JP5756300B2 - Aluminum alloy plate with excellent formability - Google Patents

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 is a hot-rolled plate or a cold-rolled plate, and refers to a tempered aluminum alloy plate such as annealing. 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 the occurrence of so-called stretcher strain (hereinafter also referred to as SS mark) during actual press molding, which is a big problem for large body panels that are molded products, especially outer panels whose appearance is important. Become.

  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 can be divided into parallel bands of parallel belt-like patterns that are generated. It is known that the former random mark is caused by yield point elongation and the latter parallel band is caused by serration on the stress-strain curve.

  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 yield elongation among SS marks.

  However, in such a method for adjusting crystal grains, if the crystal grains become too coarse, another problem arises such that the surface becomes rough 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. Also, this crystal grain adjustment method is not very effective in preventing the generation of parallel bands due to serrations on the stress-strain curve, even among SS marks.

  In addition, as a conventional method for removing the SS mark, a skin pass is previously applied to a tempered material such as an O-material (soft material) or a T4 treated material of an Al-Mg alloy plate before press molding to a large body panel. It is known to give distortion (pre-strain) due to slight processing (pre-processing) such as processing or leveling processing. This method is particularly effective for reducing random marks caused by yield elongation among SS marks. If a large number of deformation bands are formed in advance by this 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 providing such pre-processing and prevents the occurrence of SS marks, particularly random marks, there is a limit in preventing the occurrence of parallel bands due to serrations on the stress-strain curve. There is. 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 is generated on the stress-strain curve. It becomes easy. Such serrations tend to lead to the generation of wide and clear parallel bands 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 generation of random marks cannot be prevented stably and reliably. . In particular, in the case of an Al—Mg-based alloy plate with fine crystal grains that originally tends to generate random marks, random marks are remarkably generated even when pre-processing with a low processing degree 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 optimum degree of processing between the prevention of parallel band generation due to serration on the stress-strain curve and the prevention of random mark generation conflict, it is impossible to prevent both of them simultaneously.

  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, the generation of random marks due to yield elongation and the generation of a wide parallel band related to a stepwise wide serration on the stress-strain curve are suppressed. Al-Mg alloy plates that are less likely to be generated 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.

  Moreover, in the Al-Mg alloy plate, the endothermic peak height between 50 and 100 ° C of the heating curve from the solid phase obtained by measuring the thermal change in the melting process of the plate by differential thermal analysis (DSC). Is also known as an index for improving press formability. For example, in Patent Document 2, in a high Mg Al—Mg-based alloy plate manufactured by twin-roll continuous casting and having a Mg content exceeding 8 mass%, the endothermic peak height is set to 50.0 μW or more, and press formability is increased. It is improving. This is because the endothermic peak height of DSC between 50 and 100 ° C. is the presence form of an Al—Mg intermetallic compound called a β phase in an Al—Mg alloy plate structure (solid solution, stable state of precipitation). )).

JP-A-7-224364 JP 2006-249480 A

  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 that is one of the SS marks is completely suppressed. Can not. On the other hand, recent large body panels, particularly outer panels whose appearance is important, have become more demanding on the level of surface properties, and these Patent Documents 1 and 2 are insufficient as a measure for suppressing the occurrence of SS marks. It is coming.

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

In order to achieve this object, the gist of the aluminum alloy sheet excellent in formability of the present invention is, by mass%, including Mg: 3.5 to 7.0%, Zn: 0.1 to 4.0%. Furthermore, Fe: 1.0% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 0.3% or less, Zr: 0.3% or less, V: 0.3% or less Ti: 0.1% or less, Cu: 1.0% or less, respectively, the balance being an Al—Mg-based aluminum alloy plate made of Al and inevitable impurities, measured by a three-dimensional atom probe field ion microscope As the aggregate of atoms, the aggregate of atoms contains at least 20 Mg atoms or Zn atoms or both in total, and any of these Mg atoms or Zn atoms as a reference also, Mg atoms or Zn adjacent to the atom to be the reference The mutual distance between any atom of the child is not more than 0.50 nm, and comprise an aggregate of these conditions are met atoms 1 × 10 ⁴ cells / [mu] m ³ or more of the average density.

