JP6273158B2 - Aluminum alloy plate for structural materials - Google Patents

Description

  The present invention relates to a high-strength aluminum alloy plate for a structural material that has improved workability and excellent corrosion resistance. The aluminum alloy plate of the present invention is a rolled plate, which is a plate obtained by subjecting a plate produced by rolling to aging at room temperature for 2 weeks or more after solution treatment and quenching treatment, before forming into a structural material and artificial This refers to the plate before age hardening.

  In recent years, due to consideration for the global environment, social demands for reducing the weight of automobile bodies are increasing. In order to meet such demands, panels (outer panels such as hoods, doors, and roofs, inner panels), bumper force (bumper R / F), and reinforcing materials such as door beams, etc., are partly made of steel, etc. Instead of the steel material, an aluminum alloy material is applied.

  However, in order to reduce the weight of the automobile body, it is necessary to expand the application of the aluminum alloy material to automobile structural members such as frames and pillars that contribute particularly to weight reduction among the automobile members. However, these automobile structural members need to have higher strength than the above-mentioned automobile panels, for example, the required 0.2% proof stress is 350 MPa or more. In this respect, the composition and tempering (solution treatment and quenching) of the JIS to AA6000 series aluminum alloy plates, which are used in the above-mentioned automobile panels, are excellent in formability, strength, corrosion resistance, low alloy composition and recyclability. Control of the treatment, and further, the artificial age hardening treatment, is far from achieving the high strength.

Therefore, it is necessary to use a JIS or AA 7000 series aluminum alloy plate used as the reinforcing material requiring the same high strength for such a high-strength automobile structural member. However, a 7000 series aluminum alloy which is an Al-Zn-Mg series aluminum alloy is inferior in general corrosion resistance. Moreover, since it is an alloy which achieves high strength by distributing precipitates MgZn ₂ consisting of Zn and Mg at high density, there is a risk of causing stress corrosion cracking (hereinafter referred to as SCC). In order to prevent this, it is inevitable that an overaging treatment is performed and the 0.2% proof stress is used at about 300 MPa, and the characteristics as a high-strength alloy are weakened.

  For this reason, various composition control of the 7000 series aluminum alloy excellent in both strength and SCC resistance and structure control of precipitates have been conventionally proposed.

As a typical example of composition control, for example, in Patent Document 1, Mg added in excess of Zn and Mg content (stoichiometry ratio of MgZn ₂ ) of a 7000 series aluminum alloy extruded material that forms MgZn ₂ without excess or deficiency. but by utilizing the fact that contributes to high strength by adding Mg to excess over stoichiometric ratio of MgZn _2, to suppress the MgZn ₂ amount, without reducing the SCC resistance, and high strength Yes.

As a representative example of the structure control of precipitates and the like, for example, in Patent Document 2, a 7000-series aluminum alloy extruded material after artificial age hardening treatment is used for a precipitate having a particle diameter of 1 to 15 nm in crystal grains. As a result of observation by (TEM), it is made to exist at a density of 1000 to 10000 / μm ² to reduce the potential difference between the inside of the grain and the grain boundary, thereby improving the SCC resistance.

  In addition to this, although not all examples, examples of composition control to improve both the strength and SCC resistance of a 7000 series aluminum alloy extruded material and examples of structure control such as precipitates are many practical applications in extruded materials. There are many in proportion to On the other hand, there are very few examples of conventional structure control such as composition control and precipitates in a 7000 series aluminum alloy plate according to the small practical use of the plate.

  For example, in Patent Document 3, in a structural material made of a clad plate in which 7000 series aluminum alloy plates are welded together, the diameter of an aging precipitate after artificial age hardening treatment is 50 mm (angstrom) or less in order to improve strength. It has been proposed that a certain amount exists as a spherical shape. However, there is no disclosure about the SCC resistance performance, and there is no corrosion resistance data in the examples.

  Further, in Patent Document 4, the molten metal is cold-rolled after rapid solidification, and the crystal precipitates in the crystal grains of the 7000 series aluminum alloy plate after artificial age hardening treatment are measured by a 400 times optical microscope. Strength (equivalent to equivalent circle diameter with equivalent area) is set to 3.0 μm or less, average area fraction is set to 4.5% or less, and strength and elongation are improved.

  Some proposals have been made regarding the control of the texture of the plate. For example, in Patent Documents 5 and 6, in order to increase the strength and SCC resistance of a 7000 series plate for a structural material, the ingot is forged and rolled repeatedly in a warm working region after being forged. Is fine. This is because, by making the structure fine, a large tilt grain boundary having an orientation difference of 20 ° or more, which causes a potential difference between the grain boundary and the grain boundary, which causes a decrease in SCC resistance, is suppressed. This is to obtain a texture having a small angle grain boundary of 25 ° or more. However, such warm rolling is repeated because the conventional hot rolling and cold rolling methods cannot obtain a texture in which such a low-angle grain boundary is 25% or more. ing. Therefore, since the process is greatly different from the conventional method, it is difficult to say that it is a practical method for producing a plate.

