coating treatment. On the other hand, clusters (or Mg / Si clusters) formed at 50 to 150 ° C. have been shown to promote precipitation of GP zones or β ′ phases (for example, Yamada et al .: Light Metal vol. 51). , Page 215).

  Incidentally, in the said patent document 1, it is described that these clusters are conventionally analyzed by specific heat measurement, 3DAP (three-dimensional atom probe), etc. over the paragraphs 0021-0025. At the same time, in the analysis of the cluster by 3DAP, even if the existence of the cluster itself is supported by observation, the size and number density of the cluster defined in the present invention can be measured only in an unknown or limited manner. It is stated that.

  Certainly, in a 6000 series aluminum alloy, an attempt to analyze the cluster by 3DAP (three-dimensional atom probe) has been made conventionally. However, as described in Patent Document 1, even if the existence of the cluster itself is supported, the size and number density of the cluster are unknown. This is because it is unclear which of the aggregates (clusters) of atoms measured by 3DAP correlates with the BH property, and it is not clear which of the atomic aggregates greatly affects the BH property. Because there was.

  On the other hand, the present inventors clarified clusters that are greatly related to the BH property in Patent Document 2. That is, among the clusters measured by 3DAP, as specified above, a specific value that includes Mg atoms or Si atoms in total or more and a distance between adjacent atoms included in these is not more than a specific value. It was found that the cluster and the BH property are greatly correlated. The inventors have also found that by increasing the number density of atomic aggregates satisfying these conditions, high BH properties can be exhibited even when subjected to body paint baking after room temperature aging.

  According to this Patent Document 2, the presence of a cluster that includes a total of 30 or more of either Mg atoms or Si atoms, and the distance between adjacent atoms is 0.75 nm or less is BH property. Improve. In addition, even if a certain amount or more of these clusters are present, an Al—Si—Mg-based aluminum alloy plate aged at room temperature can be subjected to a low-temperature, short-time car body paint baking process at 150 ° C. for 20 minutes. The higher BH property can be exhibited.

On the other hand, the present inventors have found that, among the clusters measured by 3DAP, the presence of a large number of the clusters certainly improves the BH property, but the improvement effect is not sufficient by itself. did. In other words, it was found that the presence of a large number of the clusters is a precondition (requirement) for improving the BH property, but is not necessarily a sufficient condition.
For this reason, the present inventors have further applied for the patent document 3. This is because there is a difference (distribution) in the size (size) of clusters containing either or both of Mg atoms and Si atoms, and there is a large difference in the effect on the BH property depending on the size of the clusters. Based on the idea that there is. There is the opposite difference between the effect of the size of the cluster on the BH property that the relatively small size cluster inhibits the BH property, while the relatively large size cluster promotes the BH property. Based on this, the BH property can be further improved by reducing the relatively small size clusters and increasing the relatively large size clusters among the specific clusters. Although relatively small size clusters disappear during BH treatment (at the time of artificial age hardening), on the other hand, during this BH, the precipitation of large clusters that are highly effective in improving the strength is inhibited to lower the BH property. It is inferred that On the other hand, it is presumed that a relatively large size cluster grows during the BH treatment, promotes precipitation of precipitates during the BH treatment, and increases the BH property.

  In subsequent studies, in the Al-Mg-Si based aluminum alloy plate containing Sn, the balance of the amount of Mg and Si dissolved in the above-described aggregates (clusters) of the atoms is also BH and strength after BH treatment. It has been found that it greatly affects That is, the present invention controls the ratio of Mg, Si atoms and Mg, Si present in the matrix contained in the aggregate of atoms satisfying the above specified conditions, while increasing the strength before baking coating, This is based on the knowledge that the BH property can be increased.

  At the same time, in an Al—Mg—Si-based aluminum alloy plate containing Sn, too large clusters grow too large when grown during the BH treatment, conversely reducing the BH properties and before the BH treatment. It has also been found that the strength of the steel becomes too high and the workability deteriorates. That is, it has also been found that there is an optimally sized cluster in order to increase the BH property without degrading the workability. Although the distribution state of the size of the specific atomic aggregate is important, it has also been found that the average radius of the equivalent circle diameter, which is the average size of the specific atomic aggregate, greatly affects the BH property. That is, in order to improve the BH property of the Al—Mg—Si-based aluminum alloy plate containing Sn, the average radius of the circle-equivalent diameter of the aggregate of the specific atoms is 1.20 nm or more in the specific range. It is necessary for improving the BH property to be 50 nm or less.

(Cluster specification of the present invention)
Hereinafter, the cluster definition of the present invention will be described in detail.
As described above, the aluminum alloy plate in which the present invention defines a cluster is a rolled plate such as a hot rolled plate or a cold rolled plate, and has been subjected to tempering such as solution treatment and quenching treatment. In addition, it refers to a plate before being processed into a panel by press molding or the like (a plate before being subjected to artificial age hardening treatment such as baking coating hardening treatment). However, in order to be molded as the automobile member or the like, it is often left at room temperature for a relatively long period of about 0.5 to 4 months after the production of the plate. For this reason, even if it is the structure | tissue state of the board after standing at room temperature for this long term, it is preferable to set it as the structure | tissue prescribed | regulated by this invention. In this regard, when the property after long-term room temperature aging is a problem, it is expected that the property does not change after about 100 days of room temperature aging, and the structure does not change. It is more preferable to investigate and evaluate the structure and characteristics of the plate after 100 days or more after the progress of the series of tempering.

(Definition of the cluster of the present invention)
The structure in an arbitrary thickness center part of such an aluminum alloy plate is measured with a three-dimensional atom probe field ion microscope. In the present invention, as the clusters present in the measured tissue, first, the clusters include 10 or more of either Mg atoms or Si atoms or both in total. It should be noted that the number of Mg atoms and Si atoms contained in the aggregate of atoms is preferably as large as possible, and the upper limit is not particularly specified, but from the production limit, the upper limit of the number of Mg atoms and Si atoms contained in this cluster is Approximately 10,000 pieces.