Here, the definition of the distance of atoms in the above-mentioned atomic assembly is that any atom of Mg atoms and Zn atoms contained in the above-mentioned atomic assembly is adjacent to that atom (reference Mg atom or Zn atom). suit other atoms (Mg atom, Zn atom) of any distance to each other of one of the atoms may be equal to or less than 0.50 nm, Ru meaning der called.

  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, it is considered that not only Zn but also the difference in the structure state of the Al—Mg-based aluminum alloy plate containing Zn has a great influence on the SS mark generation state. As this organization state, the presence of a novel fine MgZn cluster or the like is expected to be effective in suppressing the SS mark.

  However, even with normal structure observation using SEM or TEM, the above-described novel fine MgZn cluster, which seems to be effective in suppressing SS marks, could not be found for Al-Mg aluminum alloy plates containing Zn. . For this reason, depending on the definition of such a novel fine MgZn cluster, the structure of an Al—Mg-based aluminum alloy plate containing Zn that is effective in suppressing the SS mark could not be specified.

  Based on this, in the present invention, an attempt was made to analyze an aggregate (cluster) of about 10 atoms by a three-dimensional atom probe field ion microscope capable of analyzing an atomic structure of less than 100 atoms. That is, for the Al-Mg-based aluminum alloy plates containing several Zn, which have different SS mark suppression properties, the difference in the existence form (existing state) of each other's aggregates was confirmed.

  As a result, the superiority or inferiority of the SS mark suppression between Al-Mg-based aluminum alloy plates containing Zn, which are not different from each other in the other material conditions, depends greatly on the presence state of the atomic aggregates defined by the present invention. It has been found that the greater the number of atomic aggregates defined by the invention, the greater the SS mark suppression effect. Here, the fact that there is no difference in other material conditions means that the above-described SS mark suppression superiority and inferiority of each plate, as well as the component composition of each other, as well as normal TEM and SEM structure observation, extraction residue method and X-ray It means that there is no difference even in analysis such as diffraction.

  In other words, in an Al—Mg-based aluminum alloy plate containing Zn, the state of existence of an aggregate of atoms containing at least either Mg atoms or Zn atoms, that is, average density, measured by a three-dimensional atom probe field ion microscope. Can serve as an index representing the relationship between the structure of the plate and the press formability represented by the SS mark characteristics of the plate.

  Even if the aggregate of atoms defined by the present invention is composed of 100 atoms as a preferred maximum number, the size is as small as about 50 Å (angstrom) at most. Therefore, even if it is a transmission electron microscope (TEM) whose maximum magnification is about 1 million times, the limit (detection limit) that can be observed is just below or below the limit. For this reason, even if it is TEM of the maximum magnification, as a practical problem, the aggregate of atoms defined by the present invention cannot be observed (detected).

  Also, for example, in the extraction residue method that is widely used to measure the amount of solid solution and the amount of precipitates of additive elements, it is possible to determine whether the precipitates have a fine size of 0.1 μm or less by using a filter having the smallest aperture size of 0.1 μm. It can be discriminated whether the precipitate has a coarse size exceeding 0.1 μm. However, even fine precipitates of 0.1 μm or less by this extraction residue method are aggregates of atoms consisting of less than 100 atoms as defined by the present invention, precipitates larger than that, or solid precipitates. It cannot be determined whether the element is dissolved.

  These facts indicate that the atoms defined in the present invention can be obtained even if the above-mentioned plates having different heat resistances are subjected to structural observation such as TEM or SEM, or analysis such as extraction residue method or X-ray diffraction. It means that it cannot be detected very much until the difference in the existence state of. It also means that even if the TEM is at the maximum magnification or the extraction residue method, it cannot be identified whether or not there is an aggregate of atoms defined by the present invention.

