JP2011231400A - Aluminum alloy plate excellent in formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Al-Mg based alloy plate having little generation of SS (stretcher-strain) mark and excellent press formability.SOLUTION: When manufacturing the specific Al-Mg based aluminum alloy plate composed of the composition containing Mg and Zn, a cooling speed in a quenching-treatment time after solution heat treatment is controlled in stages, thereby a positron annihilation life value measured by the positron annihilation method is made to be equal to or lower than a specific value, a freezing vacancy concentration is low and Mg is difficult to diffuse, so that the generation of the SS mark upon press-forming is restrained thanks to the plate structure to be difficult to generate the serration (vibration).

Description

  The present invention relates to an Al—Mg-based aluminum alloy plate that is less prone to stretcher strain marks and has excellent formability. The aluminum alloy plate referred to in the present invention refers to an aluminum alloy plate that is a hot rolled plate or a cold rolled plate and has been subjected to solution treatment and quenching treatment. Hereinafter, aluminum is also referred to as Al.

  In recent years, from the viewpoint of consideration for the global environment, social demands for weight reduction of vehicles such as automobiles are increasing. In order to meet such demands, the application of aluminum materials in place of steel materials such as steel plates is being studied as materials for automobile panels, particularly large body panels (outer panels, inner panels) such as hoods, doors, and roofs.

  Since 5000 series aluminum alloy plates (hereinafter also referred to as Al—Mg series alloy plates) such as Al—Mg based JIS 5052 alloy and JIS 5182 alloy are excellent in ductility and strength, these large body panels that have been conventionally press molded. It is used as a material for.

  However, as disclosed in Patent Document 1 and the like, if a tensile test is performed on an Al—Mg alloy, yield elongation may occur in the vicinity of the yield point on the stress-strain curve, and the yield is relatively high beyond the yield point. In some cases, a serrated or stepwise serration (vibration) occurs in the stress-strain curve depending on the amount of strain (for example, tensile elongation of 2% or more). These phenomena on the stress-strain curve cause a so-called stretcher strain (hereinafter also referred to as an SS mark) during actual press molding, which is a big problem for the large body panel which is a molded product, particularly the outer panel whose appearance is important. It becomes.

  As is well known, the SS mark is a so-called random mark having an irregular belt-like pattern such as a flame that occurs at a relatively low strain area, and about 50 ° with respect to the tensile direction at a relatively high strain area. It is divided into parallel bands of parallel strips that are generated to form It is known that the former random mark is caused by elongation at yield point, and the latter parallel band is caused by serration (vibration) on the stress-strain curve described in paragraph 0004.

  Conventionally, various methods for eliminating the SS mark in an Al—Mg alloy have been proposed. For example, usually, the finer the crystal grain size of the Al—Mg alloy plate, the more markedly the SS mark is observed. Therefore, as one method for eliminating the SS mark, a method of adjusting crystal grains to a certain degree of coarseness has been conventionally known. This method is particularly effective for reducing random marks caused by the yield elongation among SS marks.

  However, such a crystal grain adjustment method is not very effective in preventing the occurrence of the parallel band due to the serration on the stress-strain curve described in paragraph 0004 even in the SS mark. . Further, if the crystal grains become too coarse, another problem such as surface roughening occurs due to press molding. In practice, it is very difficult to prevent the rough surface of the surface at the same time as the generation of the SS mark.

In addition, as a conventional method for eliminating the SS mark, an O-material (soft material) of an Al—Mg-based alloy plate or a tempered material such as a T4 treatment material is formed in advance before press forming the large body panel. It is known to give distortion (pre-strain) due to slight processing (pre-processing) such as skin pass processing or leveling processing. This method is particularly effective for reducing random marks caused by the yield elongation among SS marks. If a large number of deformation bands are formed in advance by the pre-working, these many deformation bands function as a starting point of yielding when the Al-Mg alloy plate is press-formed. For this reason, rapid and non-uniform deformation does not occur during yielding. That is, yield elongation due to these sudden and non-uniform deformations does not occur, and random marks are also suppressed.

  In general, in an Al-Mg alloy, Mg forms a Cottrell atmosphere and fixes dislocations. Therefore, extra stress is required to cause yield during press forming. On the other hand, once the yield starts at a certain point during press forming, the deformation propagates avalanche from that point without increasing the stress, and as a result, an Al-Mg alloy Non-uniform deformation will occur abruptly within the plate. Since the deformation progresses rapidly without increasing the stress in this way, yield elongation appears on the stress-strain curve, and the rapid deformation is non-uniform. Will occur.

  However, even with a method that suppresses the occurrence of yield elongation by giving such pre-processing, and in particular prevents the generation of random marks, there is a limit to the prevention of the occurrence of the parallel bands due to serrations on the stress-strain curve. is there. In other words, if the degree of pre-working becomes too high, if a tensile test is performed on the pre-worked Al-Mg alloy plate, a step-like serration with a long strain pitch on the stress-strain curve occurs. It becomes easy. Such serration tends to lead to the generation of a wide and clear parallel band even during actual press forming, and the degree of pre-processing is naturally limited.