  In Patent Document 2, the cluster includes at least 30 Mg atoms and / or Si atoms in total. However, in the present invention, as described above, a relatively small size cluster inhibits the BH property, so that this is regulated and reduced. For this reason, in order to control this relatively small size cluster to be regulated within a measurable range, similarly to Patent Document 3, either one or both of Mg atoms and Si atoms in total are 10 or more. It is defined as including.

  In the present invention, similarly to Patent Documents 2 and 3, any one of Mg atoms and Si atoms contained in these clusters is used as a reference, and any of the other atoms adjacent to the reference atom is selected. An atom having a distance from each other of 0.75 nm or less is defined as an aggregate (cluster) of atoms defined in the present invention (satisfying the definition of the present invention). This mutual distance of 0.75 nm ensures that the distance between atoms of Mg and Si is close, guaranteeing the number density of large size clusters that have an effect of improving the BH property after aging at room temperature, and conversely, This is a numerical value that is set to regulate the cluster and control the number density to a low level. The inventors of the present invention have studied in detail the relationship between an aluminum alloy plate capable of exhibiting high BH properties in a car body paint baking process and an atomic level aggregate, and as a result, the number density of the atomic aggregate defined by the above definition is as follows. It has been experimentally found that a large is a tissue form exhibiting high BH properties. Therefore, although the technical significance of the distance between atoms of 0.75 nm is not sufficiently clarified, it is necessary to strictly guarantee the number density of atomic aggregates exhibiting a high BH property, and is defined for that purpose. It is a numerical value.

  Although the cluster prescribed | regulated by this invention has the case where both Mg atom and Si atom are included most often, it includes the case where Mg atom is included but Si atom is not included, and the case where Si atom is included but Mg atom is not included. Moreover, it is not necessarily comprised only by Mg atom or Si atom, In addition to these, Al atom is included with very high probability.

  Furthermore, Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, which are included as alloy elements and impurities, depending on the component composition of the Al—Mg—Si-based aluminum alloy plate containing Sn targeted by the present invention. Inevitably, atoms such as Zn or Ag are included in the cluster and these other atoms are counted by 3DAP analysis. However, even if these other atoms (from alloying elements and impurities) are included in the cluster, the level is smaller than the total number of Mg atoms and Si atoms. Therefore, even when such other atoms are included in the cluster, those satisfying the above-mentioned rules (conditions) function as the cluster of the present invention in the same manner as a cluster composed only of Mg atoms or Si atoms. Therefore, the cluster defined in the present invention may include any other atom as long as the above-described definition is satisfied.

  Further, according to the present invention, "the reference is based on any of the Mg atoms or Si atoms contained therein, the mutual distance between the reference atom and any one of the other adjacent atoms is 0.00. “75 mm or less” means that all Mg atoms and Si atoms present in the cluster have at least one Mg atom or Si atom having a distance of 0.75 nm or less around each other. is there.

  In the cluster of the present invention, the definition of the distance between atoms is based on any atom of Mg atoms or Si atoms included in these atoms, and all atoms among other atoms adjacent to the reference atom. The distances may not all be 0.75 nm or less, and conversely, all the distances may be 0.75 nm or less. In other words, other Mg atoms or Si atoms having a distance exceeding 0.75 nm may be adjacent to each other, and the specified distance (interval) is satisfied around a specific (reference) Mg atom or Si atom. There should be at least one other Mg atom or Si atom.

  When there is one other adjacent Mg atom or Si atom satisfying this specified distance, the number of Mg atoms or Si atoms that satisfy the distance condition is specified (reference) Mg. There are two atoms including atoms or Si atoms. In addition, when there are two adjacent Mg atoms or Si atoms satisfying the specified distance, the number of Mg atoms or Si atoms to be counted that satisfy the distance condition is a specific (reference) Mg The number is 3 including atoms or Si atoms.

  The cluster described above is a cluster generated by the temperature holding process after solution treatment and quenching at a high temperature in the tempering after rolling described above and in detail later. That is, the cluster in the present invention is an aggregate of atoms generated by solution treatment and a temperature holding treatment after quenching at a high temperature, and is a total of 10 or more of either Mg atoms or Si atoms or both. A cluster having a distance of 0.75 nm or less between the reference atom and any one of the other adjacent atoms, regardless of which Mg atom or Si atom included therein It is.

(Amount of Mg and Si in the cluster)
In the present invention, it is a cluster defined as described above (which satisfies the precondition), and Mg and Si present in all the clusters included in the entire Al—Mg—Si-based aluminum alloy plate containing Sn. The total amount of atoms is controlled by the relationship between the total amount of Mg and Si contained in the entire aluminum alloy plate. This appropriately controls the balance between the total amount of Mg and Si atoms present in the defined cluster and the total amount of Mg and Si atoms dissolved in the matrix of the aluminum alloy plate. Will be. This can increase the BH property.

For this balance control, in the present invention, the number of all Mg and Si atoms contained in a specific cluster (atomic assembly) measured on the assumption that the measurement is performed by a three-dimensional atom probe field ion microscope. N _cluster which is the sum (total amount) of N is a constant ratio with respect to N _total which is the sum (total amount) of all the measured numbers of Mg and Si atoms.

That is, the ratio of N _cluster to N _total (N _cluster / N _total ) × 100 is set in a range of 1% to 15%. Here, the ratio of N _cluster to N _total calculated by (N _cluster / N _total ) × 100 is a measure of reproducibility, as in the examples described later, by measuring multiple thicknesses at the center of the test plate. The average (average ratio) at each location.

  By making such a well-balanced structure, the strength after baking coating is 200 MPa or more and the BH property (strength difference before and after baking coating) exceeds 90 MPa after 100 days of holding the plate at room temperature after standing (room temperature standing). Can be realized.

However, the fact of the correlation between such a structure and BH property has been found experimentally, and the mechanism has not been fully elucidated. However, the average rate _{(N cluster / N total) ×} 100 is less than 1% of N _total of the above-described N _cluster, the result in which the Mg and Si solid solution in the aluminum alloy plate becomes large, precipitation strengthening by the cluster weakens Thus, the strength before baking coating is lowered due to the limit of solid solution strengthening. For this reason, the strength after baking is inevitably low.