JP 2011-144396 A JP 2010-275611 A JP-A-9-125184 JP 2009-144190 A JP 2001-335874 A Japanese Patent Laid-Open No. 2002-241882

  Thus, proposals such as composition control of 7000 series aluminum alloy excellent in both strength and SCC resistance and structure control such as precipitates or textures have been variously made in the field of extruded materials. However, for rolled plates produced by conventional methods, such as hot rolling and cold rolling of the ingot after soaking, other than special rolling or manufacturing methods such as the clad plate, rapid solidification method, warm rolling, etc. The fact is that there are not many proposals.

  And, the extruded material is completely different from the rolled plate in its manufacturing process such as a hot working process, and the resulting structure of crystal grains and precipitates is, for example, a fibrous form in which the crystal grains are elongated in the extrusion direction. , The crystal grains are basically different from a rolled plate having equiaxed grains. For this reason, proposals such as composition control in the extruded material and structure control of precipitates can be applied to 7000 series aluminum alloy plates and also to automobile structural members made of this 7000 series aluminum alloy plates. It is unclear whether it is effective in improving both strength and SCC resistance. In other words, unless it is actually confirmed, it does not leave the expected range.

  Therefore, there is still no effective means for the structure control technology excellent in both strength and SCC resistance of the 7000 series aluminum alloy plate produced by the above-mentioned conventional method, and there are many unclear points and room for clarification. is the current situation. Further, regarding the general corrosion resistance, since the potential reduction due to the addition of Zn is involved, it is necessary to reduce the Zn addition amount from the viewpoint of strength and corrosion resistance. However, when the Zn content is reduced, the corrosion resistance is improved, but the formability such as bendability, which is a necessary characteristic of the structural member, is improved, while the strength is reduced, contradicting the increase in strength, technically. This is a difficult task.

  In view of the above-described problems, the object of the present invention is to provide a 7000 series aluminum for structural members such as automobile members, which has both strength and formability and is excellent in corrosion resistance as a rolled plate produced by the conventional method. It is to provide an alloy plate.

In order to achieve this object, the gist of the aluminum alloy plate for a structural material of the present invention is, by mass, Zn: 3.0 to 6.0%, Mg: 1.5 to 4.5%, Cu: 0.00. The Zn content [Zn] and the Mg content [Mg] are [Zn] ≧ −0.3 [Mg] +4.
5 is a Al—Zn—Mg-based aluminum alloy plate having a composition consisting of Al and inevitable impurities, and the balance is subjected to differential scanning calorie when the plate is aged at room temperature after solution treatment and quenching treatment. In the analysis curve, the maximum endothermic peak temperature is 130 ° C. or less, the maximum height of the exothermic peak in the temperature range of 200 to 300 ° C. is 50 μW / mg or more, and the work hardening index n value (10 to 20%) Is 0.22 or more.

  The aluminum alloy plate referred to in the present invention is a plate produced by rolling, and the ingot is hot-rolled after soaking, further cold-rolled into a cold-rolled plate, and further subjected to solution treatment and quenching treatment, etc. The 7000 series aluminum alloy plate manufactured by the usual method to which the tempering process (T4 by the symbol according to quality) was performed. In other words, it does not include a plate manufactured by a special rolling method such as Patent Documents 5 and 6 in which an ingot is forged and then warm rolling is repeated many times.

  Further, the aluminum alloy plate referred to in the present invention defines a room temperature aged structure of the 7000 series aluminum alloy plate produced as described above, and is processed into a structural material for use as a material aluminum alloy plate. It is. For this reason, it refers to a plate after the plate manufactured as described above is aged at room temperature (room temperature standing), and before being formed into a structural material as an application and before artificial age hardening treatment.

  The present invention analyzed the room temperature aged structure of a 7000 series aluminum alloy plate having a composition in which the Mg content was increased in order to ensure strength while suppressing the Zn content in order to improve corrosion resistance, using a differential scanning calorimetry curve. . As a result, it has been found that the composition and action of the cluster formed by aging at room temperature are different in the 7000 series aluminum alloy plate having such a composition as compared with the 7000 series aluminum alloy sheet having a high Zn content.

  That is, in the 7000 series aluminum alloy plate with reduced Zn content, the clusters (atomic aggregates) generated by aging at room temperature are not only the artificial age-hardening properties (BH properties) after forming into the structural material that is used. They have also found that it contributes to the ductility (work hardening characteristics) required during the forming of structural materials. Therefore, by controlling these clusters, it is possible to improve not only the corrosion resistance but also the balance between strength and ductility (formability), and as a rolled plate manufactured by a conventional method, it has both strength and formability. In addition, a structural 7000 series aluminum alloy plate excellent in corrosion resistance such as SCC resistance can be provided.

  However, the composition, size, or density of such clusters (atomic aggregates) cannot be directly identified using ordinary observation means such as SEM or TEM at the present time. Difficult to define quantitatively.

  Therefore, in the present invention, the room temperature aged structure of the manufactured 7000 series aluminum alloy plate is controlled by controlling the cluster indirectly by analysis using a differential scanning calorimetry curve. More specifically, in the analysis based on the differential scanning calorimetry curve, the work hardening characteristics as ductility are improved as the temperature of the endothermic peak corresponding to the re-solution of clusters formed by room temperature aging is lowered. On the other hand, as the maximum height of the exothermic peak in the temperature range of 200 to 300 ° C. corresponding to the precipitate after the artificial age hardening treatment is increased, the amount of the artificial age precipitate is increased and the strength is improved.