On the other hand, even if the degree of pre-processing is reduced, the yield elongation can be suppressed to some extent, but conversely, the occurrence of the random mark can be prevented stably and reliably. Disappear. In particular, in the case of an Al-Mg alloy plate having fine crystal grains that originally tends to generate random marks, the random marks are remarkably generated even if pre-working with a low workability is performed. Moreover, in pre-machining with a low degree of processing, slight fluctuations in the thickness of the base plate depending on the location in the plate have a large effect on the variation in the degree of processing, and it is impossible to prevent the occurrence of random marks stably and reliably. It becomes a cause. Therefore, in the method of giving pre-processing, since the optimal processing degree of the prevention of the parallel band due to the serration on the stress-strain curve and the prevention of the generation of the random mark conflict, it is possible to prevent both of them simultaneously. Can not.

In addition, regarding the parallel band of the SS mark, when the strain rate at the time of press molding is high, such as at the time of mold forming by a mechanical press, for example, the generation of the parallel band is less if attention is paid to the molding speed. Known from. However, when forming with a hydraulic press machine or the like with a lower forming speed, the generation of clear parallel bands with a wide width is caused particularly in the case of Al-Mg alloy sheet materials that cause stepped serration with a large strain pitch as described above. Could not escape.

On the other hand, in Patent Document 1 described above, the generation of random marks due to the yield elongation and the generation of a wide parallel band related to the stepwise wide serration on the stress-strain curve are suppressed. Al-Mg alloy plates with less SS mark generation have been proposed. Specifically, a rolled sheet of Al-Mg alloy is subjected to solution treatment / quenching under specific conditions with rapid cooling, and then cold working as pre-processing under specific conditions is performed. Apply final annealing under conditions. Then, a final plate having an average crystal grain size of 55 μm or less and substantially free of coarse crystal grains of 150 μm or more is obtained.

In addition, in an Al-Mg alloy plate, differential thermal analysis (D
It is also known that the endothermic peak height between 50 and 100 ° C. of the heating curve from the solid phase obtained by measuring SC) can be used as an index for improving press formability. For example, in Patent Document 2, Al-Mg with a high Mg produced by twin-roll continuous casting and having a Mg content exceeding 8% by mass.
In the alloy plate, the endothermic peak height is set to 50.0 μW or more to improve press formability. This is because the endothermic peak height of the DSC between 50 and 100 ° C. is Al—Mg.
This is based on the fact that the existence form (solid solution, stability of precipitation state) of an Al—Mg intermetallic compound referred to as β phase in the steel alloy plate structure is shown.

JP-A-7-224364 JP 2006-249480 A JP 2001-116706 A JP 2004-28849 A

Light Metal, Vol. 56, No. 11 (2006), pp. 629-634, “Behavior of Atomic Vacancy in Aluminum Alloys as Seen by Positrons” “Tsushin” issued on July 1, 2009, page 4, “Introduction to SMT services”

However, in Patent Document 1, the stepped serration can be made light (described in the description of the stepped serration evaluation in the embodiment of Patent Document 1), and therefore the parallel band which is one of the SS marks is completely It cannot be suppressed. On the other hand, in recent large body panels, particularly outer panels whose appearance is important, the required level of surface properties has become more severe. In these Patent Documents 1 and 2, it is not sufficient as a measure for suppressing the occurrence of SS marks. It has become to. Patent Documents 3 and 4 and Non-Patent Documents 1 and 2 will be described later in the “Description of Embodiment” section.

In view of such problems, the object of the present invention is to simultaneously suppress the generation of random bands due to the yield elongation and the generation of parallel bands, suppress the SS mark, and press molding to an automobile panel. It is providing the Al-Mg type aluminum alloy plate excellent in property.

In order to achieve this object, the gist of the aluminum alloy sheet excellent in formability according to the present invention is, by mass%, including Mg: 0.5 to 7.0%, Zn: 0.1 to 4.0%. The balance is an Al—Mg-based aluminum alloy plate composed of Al and inevitable impurities, and a positron annihilation lifetime value measured by a positron annihilation method of sandwiching a positron beam source with a sample of the aluminum alloy plate is 210 ps or less. Suppose that

Although Al-Mg-based aluminum alloy plates have an effect of suppressing the generation of SS marks when Zn is contained, there is a large difference in the effect of suppressing the generation of SS marks even with Al-Mg-based aluminum alloy plates having the same Zn content. There is. From this, not only Zn but also Z
It is considered that the difference in the structure state of the Al—Mg-based aluminum alloy plate containing n greatly affects the SS mark generation state.

SS mark generated during press forming of Al-Mg based aluminum alloy sheet, especially stress-
It is presumed that the parallel band due to serration (vibration) on the strain curve is caused by repeated fixation and separation of free Mg atoms dissolved in the aluminum matrix. Hereinafter, the SS mark of the parallel band caused by serration (vibration) on the stress-strain curve is also simply referred to as SS mark or SS mark (serration).

On the other hand, if a novel ultrafine MgZn cluster is present in the structure of the Al—Mg-based aluminum alloy plate, it prevents the movement of free Mg atoms to dislocations during deformation by the press forming, and SS It is presumed that there is an effect of suppressing the occurrence of mark (serration). However, even with a structure observation using a 100,000-fold FE-TEM (transmission electron microscope), the Al-Mg-based aluminum alloy plate containing Zn is considered to be effective in suppressing the SS mark. Fine MgZn clusters could not be found. For this reason, S
With the EM and TEM analysis methods, the structure of the Al—Mg-based aluminum alloy plate containing Zn could not be specified.