On the other hand, the average ratio _{(N cluster / N total) ×} 100 for the N _total of the above-described N _cluster if it exceeds 15%, Mg and Si content contained in the cluster is too large, a solid solution in the aluminum alloy plate Mg and Si are reduced. For this reason, the number of strengthening phases (β ″) generated during the artificial age hardening treatment is reduced, and the BH property is liable to be lowered. For this reason, the strength after baking coating is also liable to be lowered.

(Cluster density)
In order to control the average percentage _{(N cluster / N total) ×} 100 for the N _total of the above-described N _cluster in the range of 1% to 15%, the clusters specified in the present invention 2.5 × ^{10 23} pieces / It is preferable to include it with an average number density of m ³ or more. If the average number density of the clusters is less than 2.5 × 10 ²³ / m ³ , the amount of the clusters themselves will be insufficient, and added (contained) to the clusters formed by the room temperature aging. It means that most of Mg and Si are consumed. For this reason, it becomes difficult to make the total amount of Mg and Si present in the cluster 1% or more, and the effect of improving the BH property decreases after standing at room temperature for a long time (room temperature aging). Incidentally, a preferable range of the average number density of this cluster is an average number density range of 2.5 × 10 ²³ pieces / m ³ or more and 20.0 × 10 ²³ pieces / m ³ or less.

(Size distribution rule of the present invention cluster)
In the present invention, in the Al—Mg—Si based aluminum alloy plate containing Sn, the control of the total amount of Mg and Si atoms in the cluster as described above, and further, a circle equivalent of an aggregate of atoms satisfying these conditions The average radius of the diameter is 1.20 nm or more and 1.50 nm or less. A cluster in which the total amount of atoms of Mg and Si is controlled, and the size is within the range of 1.20 nm to 1.50 nm in terms of the average radius E (r) of the equivalent circle diameter, the strength is improved during BH. Are precipitated as intermediate precipitates such as β ″ or β ′ that are highly effective (contribute to strength improvement). Accordingly, the strength is low and the workability is good at the stage of press molding or bending, and the strength becomes high only after BH.

  The average radius E (r) (nm) of the equivalent circle diameter of the aggregate of atoms is represented by E (r) = (1 / n) Σr. Here, n is the number of atomic aggregates satisfying the preconditions. r is a radius (nm) of an equivalent circle diameter of an aggregate of individual atoms satisfying the above preconditions.

Aggregates (clusters) of atoms having an average circle equivalent diameter E (r) that is too small disappear during BH treatment (at the time of artificial age hardening), and β ″ is highly effective in improving the strength during this BH. Alternatively, the precipitation of intermediate precipitates such as β ′ is suppressed to inhibit the BH property. On the other hand, an aggregate (cluster) of atoms whose average radius E (r) of the equivalent circle diameter is too large is already in the middle of β ″ or β ′ due to room temperature aging before BH treatment (before or in advance). It precipitates as a precipitate, and on the contrary, the strength before BH is excessively increased, and press formability and bending workability are hindered. In addition, if an intermediate precipitate such as β ″ or β ′ has already been deposited before the BH treatment, a new intermediate precipitate such as β ″ or β ′ is deposited during the BH. To suppress the BH property. By the way, both β ″ and β ′ are intermediate precipitation phases and both are Mg ₂ Si, but the crystal structure (arrangement of atoms) is different and difficult to express separately, so when “′” cannot be used, β ′ is called β prime, and β ″ is called β double prime.

  As described above, even if the total amount of atoms of Mg and Si is controlled, if there are many small clusters that inhibit BH, the average radius of the equivalent circle diameter becomes smaller than 1.20 nm, BH property decreases. On the other hand, even if the cluster is defined in the present invention, even if there are many large clusters that inhibit BH, the average radius of the equivalent circle diameter is larger than 1.50 nm among the above specifications, If the strength is too high, press formability and bending workability are lowered, and BH properties are also lowered.

  On the other hand, an aggregate of atoms satisfying the above-mentioned prerequisites and having a size in the range of 1.20 nm or more and 1.50 nm or less in terms of the average radius E (r) of the equivalent circle diameter, It precipitates as an intermediate precipitate such as β ″ or β ′ that is highly effective in improving the strength (contributes to improving the strength). Accordingly, the strength is low and the workability is good at the stage of press molding or bending, and the strength becomes high only after BH.

(Measurement principle and measurement method of 3DAP)
The measurement principle and measurement method of 3DAP of the present invention are disclosed in Patent Documents 2 and 3. That is, 3DAP (three-dimensional atom probe) is obtained by attaching a time-of-flight mass analyzer to a field ion microscope (FIM). With such a configuration, the local analyzer is capable of observing individual atoms on a metal surface with a field ion microscope and identifying these atoms by time-of-flight mass spectrometry. In addition, 3DAP is a very effective means for structural analysis of atomic aggregates because it can simultaneously analyze the type and position of atoms emitted from a sample. For this reason, as described above, it is used as a magnetic recording film, an electronic device, or a structure analysis of a steel material as a known technique. In addition, recently, as described above, it is also used for discrimination of the cluster of the structure of the aluminum alloy plate.

  This 3DAP uses an ionization phenomenon of sample atoms under a high electric field called field evaporation. When a high voltage necessary for the field evaporation of sample atoms is applied to the sample, the atoms are ionized from the sample surface and pass through the probe hole to reach the detector.

  This detector is a position-sensitive detector, and it is detected by measuring the time of flight to the individual ion detector along with mass analysis of individual ions (identification of elements that are atomic species). The determined position (atomic structure position) can be determined simultaneously. Therefore, 3DAP has the feature that the atomic structure at the tip of the sample can be reconstructed and observed three-dimensionally because the position and atomic species of the atom at the tip of the sample can be measured simultaneously. Further, since field evaporation occurs sequentially from the tip surface of the sample, the distribution of atoms in the depth direction from the sample tip can be examined with atomic level resolution.