  In the present invention, in order to define the clusters generated by aging at room temperature in this way, the measurement of the differential scanning calorimetry curve is not the state of the plate that has not been aged at room temperature immediately after the tempering treatment, but as a guide for two weeks or more. This is performed on the plate after aging at room temperature (room temperature standing), before forming into a structural material and before artificial age hardening.

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

Aluminum alloy composition:
First, the chemical component composition of the aluminum alloy sheet of the present invention will be described below, including reasons for limiting each element. In addition,% display of content of each element means the mass% altogether.

  The chemical composition of the aluminum alloy sheet of the present invention combines strength and formability, which are required characteristics for structural materials such as automobile members intended in the present invention, as a rolled sheet produced by a conventional method. It is a prerequisite for satisfying corrosion resistance. For this reason, the Al-Zn-Mg-Cu-based 7000 series aluminum alloy composition in the present invention is a composition in which the Mg content is increased in order to ensure strength while suppressing the Zn content in order to improve corrosion resistance. .

  From this viewpoint, the chemical composition of the aluminum alloy sheet of the present invention is, in mass%, Zn: 3.0 to 6.0%, Mg: 1.5 to 4.5%, Cu: 0.05 to 0.5. %, And the Zn content [Zn] and the Mg content [Mg] satisfy [Zn] ≧ −0.3 [Mg] +4.5, with the balance being Al and inevitable It shall consist of impurities. In addition to this composition, as transition elements, Zr: 0.05 to 0.3%, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 One type or two or more types of -0.3% may be selectively included. Further, in addition to or instead of these transition elements, Ag: 0.01 to 0.2% may be selectively included.

Zn: 3.0-6.0%
Zn, which is an essential alloy element, together with Mg, forms clusters during aging of the manufactured tempered plate at room temperature to improve work hardening characteristics and improve the workability of the structural material. In addition, an aging precipitate is formed to improve the strength during the artificial aging treatment after the forming process to the structural material. If the Zn content is less than 3.0%, the strength after the artificial aging treatment is insufficient. However, if the Zn content increases and exceeds 6.0%, the grain boundary precipitate MgZn ₂ increases and intergranular corrosion tends to occur, and the corrosion resistance deteriorates. Therefore, in the present invention, the Zn content is suppressed to be relatively small. Therefore, the Zn content is in the range of 3.0 to 6.0%, preferably 3.5 to 4.5%.

Mg: 1.5-4.5%
Mg, which is an indispensable alloy element, together with Zn forms clusters during room temperature aging of the manufactured tempered plate to improve work hardening characteristics. In addition, an aging precipitate is formed during the artificial aging treatment after the forming process to the structural material, thereby improving the strength. In the present invention, since the Zn content is suppressed to a relatively low level, the Mg content is made relatively large. If the Mg content is less than 1.5% by mass, the strength is insufficient and the work hardening characteristics are deteriorated. However, if it exceeds 4.5 mass%, the rollability of the plate is lowered, and the SCC sensitivity is enhanced. Therefore, the Mg content is in the range of 1.5 to 4.5%, preferably 2.5 to 4.5%.

Balance formula of Zn and Mg:
In the present invention, in order to ensure high strength due to the inclusion of Zn and Mg, not only the content of Zn and Mg but also the Zn content [Zn] (mass%) and the Mg content [ It is important to control the balance with [Mg] (mass%). For this reason, as a control of this balance, [Zn] and [Mg] have a balance formula of [Zn] ≧ −0.3 [Mg] +4.5, preferably [Zn] ≧ −0.5 [Mg ] Satisfy the balance equation of +5.75.

  By satisfying this [Zn] ≧ −0.3 [Mg] +4.5, the 0.2% proof stress of the structural material after the artificial age hardening treatment can be 350 MPa or more by a preferable manufacturing method described later. It becomes. Moreover, by satisfying [Zn] ≧ −0.5 [Mg] +5.75, the 0.2% proof stress of the structural material after the artificial age-hardening treatment may be set to 400 MPa or more by a preferable manufacturing method described later. It becomes possible.

  When each content of Zn and Mg is [Zn] <− 0.3 [Mg] +4.5, even if each content of Zn and Mg is within a specified range, or by a preferable manufacturing method described later, There is a possibility that the 0.2% proof stress of the structural material after the artificial age hardening treatment cannot be set to 350 MPa or more. In addition, when the Zn content and the Mg content are [Zn] <− 0.5 [Mg] +5.75, the 0.2% proof stress of the structural material after artificial age hardening can be similarly set to 400 MPa or more. There is a possibility of disappearing.

Cu: 0.05 to 0.5%
Cu has the effect of suppressing the SCC sensitivity of the Al—Zn—Mg alloy and improving the SCC resistance. It also improves general corrosion resistance. When the Cu content is less than 0.05%, the effect of improving SCC resistance and general corrosion resistance is small. On the other hand, if the Cu content exceeds 0.5%, various properties such as rollability and weldability are reduced. Therefore, the Cu content is in the range of 0.05 to 0.5%, preferably 0.4% or less.