Based on this, the present inventors can more directly measure the frequency of fixing to dislocations of Mg atoms that cause SS mark (serration) generation instead of this fine MgZn cluster. I thought it would be. That is, free M at the time of deformation by press molding
Theoretically, the diffusion rate of both Mg and Zn atoms is very small in the repetition of fixation and separation (movement) of g atoms to dislocations. Therefore, both of these atoms are frozen vacancies (atomic vacancies frozen in the material, hereinafter simply referred to as atomic vacancies) generated when the Al—Mg-based aluminum alloy plate is rapidly cooled (quenched) after solution treatment. It is thought to spread through.

That is, when the concentration of the frozen vacancies is high, it can be said that serration tends to occur because the frequency of fixation to dislocations by Mg atoms increases. On the other hand, when the concentration of the frozen vacancies is low, it can be said that the serration is difficult to occur because Mg is difficult to diffuse. In addition, MgZn clusters are preferentially generated from where there are frozen vacancies, and the frozen vacancies disappear. Therefore, when the amount of MgZn clusters generated increases, the concentration of frozen vacancies decreases and serrations are less likely to occur.
Therefore, if the concentration of frozen vacancies (atomic vacancies) in an Al—Mg-based aluminum alloy plate containing Zn is measured, the frequency of fixation of Mg atoms to dislocations causing SS mark (serration) generation That is, the SS mark property can be evaluated.

And the density | concentration of the frozen vacancy (atomic vacancy) of such an Al-Mg type aluminum alloy plate containing Zn can be measured directly by the atomic vacancy measurement by the positron annihilation method.

The positron annihilation method is used for atomic vacancies, vacancy aggregates existing in materials such as metals, semiconductors, and compounds.
Alternatively, it is a known method for extremely sensitively detecting the type and amount of nanostructure defects such as crystal lattice defects such as dislocations. In this method, a positron is incident on a material, its lifetime is measured, or the generated gamma rays are measured, so that the type and amount of the nanostructure defect can be detected very accurately.

In this positron annihilation method, positron annihilation lifetime values (unit: ps, picosecond, 10 ⁻¹² ) measured by a method of sandwiching a positron beam source with a sample of an aluminum alloy plate (sandwich method) are used. Second) shows the concentration of frozen vacancies (atomic vacancies) in the Al-Mg aluminum alloy plate containing Zn.

For this reason, the superiority or inferiority of the SS mark suppression of the Al—Mg-based aluminum alloy plates containing Zn, which are not different from each other in the other material conditions, is determined by the concentration of the frozen vacancies measured by this positron annihilation lifetime value. Prior to molding, it can be evaluated in advance. That is, the smaller the positron annihilation lifetime value is, the lower the concentration of frozen vacancies and the less difficult Mg diffuses, so that serration is less likely to occur and the SS mark property is excellent. Here, the fact that there is no difference in other material conditions means that the aluminum alloy plates having different superiority or inferiority in SS mark property (SS mark suppression), as well as the mutual component composition, structure observation such as normal TEM or SEM, or It means that there is no difference between them even by analysis such as extraction residue method and X-ray diffraction.

Incidentally, the concentration of frozen vacancies or atomic vacancies as referred to in the present invention should be measured accurately at this time depending on the structure observation such as normal TEM and SEM and other analysis methods other than the positron annihilation method. I can't.

If the frozen vacancies of the Al—Mg-based aluminum alloy plate containing Zn can be eliminated, a plate structure in which Mg atoms cannot be fixed to dislocations and Mg is difficult to diffuse. However, since the frozen vacancies themselves cannot be eliminated, in the present invention, the allowable amount of the frozen vacancy concentration is defined by the positron annihilation lifetime value.

In the present invention, the structure of an Al—Mg-based aluminum alloy plate containing Zn can be controlled at the level of nanostructure defects, and the concentration of frozen vacancies can be lowered to make the structure difficult to diffuse Mg, resulting in serration. It is difficult to improve SS mark properties.

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

Positron annihilation method:
The positron annihilation method in the present invention is, for example, used in the measurement and evaluation of the behavior of atomic vacancies in an aluminum alloy that influences age hardening in Non-Patent Document 1 described in the column of the prior art document. It is described that. More specifically, in Non-Patent Document 1, an Al-4 mass% Cu binary aluminum alloy that is age-hardened is produced by a solution treatment such as the concentration of thermal equilibrium atomic vacancies during aging or the present invention. Then, the concentration of thermal equilibrium atomic vacancies confined in the alloy by rapid quenching, that is, frozen vacancies is measured.

The method and apparatus for measuring the positron annihilation lifetime value in this positron annihilation method are also specifically described in Patent Document 3 and the like described in the column of the prior art document, and in particular, Patent Document 4 and Non-Patent Document 2
Describes a method and apparatus for measuring a positron annihilation lifetime value by a method of sandwiching a positron beam source with a sample of an aluminum alloy plate (sandwich method).