  Since this 3DAP uses a high electric field, the sample to be analyzed must be highly conductive, such as metal, and the shape of the sample is generally very fine with a tip diameter of around 100 nmφ or less. Need to be needle-shaped. For this reason, a sample is taken from the central part of the thickness of the aluminum alloy plate to be measured, and this sample is cut and electropolished with a precision cutting device to obtain a sample having an ultra-fine needle tip for analysis. Make it. As a measuring method, for example, using “LEAP3000” manufactured by Imago Scientific Instruments, a high pulse voltage of 1 kV order is applied to an aluminum alloy plate sample whose tip is shaped like a needle, and several millions from the sample tip. This is done by ionizing atoms continuously. The ions are detected by a position sensitive detector, and a pulse voltage is applied. From the time of flight from when each ion jumps out of the sample tip until it reaches the detector, mass analysis of ions (atomic species) Element identification).

  Furthermore, using the property that field evaporation occurs regularly from the tip surface of the sample, coordinates in the depth direction are given to a two-dimensional map indicating the arrival location of ions as appropriate, and analysis software “IVAS” Is used to perform three-dimensional mapping (three-dimensional atomic structure: construction of an atom map). Thereby, a three-dimensional atom map of the sample tip is obtained.

  This three-dimensional atom map is further analyzed for an aggregate (cluster) of atoms by using Maximum Separation Method, which is a method of defining atoms belonging to precipitates and clusters. In this analysis, the number of Mg atoms or Si atoms or both (total of 10 or more), the distance (interval) between adjacent Mg atoms or Si atoms, and the specific narrow interval The number of Mg atoms or Si atoms having (0.75 nm or less) is given as a parameter.

And, including any one or both of Mg atoms and Si atoms in total of 10 or more, even if any atom of Mg atoms or Si atoms contained in these is used as a reference, other atoms adjacent to the reference atom A cluster having a distance of 0.75 nm or less from any one of these atoms and satisfying these conditions is defined as an aggregate of atoms of the present invention. Then, the dispersion state of the atomic aggregates that meet this definition is evaluated, and the number density of the atomic aggregates is averaged over three or more measurement samples to obtain an average density per 1 m ³ (number / piece m ³ ) Measure and quantify.

Soite, obtaining the number N _cluster of Mg and Si atoms contained in the aggregate of all the conditions are satisfied atoms. In addition, the number N _total of all Mg and Si atoms contained in both the solid solution and the atomic aggregate, which is detected by the detector, that is, measured by 3DAP is obtained. And the ratio with respect to _Ntotal of _Ncluster is calculated | required from the formula of _Ncluster / _Ntotalx100 , and this average value (average ratio) is controlled so that it may become 1% or more and 15% or less.

Further, the inherent analysis software with the 3DAP originally, of a collection of the atoms became measured when regarded as a sphere, the radius of gyration l _g as the maximum obtained by equation the following equation 1.

In Equation (1 ₎ , lg is a radius of rotation automatically calculated by software unique to the three-dimensional atom probe field ion microscope. x, y, and z are invariant x, y, and z axes in the measurement layout of the three-dimensional atom probe field ion microscope. x _i , y _i , and z _i are the lengths of the x, y, and z axes, and are the spatial coordinates of the Mg and Si atoms that constitute the aggregate of atoms. The “X bar” in which “−” is placed on “x”, “y”, and “z” is the length of the x, y, and z axes, but is the barycentric coordinate of the aggregate of atoms. n is the number of Mg and Si atoms constituting the aggregate of atoms.

Next, converted by the relationship of the radius of gyration _{l g} a Guinier radius _{r G} the following equation number _{2, r G = √ (5/3} ) · l g.

The converted Guinier radius r _G is regarded as the radius of the atomic aggregate, and the maximum equivalent circle diameter r of each of the atomic aggregates to be measured is calculated. In addition, the number n of aggregates of atoms satisfying the preconditions is also calculated. Furthermore, the average number density (number / m ³ ) of the aggregate of atoms satisfying the preconditions can be calculated from the number n.
The measurement of the clusters by these 3DAP is carried out at 10 sites in the central part of the arbitrary thickness of the Al-Mg-Si aluminum alloy plate after the tempering, and each of the measured values (calculated values). Are averaged to obtain each average value defined in the present invention.

  Then, from the calculated maximum equivalent circle diameter r and the number n of atomic aggregates satisfying the preconditions, the average radius E (r) (nm) of the equivalent circular diameter of the atomic aggregate is calculated as follows. E (r) = (1 / n) Σr.

(Atom detection efficiency by 3DAP)
The detection efficiency of these atoms by 3DAP is currently limited to about 50% of the ionized atoms, and the remaining atoms cannot be detected. If the detection efficiency of atoms by this 3DAP changes greatly, such as improving in the future, the measurement result by 3DAP of the average number density (number / μm ³ ) of each size cluster defined by the present invention may change. There is sex. Therefore, in order to have reproducibility in this measurement, it is preferable that the detection efficiency of atoms by 3DAP is substantially constant at about 50%.

(Chemical composition)
Next, the chemical component composition of the 6000 series aluminum alloy plate will be described below. The 6000 series aluminum alloy plate targeted by the present invention is required to have excellent properties such as formability, BH property, strength, weldability, and corrosion resistance as a plate for an automobile outer plate.

  In order to satisfy such requirements, the composition of the aluminum alloy plate is, by mass, Mg: 0.2-2.0%, Si: 0.3-2.0%, Sn: 0.005-0. 3% each, and the balance consisting of Al and inevitable impurities. In addition,% display of content of each element means the mass% altogether.

  The 6000 series aluminum alloy plate targeted by the present invention is an excess Si type 6000 series aluminum alloy plate having a better BH property and a Si / Mg mass ratio of Si / Mg of 1 or more. Is preferred. 6000 series aluminum alloy sheets ensure formability by reducing the yield strength during press molding and bending, and age resistance is improved by age hardening by heating during artificial aging treatment such as paint baking treatment of panels after molding. And, it has an excellent age-hardening ability (BH property) that can ensure the required strength. Among these, the excess Si type 6000 series aluminum alloy plate is more excellent in this BH property than the 6000 series aluminum alloy plate having a mass ratio Si / Mg of less than 1.

  In the present invention, these other elements other than Mg and Si are basically impurities or elements that may be contained, and the content (allowable amount) at each element level in accordance with AA or JIS standards.