One or more of Zr: 0.05 to 0.3%, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 to 0.3% Since the transition elements of Zr, Mn, Cr, and Sc contribute to improvement in strength by refining the crystal grains of the ingot and the final product, they are selectively contained when necessary. When any one or two or more of these are contained, if the content of Zr, Mn, Cr, or Sc is less than the lower limit, the content is insufficient and the strength is lowered. On the other hand, when the contents of Zr, Mn, Cr, and Sc exceed the respective upper limits, the elongation decreases because coarse crystals are formed. Therefore, when these are contained, the contents are: Zr: 0.05 to 0.3%, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05 -0.3% of each range, preferably Zr: 0.08-0.2%, Mn: 0.2-1.0%, Cr: 0.1-0.2%, Sc: 0.1 Each range is 0.2%.

Ag: 0.01-0.2%
Ag has an effect of closely and finely precipitating aging precipitates that contribute to strength improvement by artificial aging treatment after forming processing into a structural material, and has the effect of promoting high strength. Therefore, Ag is selectively contained as necessary. . If the Ag content is less than 0.01%, the effect of improving the strength is small. On the other hand, even if the Ag content exceeds 0.2%, the effect is saturated and expensive. Therefore, the Ag content is in the range of 0.01 to 0.2%.

Other elements:
Other elements other than these are basically inevitable impurities. As a melting raw material, in addition to pure aluminum ingots, the inclusion of these impurity elements due to the use of aluminum alloy scrap is assumed (allowed), and each content within the range specified by the JIS standard of 7000 series alloys is allowed. . For example, Ti and B are impurities as a rolled plate, but also have the effect of refining the crystal grains of the ingot, so the upper limit of Ti is 0.2%, preferably 0.1%, and the upper limit of B is 0.05% or less, preferably 0.03%. Fe and Si are allowed to be contained without affecting the characteristics of the aluminum alloy rolled sheet according to the present invention as long as Fe: 0.5% or less and Si: 0.5% or less.

Organization:
Based on the above composition, the 7000 series aluminum alloy sheet structure of the present invention has the maximum endothermic peak temperature in the differential scanning calorimetry curve after room temperature aging for 2 weeks or more after solution treatment and quenching of the sheet. Is 130 ° C. or lower, and the maximum exothermic peak height in the temperature range of 200 to 300 ° C. is 50 μW / mg or higher.

  The maximum endothermic peak temperature corresponds to the re-solution of clusters formed during room temperature aging of the plate. The lower the maximum endothermic peak temperature, the lower the thermal stability of the cluster (which tends to decompose), and the easier it is to decompose by cutting dislocations during plastic deformation besides heat. Therefore, the lower the cluster stability is, the less likely it is to cause dislocation movement failure or strain concentration during plastic deformation, such as forming a plate into a structural material. As a guideline, the maximum endothermic peak temperature is 130 ° C. or less. By satisfying this rule, the stability of the cluster is low (unstable), the work hardening property is improved, and the work hardening index n value (10-20) %) Can be 0.22 or more.

  On the other hand, the higher the stability of this cluster, the higher the maximum endothermic peak temperature is higher than 130 ° C., the work hardening property is lowered, and the work hardening index n value (10 to 20%) is reduced to 0. Cannot be over 22. This is because the higher the stability of this cluster, the more difficult it is to move dislocations during plastic deformation, such as forming a plate into a structural material. Since the movement of dislocations is concentrated, strain concentration is likely to occur and work hardening characteristics are deteriorated.

  The maximum height of the exothermic peak in the temperature range of 200 to 300 ° C. corresponds to the precipitation of artificial aging treatment (artificial aging precipitate) that contributes to improvement in strength. Therefore, the higher the exothermic peak height, the greater the amount of artificial aging precipitates deposited (density) and the higher the strength. The guideline is that the maximum height of the exothermic peak in the temperature range of 200 to 300 ° C. is 50 μW / mg or more, and when the maximum height of the exothermic peak is less than 50 μW / mg, There is a high possibility that the 0.2% proof stress of the structural material cannot be 350 MPa or more.

Work hardening characteristics:
The above-described composition and structure and the preferable manufacturing method described later improve the forming processability of the 7000 series aluminum alloy plate of the present invention, but particularly ensure the bending processability used when forming a structural material. Therefore, in the present invention, a work hardening index n value (10 to 20%) is defined. That is, a 7000 series aluminum alloy plate manufactured by the above-described composition and structure and a preferable manufacturing method described later, and after hardening and quenching the plate, a work hardening index n value after aging at room temperature for 2 weeks or more as a standard (10 to 20%) is 0.22 or more.

  Work hardening is a phenomenon in which hardness increases due to plastic deformation when stress is applied during molding or the like, and is also called strain hardening. As the deformation progresses due to the molding process, the resistance increases and the hardness increases, which is work hardening. This is a characteristic value that is an index of the molding processability and is called “n value”. This n value is an index n obtained by approximating the relationship between the stress σ and the strain ε in the plastic region above the yield point. The approximate expression is the Voce equation that fits well with aluminum. The higher the n value, the easier it is to work and harden, the hardened part that has undergone plastic deformation due to molding such as bending, and the surrounding area are more likely to deform, so that molding processability such as bending is improved. On the other hand, the lower the n value, the harder the work is hardened, and the part that is initially subjected to plastic deformation, the most stressed part does not become harder, but it becomes more plastically deformed and is more likely to be constricted and fractured. Low formability such as bending.