A positron is one of elementary particles having the same mass as an electron and having a positive charge of the same absolute value as the electron, and is generated in the decay process of a specific radioisotope. When the positron (β +) is incident on a material such as an aluminum alloy, the incident kinetic energy is lost in a short time, and the subsequent behavior becomes the same thermal motion as that of a normal electron. Materials such as metals, semiconductors, and compounds
It is made up of an assembly of atoms, which is a structure in which a negatively charged electron is surrounded around a positively charged nucleus. It takes the form of a crystal with a regular three-dimensional lattice arrangement. For example, a metal crystal such as an aluminum alloy is made of an array of positively charged ions, so the positrons that enter it repel each other because of the same sign as the ions and spread between the crystal lattices. It collides with moving electric conductors, etc. and unites and disappears.

However, crystal lattice defects such as vacancies and dislocations that are deficient in atoms are relatively negatively charged, so that positive positrons are first captured in these parts, and eventually collide with electrons and coalesce and disappear. When this positron collides with an electron and disappears, the energy is 511 keV,
It emits two gamma rays whose directions are almost opposite. The time from when a positron enters a material (aluminum alloy) until it collides with the electron and disappears differs depending on whether it is in a defect-free part or when it is detected as a defect, and also depends on the shape of the defect. . Therefore, the state of the defect can be grasped by analyzing the time change from the incidence of the positron to the disappearance. In addition, γ-rays generated from collisional annihilation between positrons and electrons cause a wavelength shift due to the Doppler effect due to the movement of electrons, and the relative angle of γ-rays emitted in the opposite direction depends on the momentum of the electrons. Deviation occurs. By analyzing these, you can know the defect information in more detail,
The condition of the material can be evaluated more precisely.

Measurement method by sandwich method:
As described above, positrons are generated in the decay process of β + decay type radioisotopes, and are generally measured by using this radioisotope as a radiation source and bringing the radiation source and measured material into close contact with each other. The For example, 22Na has a long half-life, is readily available and easy to handle, is in the form of NaCl and is chemically stable, and is usually enclosed in a capsule such as a Ni foil and used as a radiation source. This 22Na emits 1.28 MeV γ-rays when β + decays, so the source is sandwiched between materials to be measured (aluminum alloy) (sandwich method) and detected by a scintillation counter, etc. After preparing a device and detecting 1.28 MeV gamma rays,
The time until 1 keV gamma ray is detected is measured. That is, since the distance between the radiation source and the material is very close, the time when 1.28 MeV γ rays are emitted is the start time when the positron material is incident, and the time when 511 keV γ rays are detected is It can be the time of disappearance. In this way, the time difference between the two is measured to obtain a positron lifetime spectrum, and a positron annihilation lifetime value (unit: ps, picosecond, ^10-12 seconds) is obtained.

Measuring device by sandwich method:
As described in each of the above-mentioned documents, the positron annihilation method measuring apparatus by the sandwich method is arranged on the same axis in the order of a positron detector, such as a positron beam source, an electromagnetic lens, an avalanche photodiode, and a material to be measured. Is done. A radiation detector such as a scintillation counter used for normal γ-ray measurement is installed in the vicinity of the material to be measured.

The flight path of the positron from the radiation source to the material to be measured can be evacuated to a vacuum, and the part where the electromagnetic lens is placed is placed in a container made of a nonmagnetic material. When positrons fly in the atmosphere, they collide with air molecules, causing energy reduction and scattering, so the source, positron detector, sample to be measured, etc. are in a vacuum with a pressure below 1 × 10 ⁻⁴ Torr. It is placed in a container that can be evacuated so that the flight path of positrons from the radiation source to the material to be measured is maintained in a vacuum. As a result, the number of positrons incident on the sample to be measured is increased, and the sensitivity can be improved and the time can be shortened.

Here, in order to efficiently measure the positron lifetime, the sandwich method was prepared by preparing two aluminum alloy samples having a size of about 1 yen in the above-described apparatus, and using these two samples, a positron beam source Comes from sandwiching. By the way, this sandwich method is called γ
-It is also called γ simultaneous measurement method.

The positron beam emitted from the positron beam source is focused by the electromagnetic lens, passes through the positron detector, and is incident on the two materials to be measured. The positron collides with electrons in the material to be measured. Disappears and emits gamma rays. At this time, a voltage pulse when the positron passes through the positron detector is used as a start signal, and when the positron disappears, detection of γ rays (511 KeV) emitted from the material to be measured is used as a stop signal, and the time between them Can be used to know the lifetime of the positron (positron annihilation lifetime value).

Incidentally, the electromagnetic lens is composed of a conductive soft iron frame and a winding coaxially arranged around the central axis, and magnetic flux generated in the gap between the magnetic pole excited by current in the winding. A lens magnetic field is constituted by the leakage magnetic field. Therefore, the effective magnetic field of the lens is limited to within the corner portion of the magnetic pole. This effective magnetic field depends on the size of the radiation source or container, but with a diameter of 2
A vertical magnetic field having a sufficient magnetic flux density can be obtained in a space of about 0 to 150 mm and a height of about 20 to 150 mm. If the intensity of the magnetic field can be about 0.1 to 0.3 T at the lens center position, positrons can be sufficiently converged.