  That is, from the viewpoint of resource recycling, in the present invention, not only high-purity Al ingots but also 6000 series alloys containing many other elements other than Mg and Si as additive elements (alloy elements) are used as melting raw materials for alloys. When a large amount of aluminum alloy scrap material, low-purity Al metal, etc. is used, the following other elements are necessarily mixed in substantial amounts. And refining itself which dares to reduce these elements raises cost, and the tolerance to contain to some extent is needed. Moreover, even if it contains a substantial amount, there is a content range that does not hinder the object and effect of the present invention.

  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. Specifically, Mn: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), Fe: 1.0% or less (excluding 0%) %), Cr: 0.3% or less (excluding 0%), Zr: 0.3% or less (excluding 0%), V: 0.3% or less (provided that 0% not included), Ti: 0.05% or less (excluding 0%), Zn: 1.0% or less (excluding 0%), Ag: 0.2% or less (excluding In addition to the basic composition described above, one or more of the above may be further contained within this range. In addition, when these elements are contained, since Cu tends to deteriorate corrosion resistance when the content is large, the Cu content is preferably 0.7% or less, more preferably 0.3% or less. Further, when Mn, Fe, Cr, Zr, and V are contained in a large amount, a relatively coarse compound is likely to be generated, and hem bendability is likely to be deteriorated. Therefore, the Mn content is preferably 0.6% or less, more preferably 0.3% or less, and the Cr, Zr, V content is preferably 0.2% or less, more preferably 0.1% or less. Do each. The content range and significance of each element in the 6000 series aluminum alloy, or the allowable amount will be described below.

Si: 0.3-2.0%
Si, together with Mg, is an important element for forming the cluster defined in the present invention. In addition, during solid solution strengthening and artificial aging treatment such as paint baking treatment, aging precipitates that contribute to strength improvement are formed to show age hardening ability and to obtain the strength (yield strength) necessary for an automobile outer panel Is an essential element. Furthermore, in the 6000 series aluminum alloy plate of the present invention, it is the most important element for combining various properties such as total elongation that affect the press formability. In addition, in order to exhibit excellent age-hardening ability in the paint baking process after forming on the panel, Si / Mg is set to a mass ratio of 1.0 or more, and Si is further added to Mg rather than the excessive Si type generally referred to. It is preferable to make the composition of 6000 series aluminum alloy excessively contained.

  If the Si content is too small, the absolute amount of Si is insufficient, so that it is not possible to form only the number density that defines the clusters defined in the present invention, and the paint bake hardenability is significantly reduced. Furthermore, it cannot combine various properties such as total elongation required for each application. On the other hand, when there is too much Si content, a coarse crystallization thing and a precipitate will be formed and bending workability, total elongation, etc. will fall remarkably. Furthermore, weldability is also significantly impaired. Therefore, Si is made 0.3 to 2.0% in range. A more preferred lower limit is 0.6%, and a more preferred upper limit is 1.4%.

Mg: 0.2-2.0%
Mg is also an important element for cluster formation as defined in the present invention together with Si. In addition, during the artificial aging treatment such as solid solution strengthening and paint baking treatment, it is essential to form aging precipitates that contribute to strength improvement together with Si, exhibit age hardening ability, and obtain the necessary proof strength as a panel Elements.

  If the Mg content is too small, the absolute amount of Mg is insufficient, so that it is not possible to form only the number density that defines the clusters defined in the present invention, and the paint bake hardenability is significantly reduced. For this reason, the proof stress required as a panel cannot be obtained. On the other hand, when there is too much Mg content, a coarse crystallized substance and a precipitate will be formed and bending workability, total elongation, etc. will fall remarkably. Therefore, the Mg content is in the range of 0.2 to 2.0%. A more preferred lower limit is 0.3%, and a more preferred upper limit is 1.0%. Further, the amount is preferably such that Si / Mg is 1.0 or more by mass ratio.

Sn: 0.005 to 0.3%
Sn traps vacancies at room temperature, thereby suppressing diffusion at room temperature and suppressing cluster formation at room temperature. For this reason, it has the effect of reducing As proof stress and improving hemmability, both at the beginning of room temperature aging (7 days) and at the end of room temperature aging (100 days). Further, since the trapped voids are released at a high temperature when baking is applied, the diffusion can be promoted and the BH property can be increased. For this reason, it is possible to increase the BH property when Sn is contained even when the cluster number density is comparable as compared with the case where Sn is not contained. When the content of Sn is too small, it can not be suppressed cluster generation in room temperature, number density or too many clusters, 15% average percentage _{(N cluster / N total) ×} 100 is for N _total of the above-described N _cluster There are cases where it exceeds. For this reason, after holding at room temperature for 100 days, the As yield strength is too high, the press formability and heme workability are inferior, the number of reinforcing phases (β ″) generated during the artificial age hardening treatment is reduced, and the BH property is low. Prone. Therefore, the Sn content is in the range of 0.005 to 0.3%. A more preferred lower limit is 0.01%, and a more preferred upper limit is 0.2%. As will be described later, the Al—Si—Mg-based aluminum alloy plate containing Sn is different in structure from that containing no Sn. However, even if Sn is contained in the same manner, this structure is different if the production conditions are different. Therefore, when a structure having an effect of suppressing room temperature aging at the high level of the present invention and improving the baking finish is obtained. Is not limited.

(Production method)
Next, a method for producing the aluminum alloy plate of the present invention will be described below. The aluminum alloy sheet of the present invention is a conventional process or a known process, and the aluminum alloy ingot having the above-mentioned 6000 series component composition is subjected to homogenization heat treatment after casting, and then subjected to hot rolling and cold rolling to obtain a predetermined process. It is manufactured by being subjected to a tempering treatment such as solution hardening and quenching.

  However, in order to control the cluster of the present invention in order to improve the BH property during these manufacturing steps, as described later, solution treatment and quenching treatment and proper quenching (cooling) stop temperature and its temperature range It is necessary to more properly control the holding in the. Also in other processes, there are preferable conditions for controlling the cluster within the specified range of the present invention.

(Dissolution, casting cooling rate)
First, in the melting and casting process, an ordinary molten casting method such as a continuous casting method and a semi-continuous casting method (DC casting method) is appropriately selected for the molten aluminum alloy adjusted to be dissolved within the above-mentioned 6000 series component composition range. Cast. Here, in order to control the cluster within the specified range of the present invention, the average cooling rate at the time of casting is as large as possible (fast) from the liquidus temperature to the solidus temperature of 30 ° C./min. Is preferred.