(Production method)
The method for producing a 7000 series aluminum alloy plate in the present invention will be specifically described below.

  In this invention, it can manufacture with the manufacturing method by the normal manufacturing process of a 7000 series aluminum alloy plate. 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 Subsequently, it is cold-rolled to obtain a cold-rolled sheet having a thickness of 3 mm or less. At this time, one or more intermediate annealings may be selectively performed before cold rolling or in the middle of cold rolling.

(Dissolution, casting cooling rate)
First, in the melting and casting process, an ordinary molten casting method such as a continuous casting method or a semi-continuous casting method (DC casting method) is appropriately selected for the aluminum alloy melt adjusted within the above-mentioned 7000-based component composition range. Cast.

(Homogenization heat treatment)
Next, the cast aluminum alloy ingot is subjected to a homogenization heat treatment prior to hot rolling. The purpose of this homogenization heat treatment (soaking) is to homogenize the structure, that is, eliminate segregation in crystal grains in the ingot structure.

  However, in the present invention, after aging at room temperature after the tempering treatment, in order to improve both the work hardening characteristics at the time of molding to the structural material and the strength after the artificial aging treatment after the molding processing to the structural material, The soaking process is performed in two or two soaking steps. Two-stage soaking means cooling after the first soaking, but it is not cooled to 200 ° C. or lower, and after stopping the cooling at a higher temperature, after maintaining at that temperature, Hot rolling is started after reheating to a high temperature. On the other hand, after the first soaking, the two-time soaking is once cooled to a temperature of 200 ° C. or less including room temperature, reheated and maintained at that temperature for a certain period of time. Start.

  In the first or first soaking step in these two or two soaking steps, the transition element compound is finely dispersed, aiming to refine the compound that affects the moldability to the structural material, In the second or second soaking step, solid solution of Zn, Mg, and Cu is promoted, aiming at work hardening characteristics at room temperature aging and strength improvement at artificial aging treatment.

  For this purpose, the soaking temperature at the first stage or the first time is controlled to 400 to 450 ° C., preferably 400 to 440 ° C. By heating and maintaining the ingot in this temperature range, a Zr compound or a compound composed of Mn, Cr, or Sc can be finely dispersed. If this soaking temperature is less than 400 ° C., a sufficient effect of miniaturization cannot be obtained, and work hardening characteristics cannot be improved during aging at room temperature. On the other hand, if it exceeds 450 ° C., these compounds are coarsened, and it is impossible to improve the work-hardening characteristics at the time of aging at room temperature. The holding time of the first stage or the first soaking may be about 1 to 8 hours.

  In addition, the second or second soaking temperature is controlled to 450 ° C. to the solidus temperature, preferably 470 ° C. to the solidus temperature. By heating and holding the ingot in this temperature range, the solid solution of Zn, Mg and Cu can be promoted, and the strength at the time of artificial aging treatment after solution treatment can be improved. When the soaking temperature is less than 450 ° C., sufficient dissolution of these elements cannot be obtained, and the work-hardening characteristics at room temperature aging and the strength after artificial aging do not increase. On the other hand, if the solidus temperature is exceeded, partial melting occurs and the mechanical properties deteriorate, so the upper limit is made the solidus temperature or lower. The holding time at the second stage or the second soaking may be about 1 to 8 hours.

(Hot rolling)
In the hot rolling, the hot rolling itself becomes difficult because burning occurs under conditions where the hot rolling start temperature exceeds the solidus temperature. On the other hand, when the hot rolling start temperature is less than 350 ° C., the load during hot rolling becomes too high, and the hot rolling itself becomes difficult. Therefore, the hot rolling start temperature is selected from the range of 350 ° C. to the solidus temperature and hot rolled to obtain a hot rolled sheet having a thickness of about 2 to 7 mm. Annealing (roughening) of the hot-rolled sheet before cold rolling is not necessarily required, but may be performed.

(Cold rolling)
In cold rolling, the hot rolled sheet is rolled to produce a cold rolled sheet (including a coil) having a desired final thickness of about 1 to 3 mm. Intermediate annealing may be performed between cold rolling passes.

(Solution treatment)
After cold rolling, solution treatment is performed as a tempering. The solution treatment is not particularly limited and may be heating and cooling using a normal continuous heat treatment line. However, in order to obtain a sufficient solid solution amount of each element and refinement of crystal grains, the holding time is predetermined at a solution treatment temperature of 450 ° C. to a solidus temperature, preferably 480 to 550 ° C. After reaching the solution treatment temperature, it is carried out in the range of 2 to 3 seconds and 30 minutes or less.

  The average cooling (temperature decrease) speed after solution treatment is not particularly limited. For cooling after solution treatment, forced cooling means such as air cooling such as fans, water cooling means such as mist, spraying, and immersion are selected or combined. Or use in hot water from room temperature to 100 ° C. By the way, the solution treatment is basically only one time, but when the room temperature age hardening has progressed too much, the solution treatment and the restoration treatment are made the above-mentioned preferable conditions in order to ensure the moldability to the automobile member. It may be applied again, and this excessive room temperature age hardening may be canceled once.