On the other hand, analysis using a three-dimensional atom probe field ion microscope is widely used for analysis of high-density magnetic recording films and electronic devices. It is also used for structural analysis in the field of steel. For example, in Japanese Patent Application Laid-Open No. 2006-29786, it is used to analyze the type and amount of elements contained in carbon-containing fine precipitates in steel materials. Japanese Patent Application Laid-Open No. 2007-254766 is also used for analysis of the amount of C and N at the interface between sulfide and Fe in steel (atom / nm ² ).

  Moreover, in the copper alloy field, the present applicant himself as Japanese Patent Application No. 2008-111611 uses an atom for improving the heat resistance of a Cu-Fe-P-based copper alloy plate using this three-dimensional atom probe field ion microscope. An index based on the provision of aggregates is proposed. That is, the average density of an aggregate of atoms including at least either Fe atom or P atom and the distance between adjacent atoms of Fe atom and P atom being 0.90 nm or less is defined as Cu atom and Fe atom. And the total number of P atoms is defined as 15 or more and less than 100.

  In this respect, in the present invention, it is an Al—Mg-based aluminum alloy plate containing Zn. Similarly, a three-dimensional atom probe field ion microscope is used to define an atomic assembly for improving formability. Yes.

  That is, in the present invention, a certain amount of ultrafine MgZn clusters are present at an average density of an aggregate of atoms including either Mg atoms or Zn atoms. As a result, 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, thereby generating stretcher strain marks. Can be suppressed.

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

(Organization)
The present inventors have found that an Al—Mg-based aluminum alloy plate has an effect of suppressing generation of SS marks when Zn is contained. However, at the same time, it has also been found that even if an Al—Mg-based aluminum alloy plate having the same Zn content is used, a phenomenon occurs in which there is a large difference in the effect of suppressing the generation of SS marks. For this reason, not only Zn but also the structure state of the Al—Mg-based aluminum alloy plate, that is, the presence state of MgZn clusters generated when Zn is included, greatly affects the SS mark generation state. It is thought that.

  For this reason, in order to confirm the existence state of such MgZn-based clusters, the present inventors have observed the structure of an Al—Mg-based aluminum alloy plate containing Zn that is excellent in press formability by suppressing the SS mark. went. Specifically, the structure was observed using a 100,000-fold FE-TEM (transmission electron microscope) most effective for measuring fine MgZn clusters in the plate structure.

  However, even with such normal structure observation by TEM, this SS mark is obtained for an Al—Mg-based aluminum alloy plate (hereinafter, also referred to as “Al—Mg—Zn-based alloy plate”) containing Zn having an excellent SS mark suppression effect. A novel fine MgZn cluster that appears to be effective in suppression could not be found (observed).

  Therefore, the present inventors have affected the SS mark suppression by the presence of a novel fine MgZn cluster that cannot be observed with these TEMs and SEMs, in other words, almost no significant difference from the solid solution state. I thought so. If such a fine MgZn cluster exists in the Al-Mg-based aluminum alloy sheet structure containing Zn, the movement of dislocations during deformation by press molding is hindered, and the effect of suppressing the generation of SS marks is achieved. This is because it is presumed that there is.

3D atom probe field ion microscope:
The above-described novel fine MgZn clusters (a collection of atoms composed of 20 or more atoms defined by the present invention) can be measured only by using a known three-dimensional atom probe field ion microscope at present.

  The three-dimensional atom probe field ion microscope (3DAP: 3D Atom Probe Field Ion Microscope, hereinafter also abbreviated as 3DAP) 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.

  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 an Al—Mg-based aluminum alloy plate containing Zn, and this sample is cut and electropolished with a precision cutting device to provide an ultra-fine needle tip for analysis. A sample is prepared. 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.