Positron annihilation lifetime value:
In the present invention, the positron annihilation lifetime value of the Zn—Al—Mg-based aluminum alloy plate measured in this way is defined as 210 ps (picosecond, 10 ⁻¹² seconds) or less. By making the structure of the Al—Mg-based aluminum alloy plate containing Zn after solution treatment and quenching as a tempering treatment after the plate production, such a positron annihilation lifetime value is obtained at the level of nanostructure defects. Organization control is possible. In other words, the concentration of frozen vacancies (atomic vacancies) is reduced and Mg
Can be made into a plate structure that is difficult to diffuse. For this reason, it is hard to generate the serration at the time of press-molding the Al-Mg type aluminum alloy plate containing Zn, and it can be set as the molding material excellent in SS mark property.

As described in the previous paragraph 0030, if the frozen vacancies (atomic vacancies) themselves can be eliminated, the Mg atoms cannot be fixed to dislocations, and a plate structure in which Mg is difficult to diffuse can be obtained. The contained Al—Mg-based aluminum alloy plate can be a molding material having excellent SS mark properties. However, due to the production limit of this aluminum alloy sheet, it is very difficult (not realistic) to eliminate the frozen holes themselves. Therefore, in the present invention, the allowable amount from the SS mark property of the frozen vacancy concentration is defined by the positron annihilation lifetime value.

That is, when this positron annihilation lifetime value exceeds 210 ps, the concentration of frozen vacancies (atomic vacancies) in the Zn-containing Al—Mg-based aluminum alloy plate after solution treatment and rapid cooling increases. For this reason, since the frequency of fixing to dislocations due to Mg atoms during press molding is increased, serration is likely to occur, and SS mark properties are degraded. Incidentally, Al-containing Zn
Control of the positron annihilation lifetime value of the Mg-based aluminum alloy plate (control of the concentration of the frozen vacancies) is performed by controlling the rapid cooling process after the solution treatment, as will be described in detail in the explanation of the manufacturing method later. The lower the positron annihilation lifetime value, the lower the positron annihilation lifetime value, the lower the concentration of frozen vacancies (atomic vacancies), and the better the positron annihilation lifetime value is. do not do. However, from the production limit, the lower limit of the positron annihilation lifetime value is approximately 190 ps.

Prevention of random marks:
In the present invention, the concentration of frozen vacancies (atomic vacancies) can be reduced to form a plate structure in which Mg is difficult to diffuse, so that random marks due to the occurrence of yield elongation can be prevented among SS marks. Therefore, in order to prevent the occurrence of this random mark, a conventional measure for applying pre-strain (pre-processing) becomes unnecessary. In other words, both the random mark generated at a portion with a relatively low amount of distortion and the parallel band generated at a portion with a relatively high amount of distortion without applying the conventional pre-strain (pre-processing). The generation of the retarder strain mark (SS mark) can be sufficiently suppressed.

Therefore, the present invention provides a material plate for automobile panels, particularly when the required level of the surface texture of the outer panel whose appearance is important is more severe, along with the generation of random marks due to the yield elongation, the stress-distortion. Generation of parallel bands related to serration on the line can be suppressed at the same time. As a result, the performance of the automobile panel material plate can be greatly improved.

(Chemical composition)
The chemical component composition of the aluminum alloy hot-rolled sheet of the present invention is basically an aluminum alloy corresponding to JIS 5000, which is an Al—Mg alloy. In addition,% display of content of each element means the mass% altogether.

Especially this invention needs to satisfy various characteristics, such as press moldability, intensity | strength, weldability, and corrosion resistance, as a raw material board for motor vehicle panels. For this reason, the hot-rolled sheet of the present invention includes, among the 5000 series aluminum alloys, in mass%, Mg: 0.5 to 7.0%, Zn: 0.1 to 4.0%, with the balance being Al and inevitable An Al—Mg aluminum alloy plate made of impurities is used.

Moreover, this Al-Mg-based aluminum alloy plate is further, in mass%, Fe: 1.0% or less, Si: 0.5% or less, Mn: 1.0% or less, Cr: 0.3% or less, It is allowed to contain one or more selected from Zr: 0.3% or less, V: 0.3% or less, Ti: 0.1% or less, and Cu: 1.0% or less. In addition, all element content is the mass%.

Mg: 0.5-7.0%
Mg enhances work hardening ability and ensures necessary strength and durability as a material plate for automobile panels. In addition, the material is uniformly plastically deformed to improve the fracture crack limit and improve the formability. It is also presumed that fine MgZn clusters are formed to suppress the generation of SS marks during press molding. If the Mg content is less than 0.5%, the strength and durability are insufficient.

On the other hand, if the Mg content exceeds 7.0%, it becomes difficult to produce a plate, and intergranular fracture is more likely to occur during press molding, which significantly reduces press formability. Therefore, the Mg content is in the range of 0.5 to 7.0%, preferably 1.5 to 6.5%.

Zn: 0.1-4.0%
Zn is necessary for forming fine MgZn clusters. It is presumed that the formation of MgZn clusters decreases the concentration of frozen vacancies (atomic vacancies) and suppresses the generation of SS marks during press forming. When Zn is less than 0.1%, such an effect is insufficient.

On the other hand, if the Zn content exceeds 4.0% by mass, the corrosion resistance deteriorates. Therefore, the Zn content is 4.0% or less and is preferably within the range of 0.1 to 4.0%. . More preferably 1
. It is in the range of 0 to 3.5%.