  When such temperature (cooling rate) control in the high temperature region during casting is not performed, the cooling rate in this high temperature region is inevitably slow. Thus, when the average cooling rate in the high temperature region becomes slow, the amount of crystallized material generated coarsely in the temperature range in this high temperature region increases, and in the plate width direction and thickness direction of the ingot. Variations in the size and amount of crystallized material also increase. As a result, there is a high possibility that the prescribed cluster cannot be controlled within the scope of the present invention.

(Homogenization heat treatment)
Next, the cast aluminum alloy ingot is subjected to a homogenization heat treatment prior to hot rolling. The purpose of this homogenization heat treatment (soaking) is to homogenize the structure, that is, eliminate segregation in crystal grains in the ingot structure. The conditions are not particularly limited as long as the object is achieved, and normal one-stage or one-stage processing may be performed.

  The homogenization heat treatment temperature is appropriately selected from the range of 500 ° C. or more and less than the melting point, and the homogenization time is 4 hours or more. When this homogenization temperature is low, segregation within the crystal grains cannot be sufficiently eliminated, and this acts as a starting point of fracture, so that stretch flangeability and bending workability are deteriorated. Thereafter, even if the hot rolling is started immediately or the hot rolling is started after cooling to an appropriate temperature, the number density of clusters defined in the present invention can be controlled.

  After performing this homogenization heat treatment, it is cooled to room temperature at an average cooling rate of 20-100 ° C / h between 300 ° C and 500 ° C, and then 350 ° C-450 ° C at an average heating rate of 20-100 ° C / h. It is possible to reheat up to this temperature and start hot rolling in this temperature range.

When the average cooling rate after the homogenization heat treatment and the subsequent reheating rate are not satisfied, there is a high possibility that a coarse Mg—Si compound is formed.

(Hot rolling)
Hot rolling is composed of an ingot (slab) rough rolling process and a finish rolling process according to the thickness of the rolled sheet. In these rough rolling process and finish rolling process, a reverse or tandem rolling mill is appropriately used.

  At this time, under conditions where the hot rolling (rough rolling) start temperature exceeds the solidus temperature, burning occurs and thus the hot rolling itself becomes difficult. On the other hand, when the hot rolling start temperature is less than 350 ° C., the load during hot rolling becomes too high, and the hot rolling itself becomes difficult. Therefore, the hot rolling start temperature is set to 350 ° C. to the solidus temperature, more preferably 400 ° C. to the solidus temperature.

(Hot rolled sheet annealing)
Annealing (roughening) of the hot-rolled sheet before cold rolling is not always necessary, but it can be performed to further improve properties such as formability by refining crystal grains and optimizing the texture. good.

(Cold rolling)
In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final thickness. However, in order to further refine the crystal grains, the cold rolling rate is desirably 60% or more, and intermediate annealing may be performed between the cold rolling passes for the same purpose as the roughening. .

(Solution and quenching)
After cold rolling, a solution hardening treatment is performed. The solution treatment and quenching treatment may be heating and cooling by a normal continuous heat treatment line, and is not particularly limited. However, since it is desirable to obtain a sufficient solid solution amount of each element and, as described above, it is desirable that the crystal grains are finer, a heating rate of 5 ° C. is applied to a solution treatment temperature of 520 ° C. or higher and a melting temperature or lower. It is desirable to carry out under the condition of heating at 0 / second or more and holding for 0-10 seconds.

Further, from the viewpoint of suppressing the formation of coarse grain boundary compounds that deteriorate the moldability and hemmability, it is desirable that the average cooling rate from the solution temperature to the quenching stop temperature is 3 ° C./s or more. If the cooling rate for solution treatment is low, coarse Mg ₂ Si and simple substance Si are generated during cooling, and formability deteriorates. Moreover, the amount of solid solution after solution forming falls, and BH property will fall. In order to ensure this cooling rate, the quenching treatment is performed by selecting water cooling means and conditions such as air cooling such as a fan, mist, spray, and immersion, respectively.

(Reheating treatment)
A reheating treatment is performed after the solution hardening treatment. This reheating treatment is performed in two stages, and the first stage is performed within a temperature range of an ultimate temperature (heating temperature) of 80 to 250 ° C. with a holding time of several seconds to several minutes. The cooling after the first-stage reheating treatment may be allowed to cool or may be forcibly quenched by using the cooling means at the time of solution hardening in order to increase production efficiency. Next, after the cooling after the first stage reheating treatment is completed, the second stage reheating is held within the temperature range of 70 to 130 ° C. within the holding time at room temperature within 24 hours. The time is in the range of 3 to 48 hours. If the holding time at room temperature after the end of cooling after the first stage reheating treatment exceeds 24 hours, aging at room temperature proceeds too much and the effect of the second stage reheating treatment is impaired.

  When deviating from such reheating conditions, the total content of Mg and Si contained in the above-described aggregate of atoms is the total content of Mg and Si contained in the aluminum alloy plate. It becomes difficult to set it to 10% or more and 30% or less. For example, when the ultimate temperature of the first stage reheating is less than 100 ° C. or the ultimate temperature of the second stage reheating is less than 70 ° C., Mg—Si clusters that promote BH properties are not sufficiently generated. On the other hand, if the reheating temperature is too high, a part of intermetallic compound phases such as β ″ and β ′ different from the clusters are formed, so the number density of the clusters tends to be less and the BH property is too low. End up. Further, due to β ″ and β ′, the moldability tends to deteriorate.

  The cooling to the room temperature after the second stage reheating treatment may be allowed to cool or may be forcibly quenched using the cooling means at the time of quenching in order to increase production efficiency. That is, because the clusters defined in the present invention have uniform or similar sizes are exhausted by the temperature holding treatment, forced rapid cooling as in the conventional reheating treatment and complicated control of the average cooling rate over several stages are not possible. It is unnecessary.

  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.

  Next, examples of the present invention will be described. 6000 series aluminum alloy plates having different compositions and cluster conditions defined in the present invention were prepared according to the two-stage reheating treatment conditions after the solution treatment and the quenching treatment. Then, the structure (cluster) and strength, BH property (coating bake hardenability), press formability and hemming property as bending workability after being kept at room temperature for 100 days in each example were also evaluated.