  The three-dimensional atom map is further analyzed for an aggregate of atoms (cluster) using a maximum separation method which is a method of defining atoms belonging to precipitates and clusters. In this method, the maximum distance dmax between specified solute atoms and the minimum number of atoms Nmin constituting the cluster are given as parameters. In this analysis, the cluster is defined with the maximum interval dmax between adjacent Mg and Zn atoms being 0.50 nm and the total minimum number Nmin of Mg and Zn atoms being 20. From this result, the cluster dispersion state is evaluated, and the number density of the clusters (the specified average density when the number of measurement samples is 3 or more) is quantified.

(Atom detection efficiency by 3DAP)
However, 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 3DAP is greatly changed, such as an improvement in the future, the measurement result by 3DAP of the average number density (pieces / μm ³ ) of the aggregate of atoms defined by the present invention may change. There is sex. Therefore, in order to give reproducibility to the measurement of the average number density of the aggregate of atoms, it is preferable that the detection efficiency of atoms by 3DAP is substantially constant at about 50%.

(Definition of atomic assembly)
In the present invention, the aggregate (cluster) of the atoms defined in this claim includes at least either an Mg atom or a Zn atom. In addition, an aggregate of atoms including a total of 20 or more of either or both of Mg atoms and Zn atoms, and any of these atoms of Mg or Zn atoms as a reference, The average number density (pieces / μm ³ ) of an aggregate of atoms having a distance of 0.50 nm or less between the adjacent atom and any one of adjacent Mg or Zn atoms is 1 × 10 ⁴ / Μm ^{3 and} the average density is specified to be included.

  This aggregate of atoms is not necessarily composed of only two atoms of Mg atom and Zn atom. In addition to Mg atoms, Zn atoms, or Mg atoms and Zn atoms, other elements, particularly Si atoms, Cu atoms, and the like may be included.

  Depending on the component composition of the Al—Mg-based aluminum alloy containing Zn, the atoms such as Si atoms, Cu atoms, or atoms such as Fe, Mn, Cr, Zr, V, and Ti contained as alloy elements and impurities in the aggregate of atoms This is because the case where these other atoms are counted by 3DAP analysis inevitably occurs. However, even if other atoms (from alloy elements and impurities) are included in the aggregate of atoms, the level is smaller than the total number of Mg atoms and Zn atoms. Therefore, even when such other atoms are included in the aggregate, those satisfying the conditions of the prescribed distance between the Mg atom and the Zn atom and the prescribed total number are Mg atoms as the aggregate of atoms of the present invention. And functions in the same manner as an aggregate of atoms consisting only of Zn atoms. Therefore, when the number of atoms within the adjacent distance is satisfied, even when other atoms are included in the aggregate, it is counted as the aggregate of atoms of the present invention and does not satisfy the condition for the number of atoms within the adjacent distance. In the case, it is not counted as an aggregate of atoms according to the present invention.

  In this respect, the atoms adjacent to each other in the aggregate of atoms of the present invention may be not only atoms different from Mg atoms and Zn atoms, but also Mg atoms and Zn atoms. For example, in an aggregate of atoms, either Mg atom or Zn atom is not detected and is zero (even if only Mg atom or Zn atom), or between Mg atoms or Zn atoms If any of the atoms satisfies the adjacent distance (0.50 nm or less) and the number (20 or more), it is an aggregate of atoms defined by the present invention, and an aggregate of atoms defined by the present invention. As an average number density. Therefore, in the measurement by 3DAP analysis, even if the number of atoms within adjacent distances satisfies the prescribed number, this aggregate of atoms does not contain any Mg atom or Zn atom If so, it is not an aggregate of atoms defined by the present invention, and does not count. That is, the aggregate of atoms defined in the present invention necessarily includes both Mg atoms and Zn atoms, or atoms of either Mg atoms or Zn atoms.

Here, as described in paragraph 0019, the definition of the atomic distance in the atomic aggregate is that any atom of the Mg atom and Zn atom included in the atomic aggregate is the atom (reference Mg atom). and Zn atom) and the adjacent other atoms (Mg atom, of the Zn atom), not good if the distance of each other with any one of the atom is 0.50nm or less.