Incidentally, in an Al—Mg-based aluminum alloy plate, Zn as an additive element is usually C
Along with u, it is recognized as an effective element that improves the strength by precipitation strengthening. Moreover, in the said patent document 1, Zn is recognized as an element effective also in suppression of SS mark. But,
The point which suppresses generation | occurrence | production of SS mark at the time of press molding by the combination with the manufacturing conditions mentioned later like this invention is not necessarily well-known.

Other elements:
In the present invention, as other elements, Fe, Si, Mn, Cr, Zr, V, Ti,
It is allowed to contain one or more selected from Cu. These elements are impurity elements whose content increases as the amount of aluminum alloy scrap (ratio to aluminum metal) increases as a melting raw material. In other words, from the viewpoint of recycling Al alloy plates, not only high-purity aluminum bullion but also 5000 series alloys, other Al alloy scrap materials, and low-purity Al bullion are used as melting raw materials. The amount (content) of these elements inevitably increases. Then, reducing these elements to, for example, below the detection limit itself increases the cost, and it is necessary to allow a certain amount of inclusion.

Further, when these elements are contained in a small amount, they also have an effect of refining crystal grains. A
The rough surface of the l-Mg-based aluminum alloy sheet during press forming has an average crystal grain size of 50 μm.
This is likely to occur when the crystal grain size is large, such as exceeding, and the smaller the crystal grain size of the plate, the better. These elements are also contained in small amounts, and have the effect of improving the formability limit.

However, on the other hand, if the content of these elements is increased, too, as a harmful effect of these elements, coarse crystallized substances and precipitates resulting from these elements increase, which tends to be the starting point of destruction.
On the other hand, press formability is reduced. Furthermore, the crystal grain size becomes too fine, and if it is less than 25 μm, an SS mark is likely to appear. Therefore, when these elements are contained, Fe: 1.0% or less, Si: 0.5% or less, Mn: 1.0% or less, Cr: 0.3% or less, Zr: 0.00%, respectively. The range is 3% or less, V: 0.3% or less, Ti: 0.1% or less, and Cu: 1.0% or less.

(Production method)
The manufacturing method of the board of this invention is demonstrated concretely below.

In the present invention, 5182, 5082, 5083, 5 until the rolling process before solution treatment.
It can be manufactured by a manufacturing method according to a normal manufacturing process of an Al-Mg alloy for forming containing about 4.5% Mg such as 056. That is, an aluminum alloy hot-rolled sheet having a thickness of 1.5 to 5.0 mm is manufactured through normal manufacturing processes such as casting (DC casting or continuous casting), homogenization heat treatment, and hot rolling. The At this stage, it may be a product plate, and further cold rolling while selectively performing one or more intermediate annealings before or during cold rolling, or in the middle of cold rolling,
It is good also as a product board of the cold-rolled board whose board thickness is 1.5 mm or less.

Solution treatment (final annealing):
In order to obtain a plate having the structure of the present invention, these hot-rolled or cold-rolled plates having the required thickness obtained as described above are subjected to rapid heating and rapid cooling as final annealing. Conversion
Quenching is performed. A material subjected to such solution treatment and quenching treatment, so-called T4 treatment material, is excellent in balance between strength and formability as compared with a batch annealed material with relatively gentle heating and cooling. In addition, frozen vacancies (atomic vacancies) are introduced during the quenching process following the solution treatment.

Here, although the appropriate value of solution treatment temperature changes with specific alloy compositions, it needs to be in the range of 400 degreeC or more and 570 degrees C or less. The retention at the solution treatment temperature is 180.
Must be within seconds (3 minutes). When the solution treatment temperature is less than 400 ° C., the alloy elements are not sufficiently dissolved, and the strength and ductility may be lowered. On the other hand, if the solution treatment temperature exceeds 570 ° C., the crystal grains become excessively coarse, which causes problems such as deterioration of moldability and generation of rough skin during molding. On the other hand, if the retention time at the solution treatment temperature exceeds 180 seconds, problems such as deterioration of moldability and generation of rough skin during molding due to excessive coarsening of crystal grains occur.

Quenching process:
Satisfy the positron annihilation lifetime value of the present invention, lower the concentration of frozen vacancies (atomic vacancies),
In order to obtain a plate structure in which Mg is difficult to diffuse, the quenching treatment conditions after the solution treatment are important.
In other words, during the quenching after solution treatment, the rapid cooling to the low temperature range, which is the normal condition, is divided into three stages: slow cooling in the high temperature range and low temperature range, and rapid cooling in the intermediate temperature range. It is necessary to set the cooling rate.

First, in the slow cooling in the high temperature region, the plate is cooled at a rate of 2 to 5 ° C./second until the temperature of the plate reaches a temperature range of 325 to 375 ° C. If the cooling rate is less than 2 ° C./second, coarse precipitates are formed at this point, the amount of clusters generated thereafter is insufficient, the frozen pore concentration increases, and SS
Marks may be generated. On the other hand, if it exceeds 5 ° C./second, atomic vacancies that are present at the solution solution temperature are frozen as they are, and the concentration of frozen vacancies increases.