  And the aluminum alloy plate of this invention is shape-processed into a motor vehicle member as a raw material, and is assembled as a motor vehicle member. Further, after being molded into an automobile member, it is subjected to a separate artificial age hardening treatment to obtain an automobile member or an automobile body.

Artificial age hardening treatment:
The 7000 series aluminum alloy plate of the present invention is made 350 MPa or more, preferably 400 MPa or more with a desired strength and 0.2% proof stress as a structural material such as an automobile member by an artificial age hardening treatment after forming the structural material. The The time point at which this artificial age hardening treatment is performed is preferably after the forming process of the material 7000 series aluminum alloy plate to the automobile member. This is because the 7000 series aluminum alloy plate after the artificial age hardening treatment has high strength but has low formability, and may not be formed depending on the complexity of the shape of the automobile member.

  The temperature and time conditions of this artificial age hardening treatment are within the range of general artificial aging conditions (T6, T7) from the desired strength, the strength of the 7000 series aluminum alloy plate of the material, or the progress of room temperature aging. It is decided freely. By the way, exemplifying the conditions of artificial age hardening treatment, an aging treatment at 100 to 150 ° C. is performed for 12 to 36 hours (including an overaging region) in the case of one-stage aging treatment. In the two-stage process, the first-stage heat treatment temperature is in the range of 70 to 100 ° C. for 2 hours or longer, and the second-stage heat treatment temperature is in the range of 100 to 170 ° C. for five hours or longer (over-aged region). Select from).

  The 7000 series aluminum alloy cold-rolled sheet of the Al-Zn-Mg-Cu system component composition shown in the following Tables 1 and 2 was manufactured by changing various structures analyzed by the DSC curve. About these manufactured cold-rolled sheets, the maximum endothermic peak temperature in the DSC curve when the plate is aged at room temperature after solution treatment and quenching treatment, the maximum height of the exothermic peak in the temperature range of 200 to 300 ° C., work hardening index n Values (10-20%) were measured. In addition, mechanical properties such as strength after artificial age hardening and general corrosion resistance were also evaluated. These results are shown in Tables 3 and 4 below.

  The structure of the cold-rolled sheet was mainly controlled by changing various soaking conditions shown in Tables 3 and 4. Specifically, in common with each example, a 7000 series aluminum alloy molten metal having each component composition shown in Tables 1 and 2 below was DC-cast to obtain an ingot of 45 mm thickness × 220 mm width × 145 mm length. The ingot was subjected to two-stage soaking or twice soaking under the conditions of Tables 3 and 4. After the first soaking, the second stage soaking is cooled to 250 ° C., once the cooling is stopped at that temperature, reheated and held at the second stage soaking temperature, and cooled to the hot rolling start temperature. The hot rolling was started. After the first soaking, the second soaking was once cooled to room temperature, reheated and held at the second soaking temperature, cooled to the hot rolling start temperature, and then started hot rolling. The only soaking process in Tables 3 and 4 does not perform the second reheating after cooling, but keeps the soaking temperature and time as usual, then cools to the hot rolling start temperature and heats. Started.

  After these soaking processes, hot rolling was performed at the starting temperatures shown in Tables 3 and 4 to produce hot-rolled sheets having a thickness of 5 mm. The hot-rolled sheet was subjected to a roughing treatment in which forced air cooling was performed after holding at 500 ° C. for 30 seconds, and cold rolling was performed to 2 mm. This cold-rolled sheet was subjected to a solution treatment at 500 ° C. for 1 minute in common with each example, and after this solution treatment, forced air cooling was performed to cool to room temperature to obtain a T4 tempered material. A plate-like test piece was collected from the solution-treated aluminum alloy plate that had been aged at room temperature for two weeks in common with each example, and subjected to DSC measurement and tensile test. Each characteristic was investigated as follows.

DSC measurement (differential thermal analysis):
The DSC measurement (differential thermal analysis) conditions were the same under the following conditions in common with each example.
Test apparatus: Seiko Instruments DSC220C,
Standard material: pure aluminum,
Sample container: pure aluminum,
Temperature rising condition: 15 ° C./min,
Atmosphere (in sample container): Argon gas (gas flow rate 50 ml / min),
Test sample weight: 24.5 to 26.5 mg.
In addition, the sampling by differential thermal analysis is performed from 10 points including the front end, the center, and the rear end in the longitudinal direction of the aluminum alloy plate after aging at room temperature, and the measured values are averaged. Turned into.

  After each obtained differential thermal analysis profile (μW) was divided by the sample weight and normalized (μW / mg), the differential thermal analysis profile was horizontal in the 0-100 ° C. section of the differential thermal analysis profile. The region to become the reference level of 0, and the maximum exothermic peak in the temperature range of 200 to 300 ° C. was measured as the maximum height of the exothermic peak from this reference level.

  In addition, the artificial age hardening treatment after molding to an automobile member was simulated, and the aluminum alloy plate after the room temperature aging was subjected to an artificial age hardening treatment of 90 ° C. × 3 hr + 140 ° C. × 8 hr as a T6 treatment. A plate-like test piece was collected from the central part of the aluminum alloy plate after the artificial age hardening treatment, and the mechanical properties and corrosion resistance were investigated as follows. These results are also shown in Tables 3 and 4, respectively.