  Such an aggregate of atoms of the present invention is presumed to be formed by the diffusion of Mg atoms and Zn atoms in long-term annealing at a very low temperature following the solution quenching process described later.

(Average density of aggregate of atoms)
In the present invention, the aggregate of atoms defined as described above and measured by 3DAP analysis is an average of 1 × 10 ⁴ pieces / μm ³ or more in an Al—Mg-based aluminum alloy sheet structure containing Zn. Be present at density.

  Thereby, generation | occurrence | production of SS mark which is the press formability of the Al-Mg type aluminum alloy plate containing Zn can be suppressed. That is, as the number of atomic aggregates increases, the atomic vacancies (vacancy) of the Al—Mg-based aluminum alloy plate containing Zn can be blocked (trapped) by the atomic aggregates. The existence of atomic vacancies is considered to promote the generation and propagation of parallel bands due to serrations on the stress-strain curve. For this reason, when the atomic vacancies are blocked by the aggregate of atoms, the generation and propagation of the SS mark are suppressed. In addition, this can simultaneously suppress the generation of random marks due to the occurrence of yield elongation and the generation of parallel bands. Therefore, according to the present invention, generation of SS marks can be comprehensively suppressed, and press formability can be improved.

On the other hand, when the average density of the atomic aggregate is less than 1 × 10 ⁴ / μm ³ , the atomic aggregate is too small, and the atomic vacancies (vacancy) of the Al—Mg-based aluminum alloy plate containing Zn. ) And Mg atoms and Zn atoms cannot be diffused and blocked. For this reason, many atomic vacancies remain as they are, and the generation of SS marks due to the atomic vacancies and the promotion of propagation of SS marks cannot be stopped, and the generation of SS marks in an Al—Mg-based aluminum alloy plate containing Zn occurs. Cannot be sufficiently suppressed. Note that the upper limit of the average density of the aggregate of atoms is not particularly limited, but about 1 × 10 ⁶ / μm ³ is assumed from the viewpoint of manufacturing limitations.

  Here, the total number of Mg atoms and Zn atoms of the aggregate of atoms of the present invention is 20 or more. If the total number is less than 20, the size of the aggregate of atoms is too small. This is because the blocking effect (trap) of atomic vacancies is reduced. On the other hand, it is difficult for the manufacturing method of the present invention to set the total number of Mg atoms and Zn atoms to a large number of 100 or more. In addition, if the total number of Mg atoms and Zn atoms is 100 or more, the aggregates (clusters) of atoms defined in the present invention do not become a coarse precipitate, There is a possibility that the atomic aggregate itself defined by the invention and its effect may be lost. Therefore, the total number of Mg atoms and Zn atoms is 20 or more, preferably 20 or more and less than 100.

Prevention of random marks:
In the present invention, generation of random marks due to 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 stretcher strains of the random mark that occurs at a relatively low strain area and the parallel band that occurs at a relatively high strain area without applying the conventional pre-strain (pre-processing). The generation of marks (SS marks) can be sufficiently suppressed.

  The present invention provides a serration on a stress-strain curve as a result of generation of random marks due to yield elongation even when the required level of surface properties of an outer panel whose appearance is particularly important as an automotive panel material plate becomes more severe. The generation of parallel bands related to can be simultaneously suppressed. 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. Further, it is presumed that an ultrafine MgZn cluster is formed to suppress generation of SS marks during press molding. If the content of Mg is less than 0.5%, these effects of containing Mg will be insufficient. In addition, the ultrafine MgZn clusters are also insufficient, and the average number density of the aggregate of atoms measured by 3DAP analysis does not exceed 1.0 × 10 ⁻⁴ / nm ³ .