Then, it switches to the rapid cooling in an intermediate temperature range in the temperature range of 325-375 degreeC. In the rapid cooling in the intermediate temperature range, the cooling rate is 5 ° C./second or more from the temperature (within the switching temperature range) where the temperature of the plate has finished the slow cooling in the high temperature range to the temperature range of 125 to 75 ° C. The cooling rate is large. When the cooling rate here is less than 5 ° C./second, coarse precipitates are generated during cooling,
Thereafter, the amount of clusters to be generated is insufficient, the frozen hole concentration becomes high, and SS marks tend to be generated.

And it switches to the slow cooling of a low temperature range further in the said 125-75 degreeC temperature range. In the slow cooling in the low temperature range following the rapid cooling, the cooling rate from the temperature at which the rapid cooling in the intermediate temperature range is finished (within the switching temperature range) to room temperature is set to a small cooling rate of less than 1 ° C./second. In order to achieve such slow cooling, the lower limit value of the cooling rate is not particularly determined, but it is preferably 0.01 ° C./min or more in consideration of the production efficiency and the limit of processing equipment. If the cooling rate is 1 ° C./second or more, it is presumed that the freezing pore concentration becomes high without generating clusters in an amount that can reliably prevent the occurrence of the SS mark.

Such solution treatment and quenching with cooling may be performed continuously using a continuous annealing line (CAL) or the like, and the cooling rate of each region may be controlled by combining forced air cooling or mist cooling. Alternatively, a salt bath or the like may be used for heating, and a batch method using a combination of water quenching, oil quenching, forced air cooling, mist cooling, or the like for cooling. Incidentally, when solution treatment / quenching using the CAL is carried out, the general heating and cooling rates from room temperature to the solution treatment temperature are both about 5 to 100 ° C./second.

By combining such a solution treatment condition and a special quenching process condition of subsequent three-stage cooling, an Al—Mg based aluminum alloy plate containing Zn satisfies the positron annihilation lifetime value of the present invention, By reducing the concentration of holes (atomic vacancies), a plate structure in which Mg is difficult to diffuse can be obtained. Thereby, the effect of increasing the limit strain amount of the Al—Mg-based aluminum alloy plate containing Zn is enhanced, the serration on the stress-strain curve is suppressed, and the parallel band resulting from this is suppressed, and the stretcher strain mark Can be suppressed. In addition, among the SS marks, the generation of random marks due to the occurrence of yield elongation can be prevented.

In addition, after this solution treatment / quenching treatment, a skin pass or a tension leveler passing plate may be performed in order to control the shape of the plate and remove residual stress. Subsequent additional annealing or aging treatment is unnecessary because of the effect of suppressing SS mark generation, and is not performed in the present invention.

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. After manufacturing the Al-Mg type alloy plate of each composition of the invention example shown in Table 1 and a comparative example, and tempering and manufacturing on the conditions shown in Table 2 (continuation of Table 1), the structure | tissue of this plate after this tempering The mechanical properties were measured and evaluated. These results are also shown in Table 2. In addition, "-" description of element content in Table 1 shows that the content of the element is below a detection limit.

Each manufacturing method (condition) of a hot rolled sheet and a cold rolled sheet was performed under the same common conditions in each example. That is, a 50 mm thick ingot cast by book mold casting was subjected to a homogenization heat treatment at 480 ° C. for 8 hours, and then hot rolling was started at 400 ° C. The plate thickness was a 3.5 mm hot rolled plate.
The hot-rolled sheet was cold-rolled to a thickness of 1.35 mm, and then heated at 400 ° C. and 1
Intermediate annealing for 0 second was performed, and cold rolling was further performed to obtain a cold rolled sheet having a thickness of 1.0 mm.

As shown in Table 2, these cold-rolled plates were subjected to solution treatment and quenching treatment under different conditions. This solution treatment and quenching process are performed continuously using a continuous annealing line (CAL), etc., and forced air cooling and mist cooling are used separately, and the line speed of the plate and the air volume are controlled in each temperature range, and quenching is performed. The cooling rate during processing was controlled. Thus, the concentration of frozen vacancies (atomic vacancies) or the positron annihilation lifetime value in each example was controlled.

In addition, the skin pass as a cold working to give a pre-strain after the solution treatment and the quenching treatment, the room temperature aging treatment, the artificial aging treatment and the like after that were not performed at all.

A test piece (1 mm thickness) was cut out from the plate after solution treatment and quenching treatment, and the positron annihilation lifetime value, SS mark characteristic and mechanical characteristic of this test piece (plate after tempering) were measured and evaluated, respectively. did.

(Positron annihilation lifetime measurement)
By using the measurement method and apparatus described above, the positron annihilation lifetime value of each test piece is obtained by a positron annihilation method in which a positron beam source is sandwiched between test pieces (samples) of an aluminum alloy plate after solution treatment and quenching treatment. It was measured. These actual measurements were entrusted to Sumitomo Metal Technology Co., Ltd. (Kansai Division), which has the measurement device described in Non-Patent Document 2 and has measurement know-how.

(Mechanical properties)
As an investigation of the mechanical properties of the plate, a tensile test of each of the above test pieces was performed, and the tensile strength (MPa
) And elongation (%) were measured. The test conditions are JISZ22 perpendicular to the rolling direction.
A No. 5 test piece (25 mm × 50 mmGL × plate thickness) was sampled and subjected to a tensile test. The tensile test is based on JISZ2241 (1980) (metal material tensile test method) at room temperature of 20 ° C.
The test was conducted. At this time, the crosshead speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke.