(Mechanical properties)
In each example, the mechanical properties were commonly measured by performing a room temperature tensile test in the direction perpendicular to the rolling direction of each plate-like specimen and measuring 0.2% proof stress (MPa) and total elongation (%). The room temperature tensile test was performed at a room temperature of 20 ° C. based on JIS2241 (1980). The tensile speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke.

(N value)
The n value was measured by performing a room temperature tensile test in the direction perpendicular to the rolling direction, using the plate-like test piece after the artificial age hardening treatment as a JIS No. 5 tensile test piece (distance between gauge points 50 mm). Then, the true stress and true strain are calculated from the end point of yield elongation, plotted on a logarithmic scale with the horizontal axis being strain and the vertical axis being stress, and the gradient of the straight line represented by the measurement point is the nominal strain of 10% and 20%. To obtain an n value (10 to 20%).

(Intergranular corrosion sensitivity)
For general corrosion resistance evaluation, a grain boundary corrosion susceptibility test in accordance with the provisions of the former JIS-W1103 was performed on the plate-like test pieces (three test pieces) after the artificial age hardening treatment. The test condition was that the test piece was immersed in an aqueous nitric acid solution (30% by mass) for 1 minute at room temperature, then immersed in an aqueous sodium hydroxide solution (5% by mass) at 40 ° C. for 20 seconds, and then immersed in an aqueous nitric acid solution (30% by mass). The surface of the test piece was cleaned by immersion for 1 minute at room temperature. Thereafter, a current having a current density of 1 mA / cm2 was allowed to flow for 24 hours in a state immersed in an aqueous sodium chloride solution (5% by mass), and then the sample was pulled up, and then the cross section of the test piece was cut and polished, and an optical microscope was used. Using, the corrosion depth from the sample surface was measured. The magnification was x100, and a corrosion depth of 200 μm or less was evaluated as “◯” as minor corrosion. Moreover, the case where it exceeded 200 μm was evaluated as “×” as large corrosion.

  As is apparent from Tables 1 and 3, each invention example is within the composition range of the aluminum alloy of the present invention, and is manufactured within the range of the preferable manufacturing conditions described above.

  As a result, in the differential scanning calorimetry curve when this plate was aged at room temperature after solution treatment and quenching treatment, the maximum endothermic peak temperature was 130 ° C. or lower, and the maximum exothermic peak in the temperature range of 200 to 300 ° C. The height is 50 μW / mg or more and satisfies the tissue regulations analyzed by the DSC curve.

For this reason, even after aging at room temperature, the work hardening index n value (10 to 20%) is as high as 0.22 or more, excellent in ductility, and excellent in workability to a structural material. At the same time, even after room temperature aging, the BH property is excellent and the strength is high. It also has excellent corrosion resistance.
Among these invention examples, the composition example satisfying [Zn] ≧ −0.3 [Mg] +4.5 has a 0.2% proof stress of 350 MPa or more after artificial age hardening treatment, and [Zn] ≧ − In the case of Invention Examples 2, 4, 6, 8-12, 16-19, and 21 that satisfy both 0.5 [Mg] +5.75, the 0.2% proof stress after artificial age hardening is 400 MPa or more. is there.

  On the other hand, as shown in Tables 2 and 4, each comparative example does not have both workability and strength because the alloy composition is out of the scope of the present invention or the manufacturing conditions are out of the preferred range.

  In Comparative Examples 22 to 25 of Table 4, the contents of Zn and Mg are within the specified ranges as in Alloy Nos. 22 to 25 of Table 2, but [Zn] ≧ −0.3 [Mg] +4.5 And [Zn] ≧ −0.5 [Mg] +5.75 are not satisfied. For this reason, although it manufactured within preferable manufacturing conditions including soaking, it does not satisfy the structure rule analyzed by the DSC curve, and the work hardening index n value (10 to 20%) after aging at room temperature is 0. However, the 0.2% proof stress after the artificial age hardening treatment is less than 350 MPa, so that both workability and strength cannot be achieved.

  In Comparative Example 26 in Table 4, as shown in Alloy No. 26 in Table 2, Zn is outside the lower limit. In Comparative Example 27, Zn exceeds the upper limit as shown by Alloy No. 27 in Table 2. In Comparative Example 28, Cu is outside the lower limit as shown by Alloy No. 28 in Table 2. In Comparative Example 29, Cu exceeds the upper limit as shown by Alloy No. 29 in Table 2. For this reason, although it is manufactured within preferable manufacturing conditions including soaking, it does not satisfy the structure rule analyzed by the DSC curve, and the work hardening index n value (10 to 20%) after aging at room temperature is 0. .22 or less, and it does not have both workability and strength. Comparative Example 27 has too much Zn, and Comparative Example 28 has too little Cu, both of which have poor corrosion resistance.