  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 presumed to form a new ultrafine MgZn cluster and suppress the generation of SS marks during press molding. When Zn is less than 0.1%, the effect of suppressing the generation of SS marks during press molding is insufficient. Also, the amount of new ultrafine MgZn clusters 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 in the range of 0.1 to 4.0%. More preferably, it is in the range of 1.0 to 3.5%.

  Incidentally, in an Al—Mg-based aluminum alloy plate, Zn as an additive element is generally recognized as an effective element for improving the strength by precipitation strengthening together with Cu. Moreover, in patent document 1, Zn is recognized as an element effective also in suppression of SS mark. However, as in the present invention, it is not publicly known that an ultrafine MgZn cluster is formed in combination with manufacturing conditions described later to suppress the generation of SS marks during press forming.

Other elements:
In the present invention, it is allowed to contain one or more selected from Fe, Si, Mn, Cr, Zr, V, Ti and Cu as other elements. 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. Roughness during press forming of an Al—Mg-based aluminum alloy plate is likely to occur when the crystal grain size is large, such as when the average crystal grain size of the plate exceeds 50 μm, 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 increases, the adverse effects of these elements also increase the number of coarse crystals and precipitates resulting from these elements, which tend to be the starting point of destruction. Reduces moldability. 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, until the rolling process before the solution treatment, it is manufactured by a manufacturing method according to a normal manufacturing process of an Al-Mg alloy for forming containing about 4.5% of Mg such as 5182, 5082, 5083, and 5056. Is possible. 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, a product plate may be used. Further, it is further cold-rolled while selectively performing one or more intermediate annealings before or during cold rolling, and the plate thickness is 1.5 mm or less. It is good also as the product board of the cold-rolled sheet.

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. Convert to quenching. 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, 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. Further, it is necessary to keep the solution treatment temperature within 180 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:
At the time of quenching after the solution treatment, it is necessary to cool the plate from the solution temperature to the subsequent low-temperature annealing temperature at a cooling rate of 10 ° C./second or more. When the cooling rate is less than 10 ° C./second, coarse precipitates are generated during cooling, and after this, low temperature annealing is applied to form the final plate, resulting in insufficient cluster generation and SS marks. Such solution heating / quenching with rapid heating or rapid cooling may be performed continuously using a continuous annealing line (CAL) or the like, or a salt bath or the like for heating, water quenching for cooling, oil You may carry out by a batch type using quenching, forced air cooling, etc. Here, when solution treatment / quenching using CAL is performed, the general heating and cooling rates from room temperature to the solution treatment temperature are both about 5 to 100 ° C./second.

Low temperature annealing:
Subsequent to this quenching treatment (rapid cooling), low-temperature annealing is continuously performed in a range of 30 ° C. to 50 ° C. for 24 hours or more without lowering the plate temperature to room temperature. For this purpose, when the temperature of the plate is in the range of 30 ° C. or more and 50 ° C. or less, the cooling in the quenching process (rapid cooling) is stopped, and the plate (coil) is kept in this temperature range of 30 ° C. or more and 50 ° C. or less. ) For 24 hours or more.

Thus, after solution treatment and quenching treatment, it is possible to perform low-temperature annealing continuously without cooling the plate to room temperature, and as a plate of the present invention, a set of atoms measured by a three-dimensional atom probe field ion microscope It is important to make the average density of the body 1 × 10 ⁴ pieces / μm ³ or more.

The temperature of 30 to 50 ° C. in this low-temperature annealing is significantly lower than the normal higher aging precipitation temperature. This is because the supersaturated solid solubility after the solution hardening treatment is increased at a lower annealing temperature, so that ultrafine clusters are stably formed. If this low-temperature annealing temperature exceeds 50 ° C. and is too high, coarse MgZn-based precipitates are dispersed at a low density, so that the average density of the atomic aggregate measured by a three-dimensional atom probe field ion microscope is 1 × 10. Not more than ⁴ / μm ³ In addition, coarsening of the second phase particles containing Mg, Cu or other alloy additive elements occurs at the grain boundaries or the like, resulting in a decrease in ductility, formability or corrosion resistance.