  The cluster conditions are the total amount of Mg and Si atoms present in the aggregate of atoms, the average radius of the equivalent circle diameter, and the average number density. And this aggregate of atoms includes a total of 10 or more of either Mg atoms or Si atoms, and any of the atoms of Mg atoms or Si atoms contained therein is used as a reference, Is an aggregate of atoms satisfying the condition that the distance between each atom and any one of the adjacent atoms is 0.75 nm or less.

  The specific production conditions for the aluminum alloy plate were as follows. Aluminum alloy ingots having respective compositions shown in Table 1 were commonly melted by DC casting. At this time, in common with each example, the average cooling rate during casting was set to 50 ° C./min from the liquidus temperature to the solidus temperature. In the display of the content of each element in Table 1 showing the composition of the 6000 series aluminum alloy plate of each example, the display in which the numerical value in each element is blank, the content is below the detection limit and includes these elements No 0%.

  Subsequently, the ingot was subjected to soaking at 540 ° C. for 4 hours in common with each example, and then hot rough rolling was started. And in each example, it was hot rolled to a thickness of 3.5 mm in the subsequent finish rolling to obtain a hot rolled sheet. The aluminum alloy sheet after hot rolling is commonly used in each example, and after subjecting to 500 ° C. × 1 minute of rough annealing, cold rolling is performed at a processing rate of 70% without intermediate annealing in the middle of the cold rolling pass, In each example, a cold-rolled plate having a thickness of 1.0 mm was used.

  Furthermore, each cold-rolled sheet was subjected to a solution treatment in a 560 ° C. glass stone furnace in common with each example, held for 10 seconds after reaching the target temperature, and quenched by water cooling. After this quenching treatment was completed, the first stage pre-aging treatment at 100 to 250 ° C. was performed under the conditions shown in Table 2, and the solution was cooled to room temperature. Thereafter, a second stage pre-aging treatment was performed at 70 to 130 ° C., and the mixture was cooled to room temperature by water cooling. Here, in this example, after the first and second stage reheating treatments, cooling is performed by water cooling, but a similar structure can be obtained even if this cooling is allowed to cool.

  A test plate (blank) was cut out from each plate after being left at room temperature for 100 days after the tempering treatment, and the structure and strength (AS proof stress) of each test plate were measured. The tissue observation using the 3DAP was performed only on the sample 100 days after the tempering treatment. These results are shown in Table 3.

(cluster)
The structure in the plate thickness direction cross section of the plate thickness central portion of the test plate after aging at room temperature for 100 days was analyzed by the 3DAP method, and the number density of clusters defined by the present invention in each of the analysis methods described above ( × 10 ²³ pcs / m ³ ), the average radius of equivalent circle diameter (nm), the total number of all Mg and Si atoms contained in the _cluster N _cluster , and the total number of Mg and Si atoms measured. The ratio with respect to a certain N _{total was calculated |} required.

  These results are shown in Table 3. In Table 3, the cluster condition defined in the present invention includes a total of 10 or more of either Mg atoms or Si atoms, and is simply simplified as “Mg, 10 Si atoms or more”. It is described. Moreover, even if any atom of Mg atom or Si atom contained in these is used as a reference, the mutual distance between the reference atom and any one of the other adjacent atoms is 0.75 nm or less, It is simply described as “distance 0.75 nm or less”.

These 3DAP measurements were performed by cutting three square columns 30 mm long, 1 mm wide, and 1 mm thick from a 1 mm thick test plate at intervals of 1 mm in the width direction. The prism was thinned by electropolishing to produce a needle-like sample with a tip radius of 50 nm. For this reason, the measurement location measures the vicinity of the center of the plate thickness. A 3DAP measurement was performed on an aluminum alloy plate sample with the tip shaped like a needle using “LEAP3000” manufactured by Imago Scientific Instruments. Then, for each of the three prisms, the number density of clusters (× 10 ²³ / m ³ ), the average radius of equivalent circle diameter (nm), and the total number of all Mg and Si atoms contained in the cluster The ratio of N _cluster to N _total which is the number of all measured Mg and Si atoms was determined and averaged. Therefore, each value in the present embodiment is an average value of the number of measurements N = 3. Incidentally, the measurement volume by the 3DAP method is approximately 1.0 × 10 ⁻²² to 10 ⁻²¹ mm ³ .

(Paint bake hardenability)
As mechanical properties of each test plate after aging at room temperature for 100 days, 0.2% proof stress (As proof stress) and 0.2% proof stress after aging at 185 ° C. for 20 minutes (after BH) (BH post-proof strength) was similarly determined by a tensile test. And the BH property of each test plate was evaluated from the difference (increased yield strength) between these 0.2% proof stresses.

  In the tensile test, No. 5 test pieces (25 mm × 50 mmGL × plate thickness) of JISZ2201 were sampled from the respective test plates and subjected to a tensile test 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. The test piece for measuring the yield strength after the BH was subjected to the BH treatment after giving a pre-strain of 2% simulating press forming of the plate to the test piece by the tensile tester.

(Heme workability)
Hem workability was measured only for each test plate after standing at room temperature for 7 days or 100 days after the tempering treatment. In the test, a strip-shaped test piece with a width of 30 mm was used, and after bending 90 ° with an internal bend R of 1.0 mm by a down flange, a 1.0 mm thick inner was sandwiched, and the bent portion was further bent inwardly to about 130 degrees. Pre-hem processing was performed, and flat hem processing was performed in which the end was closely attached to the inner by bending 180 degrees.

The surface state of the flat hem bent portion (edge curved portion) such as rough skin, minute cracks, and large cracks was visually observed and visually evaluated according to the following criteria.
0: No cracking, rough skin, 1: Mild rough skin, 2; Deep rough skin, 3: Small surface crack, 4; Continuous surface crack, 5: Break

Examples of the invention are shown in Alloy Nos. 0 to 9 in Table 1 and Nos. 0, 1, 7, 13, and 19 to 24 in Table 2, respectively. Manufacturing and tempering processing. For this reason, as shown in Table 3, each of these inventive examples satisfies the cluster conditions defined in the present invention. That is, satisfying cluster default invention, preferably the average number density (3.0 × ^{10 24} atoms / ^{m 3} or higher) while satisfying the percentage for the N _total of N _cluster (N _cluster / N _total ) × 100 is 1% or more and 15% or less, and the average radius of the equivalent circle diameter is 1.20 nm or more and 1.50 nm or less.