  Comparative Examples 30 to 32 in Table 4 are manufactured out of the preferable manufacturing condition range although the invention example aluminum alloy of Alloy No. 2 in Table 1 is used. Comparative Example 30 is a soaking process only once (corresponding to the second soaking). In Comparative Example 31, the first soaking temperature is too low. In Comparative Example 32, the second soaking temperature is too low. For this reason, the comparative example in which these soaking conditions deviate from the preferable range does not satisfy the structure rule analyzed by the DSC curve, and the work hardening index n value (10 to 20%) after aging at room temperature is less than 0.22. In other words, the 0.2% proof stress after the artificial age hardening treatment is less than 350 MPa, so that both workability and strength are not achieved.

  In Comparative Example 33 of Table 4, the contents of Zn and Mg are within the specified ranges as shown in Alloy No. 30 of Table 2, but [Zn] ≧ −0.3 [Mg] +4.5 and [Zn] Both of the relations ≧ −0.5 [Mg] +5.75 are not satisfied. For this reason, although it manufactured within preferable manufacturing conditions including soaking, it does not satisfy the structure rule analyzed by the DSC curve, and the work hardening index n value (10 to 20%) after aging at room temperature is 0. .22 level, but 0.2% proof stress after artificial age hardening treatment is less than 350 MPa, and it does not have both workability and strength.

  In Comparative Examples 34, 35, and 36 in Table 4, the Mg content is outside the lower limit as shown in Alloy Nos. 31, 32, and 33 in Table 2. Therefore, even if both of the relations [Zn] ≧ −0.3 [Mg] +4.5 and [Zn] ≧ −0.5 [Mg] +5.75 are satisfied, soaking is included. Even if it is manufactured within the preferable manufacturing conditions, it does not satisfy the organization rules analyzed by the DSC curve. As a result, the work hardening index n value (10 to 20%) after aging at room temperature is 0.21 to 0.22 level, but the 0.2% proof stress after artificial age hardening is less than 350 MPa. It cannot combine strength.

  In Comparative Examples 37, 38, and 39 in Table 4, as shown in Alloy Nos. 34, 35, and 36 in Table 2, the Zn content is outside the upper limit. For this reason, even if both [Zn] ≧ −0.3 [Mg] +4.5 and [Zn] ≧ −0.5 [Mg] +5.75 are satisfied, including soaking, Even if it is manufactured within the preferable manufacturing conditions, it does not satisfy the tissue regulations analyzed by the DSC curve. As a result, the work hardening index n value (10 to 20%) after aging at room temperature is at the 0.21 level, and it does not have both workability and strength. Moreover, these comparative examples have too much Zn and have poor corrosion resistance.

  As for the comparative examples 40-43 of Table 4, as the alloy numbers 37-40 of Table 2, the content of Mg has deviated from the upper limit. Therefore, even if both of the relations [Zn] ≧ −0.3 [Mg] +4.5 and [Zn] ≧ −0.5 [Mg] +5.75 are satisfied, Even if it is manufactured within the preferable manufacturing conditions, it does not satisfy the tissue regulations analyzed by the DSC curve. As a result, Comparative Examples 40 and 41 having relatively high Zn have a low work hardening index n value (10 to 20%) after room temperature aging at a low level of 0.21, and do not have both workability and strength. In Comparative Examples 42 and 43 with relatively low Zn, the work hardening index n value (10 to 20%) after aging at room temperature is 0.22, but the 0.2% proof stress after artificial aging hardening is less than 350 MPa. Thus, it does not have both workability and strength. Moreover, these comparative examples have too much Mg and are inferior in corrosion resistance.

  The above results support the critical significance of the requirements of the present invention for the aluminum alloy sheet of the present invention to have both high strength, high ductility (formability) and SCC resistance.

  As described above, the present invention can provide a 7000 series aluminum alloy plate for automobile members having both strength, formability, and corrosion resistance. Therefore, the present invention is also suitable for automobile structural materials such as frames and pillars that contribute to weight reduction of the vehicle body, and structural materials for other uses.

Claims (4)

In mass%, Zn: 3.0-6.0%, Mg: 1.5-4.5%, Cu: 0.05-0.5%, respectively, and Zn content [Zn] and Mg The content [Mg] of the Al—Zn—Mg-based aluminum alloy plate having a composition satisfying [Zn] ≧ −0.3 [Mg] +4.5, with the balance being Al and inevitable impurities. In the differential scanning calorimetry curve of this plate, the maximum endothermic peak temperature is 130 ° C. or lower, and the maximum height of the exothermic peak in the temperature range of 200 to 300 ° C. is 50 μW / mg or more. An aluminum alloy sheet for a structural material having an index n value (10 to 20%) of 0.22 or more.   The aluminum alloy plate is further mass%, Zr: 0.05 to 0.3%, Mn: 0.1 to 1.5%, Cr: 0.05 to 0.3%, Sc: 0.05. The aluminum alloy plate for structural materials according to claim 1, comprising one or more of ˜0.3%.   The aluminum alloy plate for structural materials according to claim 1 or 2, wherein the aluminum alloy plate further contains Ag: 0.01 to 0.2% by mass.   The Zn content [Zn] and the Mg content [Mg] of the aluminum alloy plate satisfy the relationship [Zn] ≧ −0.5 [Mg] +5.75, and 0 after the artificial age hardening treatment. The aluminum alloy plate for a structural material according to any one of claims 1 to 3, having a 0.2% proof stress of 400 MPa or more.