  Moreover, even if this low-temperature annealing temperature is less than 30 ° C., or even in an appropriate temperature range of 30 ° C. to 50 ° C., if the holding time is less than 24 hours, atom diffusion becomes insufficient. For this reason, in any case, it takes too much time to form ultrafine clusters, and the effect of low-temperature annealing is reduced, which is insufficient as industrial conditions.

  In order to perform this low-temperature annealing continuously without cooling the plate to room temperature after solution treatment and quenching treatment, forced air cooling or forced cooling (rapid cooling) such as mist when the plate temperature reaches 30-50 ° C Or is immersed in a warm water bath or oil bath with a bath temperature of 30 to 50 ° C. to cool (rapidly cool). And after that, the plate or coil is immediately moved into the furnace or covered and kept at the temperature, or the plate is moved into the furnace after reheating or covered and kept at the temperature. Apply low temperature annealing.

  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.

By a special combination of such solution treatment / quenching treatment and low-temperature annealing, the structure of the Al—Mg-based aluminum alloy plate containing Zn can be made into an atomic aggregate defined in the present invention. That is, the aggregate of atoms includes at least either Mg atom or Zn atom, the distance between these adjacent atoms is 0.50 nm or less, and the total number of Mg atoms and Zn atoms Are composed of 20 or more, and this aggregate of atoms can be included at an average density of 1 × 10 ⁴ / μm ³ or more. And 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, 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.

  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 Al-Mg type alloy plates having the respective compositions of the invention examples and comparative examples shown in Table 1 and tempering and manufacturing under the conditions shown in Table 2, the structure and mechanical properties of the tempered plates are shown. Each was 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. However, Invention Example 2 in Tables 1 and 2 is a reference example in which the Mg content deviates from the lower limit of the scope of the present invention.

  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. This hot-rolled sheet is cold-rolled to a thickness of 1.35 mm, then subjected to intermediate annealing at 400 ° C. for 10 seconds in a glass furnace, and further cold-rolled to obtain a cold-rolled sheet having a thickness of 1.0 mm It was.

  As shown in Table 2 together with the alloy numbers in Table 1, these cold-rolled plates were subjected to solution treatment and quenching treatment under different conditions, and subsequent continuous low-temperature annealing treatment under various conditions. This low-temperature annealing is performed by immersing the quenching process in an oil bath having a bath temperature of 30 to 50 ° C., and after the plate temperature reaches 30 to 50 ° C., the plate is kept at 30 to 50 ° C. without being cooled to room temperature. The temperature was kept in the furnace, and this low temperature annealing was performed. In addition, after the solution treatment and the quenching treatment, a skin pass as a cold working to give a pre-strain, a subsequent room temperature aging treatment, an artificial aging treatment and the like are not performed.

  A test piece (thickness 1 mm) was cut out from the plate after these low-temperature annealing treatments, and 3DAP measurement, structure and mechanical properties of this test piece (plate after tempering) were measured and evaluated. These results are shown in Table 2, respectively.

(Tissue measurement by 3DAP)
The average density of the aggregate of atoms defined in the present invention was measured by a measurement method using a three-dimensional atom probe field ion microscope and analysis analysis software (measurement method described in detail after paragraph 0037).

(Mechanical properties)
As an investigation of the mechanical properties of the plate, the above test pieces were subjected to a tensile test, and tensile strength (MPa) and elongation (%) were measured. As test conditions, a No. 5 test piece (25 mm × 50 mmGL × sheet thickness) of JISZ2201 in a direction perpendicular to the rolling direction was sampled and subjected to a tensile test. The tensile test was performed at room temperature of 20 ° C. based on JISZ2241 (1980) (metal material tensile test method). 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, for the SS mark generation evaluation as the press formability of the plate, the yield elongation (%) and the amount of strain in which serrated serration on the stress-strain curve is generated during this tensile test (critical strain amount:% ).