(SS mark generation evaluation)
At the same time, in order to evaluate the occurrence of SS marks as the press formability of the plate, yield elongation (%) during the tensile test and the amount of strain that generates serrated serrations on the stress-strain curve (critical strain: %). As a standard for suppressing the occurrence of the SS mark, the critical strain is 8% or more. The higher the critical strain, the better. In the present invention, the upper limit is not particularly defined. However, from the production limit, the upper limit of the critical strain is about 20%.

(Press formability evaluation)
Further, as an evaluation of the stretch formability which is a problem in the outer panel, a stretch forming test was conducted.

The overhang forming test uses a ball head overhang punch having a diameter of 101.6 mm, a length of 180 mm, a width of 1
R-303P was applied as a lubricant to a 10 mm test piece, an overhang forming test was performed at a forming speed of 4 mm / S and a wrinkle holding load of 200 kN, and the occurrence of cracks in the molded product was visually observed. Then, regardless of the size of the crack, the case where no crack was generated was evaluated as ◯, and the case where crack was generated was evaluated as x.

As shown in Table 1, Examples 1 to 7 of the solution treatment and quenching treatment satisfy the Al-Mg based aluminum alloy composition rule of the present invention and the cooling rate condition of the quenching treatment is the above three stages. Manufactured under preferred manufacturing conditions. As a result, the measured positron annihilation lifetime value is 21
It is 0 ps or less, and the freezing vacancy concentration is low, the Mg is difficult to diffuse, and the structure is difficult to generate serration.

Thus, in each of the inventive examples, the critical strain of serration generation on the stress-strain curve of the aluminum alloy plate is 8% or more, and the high one is 10% or 15% or more. And
No cracks occurred in the overhang forming test. Moreover, these excellent SS mark characteristics or stretch formability (indicated as “press formability” in Table 2) are excellent machines such as tensile strength and elongation of 5000 series aluminum alloy plates such as JIS 5052 alloy and JIS 5182 alloy. This can be achieved without degrading general characteristics.

On the other hand, although Comparative Examples 8-12 are the same alloy compositions as Invention Example 1, as shown in Table 2, the tempering conditions are out of the preferred range.
In Comparative Example 8, the solution treatment temperature is too low, and the three-stage cooling cannot be performed by the quenching treatment.
Comparative Example 9 is a quenching treatment, and the cooling rate in the slow cooling in the high temperature range is less than 2 ° C / second.
In Comparative Example 10, the cooling rate in the slow cooling in the high temperature region exceeds 5 ° C./second in the quenching process.
In Comparative Example 11, the cooling rate in quenching in the rapid cooling in the intermediate temperature range is less than 5 ° C./second.
In Comparative Example 12, the cooling rate in the slow cooling in the low temperature region exceeds 1 ° C./second in the quenching process.

As a result, in Comparative Examples 8 to 12, when the measured positron annihilation lifetime value exceeds 210 ps in common and the concentration of frozen vacancies is high, the frequency of fixing to dislocations due to Mg atoms increases, so serration The organization is easy to get up. As a result, in these comparative examples, although mechanical properties such as strength and elongation are not significantly different from those of the invention example, the critical strain of serration generation on the stress-strain curve of the aluminum alloy plate is as low as less than 8%. For this reason, the SS mark characteristic or the overhang formability is remarkably low as compared with the inventive examples.

In Comparative Examples 13 and 14, as shown in Tables 1 and 2, the tempering conditions are within a preferable range, but the alloy composition is outside the scope of the invention. The comparative example 13 has too little Zn content. Comparative Example 14 has too much Mg content.

As a result, Comparative Example 13 has a measured positron annihilation lifetime value exceeding 210 ps and a structure in which serration is likely to occur. The critical strain of the serration generation is as low as less than 8%. Remarkably low. Also, the strength is low. In Comparative Example 14, the measured positron annihilation lifetime value is 210 ps or less, and the SS mark characteristic is not much different from that of the inventive example, but the elongation is low and the stretch formability is remarkably lower than that of the inventive example.

The above examples support the critical significance of combining the SS mark characteristics, the stretch formability, the mechanical characteristics, and the like, such as the requirements of the present invention or preferred manufacturing conditions.

As described above, according to the present invention, it is possible to provide an Al—Mg-based aluminum alloy plate containing Zn with less SS mark generation and excellent press formability. As a result, the application of the Al—Mg-based aluminum alloy plate to many uses such as the automobile described above, which is used by press-molding the plate, is expanded.

Claims (2)

An Al—Mg-based aluminum alloy plate containing, by mass%, Mg: 0.5 to 7.0%, Zn: 0.1 to 4.0%, the balance being Al and inevitable impurities, An aluminum alloy plate excellent in formability, characterized in that a positron annihilation lifetime value measured by a positron annihilation method of sandwiching a positron beam source with a sample of an alloy plate is 210 ps or less. As an index indicating the formability of the aluminum alloy plate, the stress of the aluminum alloy plate −
The aluminum alloy sheet excellent in formability according to claim 1, wherein the critical strain for occurrence of serration on the strain curve is 8% or more.