  As a result, as shown in Table 3, each invention example is excellent in BH property and has a relatively low As proofing property even after long-term aging at room temperature such as 100 days. Excellent and excellent heme workability. That is, according to the example of the present invention, even when the vehicle body is baked after aging at room temperature for a long period of 100 days, higher BH properties with a proof stress difference of 100 MPa or more, press formability and bending workability are obtained. It can be seen that an Al—Si—Mg-based aluminum alloy plate that can be exhibited can be provided.

Inventive alloy examples 1, 2, and 3 in Table 1 are used in Comparative Examples 2 to 6, 8 to 12, and 14 to 18 in Table 2. However, in each of these comparative examples, as shown in Table 2, the two-stage reheating treatment conditions after completion of the solution treatment and the quenching treatment are out of the preferable conditions.
In Comparative Examples 2, 8, and 14, the reheating process is the first stage only.
In Comparative Examples 3, 9, and 15, the reheating temperature at the first stage is too low.
In Comparative Examples 4, 10, and 16, the reheating temperature at the first stage is too high.
In Comparative Examples 5, 11, and 17, the reheating temperature at the second stage is too high.
In Comparative Examples 6, 12, and 18, the reheating temperature at the second stage is too low.
Therefore, in each of these comparative examples, as shown in Table 3, the average radius of the equivalent circle diameter of the aggregate of atoms is less than 1.20 nm, more than 1.50 nm, or N _cluster / N _total × 100 The average ratio of Mg and Si atoms contained in the atomic assembly calculated by (1) is less than 1% or exceeds 15%, which is not within the scope of the present invention. As a result, compared to Inventive Examples 1, 2, and 3 having the same alloy composition, as the As yield strength after holding at room temperature for 100 days is relatively high, it is inferior in press formability and hem workability to automobile panels, etc. , BH property is inferior.

Moreover, although the comparative examples 25-34 of Table 2 are manufactured in the preferable range including the said tempering process, the alloy numbers 10-19 of Table 1 are used, and essential elements Mg, Si, and Cu are used. Each content is out of the range of the present invention, or the amount of impurity elements is too large. As a result, as shown in Table 3, these comparative examples are inferior in BH property and hemming property as compared with each invention example. In particular, Comparative Example 27 in Table 3 Sn is too small, the number density of the clusters has become much, the average ratio _{(N cluster / N total) ×} 100 be higher than 15% of N _total of the above-described N _cluster Too much. As a result, the room temperature aging is not suppressed, the As yield strength after holding at room temperature for 100 days is too high, the press formability and the hemmability are inferior, and the BH resistance is less than 100 MPa in terms of increased yield strength. Further, Comparative Example 28 having too much Sn was cracked during hot rolling, and the plate itself could not be produced.

The comparative example 25 is the alloy 10 of Table 1, and there is too little Si.
The comparative example 26 is the alloy 11 of Table 1, and there is too much Si.
Comparative example 27 is alloy 12 of Table 1, and there is too little Sn.
The comparative example 28 is the alloy 13 of Table 1, and there is too much Sn.
The comparative example 29 is the alloy 14 of Table 1, and there is too much Fe.
The comparative example 30 is the alloy 15 of Table 1, and there is too much Mn.
The comparative example 31 is the alloy 16 of Table 1, and there is too much Cu.
The comparative example 32 is the alloy 17 of Table 1, and there is too much Cr.
Comparative Example 33 is Alloy 18 in Table 1 and contains too much Ti and Zn.
The comparative example 34 is the alloy 19 of Table 1, and Zr and V are too much.

  From the results of the above examples, it is necessary to satisfy the cluster conditions defined in the present invention in order to exhibit higher BH properties and post-BH yield strength even when the strength before baking coating is increased. It is confirmed that there is. Moreover, the critical significance or the effect of each requirement of the component composition in this invention or preferable manufacturing conditions for obtaining such cluster conditions, BH property, etc. is supported.

  According to the present invention, it is possible to provide a 6000 series aluminum alloy plate that can exhibit higher BH properties even when it is aged at room temperature when the strength before baking coating is increased. As a result, automotive panel materials, automotive frame members or structural members, pillars such as center pillars, arms such as side arms, or reinforcing materials such as bumper reinforcements and door beams, It is suitable for use as a thin plate for skeleton members and structural members other than automobiles.

Claims (2)

In mass%, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0%, Sn: 0.005 to 0.3%, respectively, with the balance being Al and inevitable impurities Al -Mg-Si based aluminum alloy plate, where N _total is the sum of the number of all Mg atoms and Si atoms measured by a three-dimensional atom probe field ion microscope, while this three-dimensional atom probe field ion microscope As a set of atoms measured by the above, it contains at least 10 Mg atoms or Si atoms or both in total, and any of these Mg atoms or Si atoms is used as a reference. The number of all Mg atoms and Si atoms contained in all of the atomic aggregate satisfying the condition that the distance between each atom and any one of the other adjacent atoms is 0.75 nm or less. N _cluster When the percentage for the N _total of _{N cluster (N cluster / N total} ) × 100 is 1% or more, 15% or less, and the average radius of the circle equivalent diameter of the aggregate of the atoms 1. An aluminum alloy plate excellent in bake coating curability characterized by being 20 nm or more and 1.50 nm or less.   The aluminum alloy plate further comprises Mn: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), Fe: 1.0% or less ( However, 0% is not included), Cr: 0.3% or less (however, 0% is not included), Zr: 0.3% or less (however, 0% is not included), V: 0.3% or less (However, not including 0%), Ti: not more than 0.1% (however, not including 0%), Zn: not more than 1.0% (however, not including 0%), Ag: 0.2% The aluminum alloy sheet excellent in bake coating curability according to claim 1, comprising one or more of the following (excluding 0%).