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

  In the overhang forming test, a ball head overhang punch having a diameter of 101.6 mm was used, R-303P was applied as a lubricant to a test piece having a length of 180 mm and a width of 110 mm, a forming speed of 4 mm / S, and a wrinkle holding load of 200 kN. An overhang molding test was performed, 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 Tables 1 and 2, each of the inventive examples satisfies the composition rule of the present invention and is manufactured under preferable manufacturing conditions. As a result, as shown in Table 3, each invention example has an assembly of atoms within a specified range. That is, at least one of Mg atoms and Zn atoms is included, the distance between these adjacent atoms is 0.50 nm or less, and the total number of Mg atoms and Zn atoms is 20 or more. This aggregate of atoms is included at an average density of 1 × 10 ⁴ / μm ³ or more.

  Thus, as shown in Table 2, 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 10.0% or 15.0% or more. . Further, 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, as for Comparative Examples 22-28, using the alloy number 1 of Table 1 which is the same as that of Invention Example 1, as shown in Table 2, the tempering conditions are out of the preferred ranges.

  In Comparative Example 22, the plate was not cooled to room temperature during quenching (rapid cooling), and then subjected to low-temperature annealing, but the temperature of the plate dropped midway, and the low-temperature annealing temperature was too low. Insufficient atom diffusion.

  In Comparative Example 23, the plate was not cooled to room temperature during quenching (rapid cooling), and then subjected to low-temperature annealing, but the low-temperature annealing temperature was too high.

  In Comparative Example 24, the solution treatment temperature is too low.

  In Comparative Examples 25 and 26, the low-temperature annealing time is too short.

  In Comparative Examples 27 and 28, the cooling rate of the quenching process is too small.

  On the other hand, in Comparative Examples 29 to 30, the tempering conditions are in a preferable range, but the alloy composition in Table 1 is outside the scope of the invention. Comparative Example 29 has too little Zn content (Alloy 15 in Table 1). Comparative Example 30 has too little Mg content (Alloy 16 in Table 1). Comparative Example 31 has too much Mg content (Alloy 17 in Table 1).

  As a result, as shown in Table 2, in each comparative example, except for the comparative example 31, the aggregate of atoms deviates from the specified range, and the critical strain for occurrence of serration on the stress-strain curve of the aluminum alloy plate is as low as less than 8%. . For this reason, each comparative example, including the comparative example 31 in which the content of Mg is too high, is significantly mechanical, such as strength and elongation, SS mark characteristics or stretch formability, as compared to the inventive examples. Inferior.

  The above examples support the critical significance of each requirement or preferred condition of the present invention for the SS mark characteristics.

  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 formability. As a result, the application of the Al—Mg-based aluminum alloy plate to many uses such as automobiles, which are used by pressing the plate, is expanded.

Claims (2)

In mass percent, Mg: 3.5-7.0%, Zn: 0.1-4.0%, Fe: 1.0% or less, Si: 1.0% or less, Mn: 1.%. 0% or less, Cr: 0.3% or less, Zr: 0.3% or less, V: 0.3% or less, Ti: 0.1% or less, Cu: 1.0% or less, and the remainder An Al—Mg-based aluminum alloy plate made of Al and unavoidable impurities, and the atomic aggregate measured by a three-dimensional atom probe field ion microscope is either an Mg atom or a Zn atom. Or a total of 20 or more of them, and any atom of Mg atoms or Zn atoms contained in the Mg atom or Zn atom as a reference, and any atom of Mg atom or Zn atom adjacent to the reference atom The distance between each other is 0.50 nm or less. The fill assembly for a nuclear to 1 × 10 ⁴ cells / [mu] m superior aluminum alloy sheet in formability, characterized in that it comprises at least ^three average density.   2. The aluminum alloy plate having excellent formability according to claim 1, wherein a critical strain of serration generation on the stress-strain curve of the aluminum alloy plate is 8% or more as an index indicating the formability of the aluminum alloy plate.