JP7046780B2 - A method for manufacturing a 7000 series aluminum alloy member. - Google Patents

Description

The present invention relates to a method for manufacturing a 7000 series aluminum alloy member, and more particularly to a method for manufacturing a 7000 series aluminum alloy member by machining a 7000 series aluminum alloy extruded profile.

The aluminum alloy extruded profile can be used to manufacture hollow aluminum alloy members (particularly long members) having an arbitrary cross-sectional shape and length, and can be used for automobile members (skeleton members, energy absorbing members, etc.). Active recruitment is expanding. There are rockers (side sills), side members, pillars, etc. as skeleton members of automobiles, and door beams, bumper reinforcing materials, roof reinforcing materials, etc. as energy absorbing members.
In most cases, such aluminum alloy members are basically required to have rigidity and strength against a bending moment. Therefore, the extruded profile as a material often has a hollow cross section that is lightweight and has excellent bending strength and bending rigidity. Examples of such a hollow cross section include a substantially rectangular cross section and a cross section having one or two middle ribs inside a substantially rectangular contour.

The general manufacturing process for aluminum alloy members made from aluminum alloy extruded profiles is 1) melt casting, 2) hot extrusion, 3) tensile straightening (cold), 4) cutting, and 5) artificial aging treatment. (Heat treatment type alloy only). When the aluminum alloy member is for automobiles, further bending, variable cross-section processing, drilling, and other processing are required, which are performed after the step 4).
The aluminum alloy extruded profile formed by hot extrusion is bent or twisted due to the difference in the flow velocity of the metal from the extruded die and the non-uniform cooling rate in the cross section. The thinner the aluminum alloy extruded profile and the asymmetrical cross section, the greater the tendency for bending and twisting. Further, in water cooling with a high cooling rate, bending and twisting become more remarkable than in air cooling. In order to correct such bending and twisting, the aluminum alloy extruded profile is subjected to cold tensile straightening immediately after extrusion, while remaining long. The larger the amount of stretch in tensile straightening, the smaller the cross-sectional area of the extruded aluminum alloy profile. The strain (plastic strain) applied to the extruded aluminum alloy profile by tensile straightening is set to about 1% or less in order to minimize the decrease in wall thickness and the change in cross-sectional shape.

Generally, the higher the strength level of an aluminum alloy, the higher the weight reduction effect. Therefore, for example, development of a high-strength aluminum alloy is being promoted for automobile members.
Typical high-strength aluminum alloys include 6000 series (Al-Mg-Si- (Cu) series) and 7000 series (Al-Zn-Mg- (Cu) series), which are precipitation-curing alloys. Generally, a 6000 series aluminum alloy has a 0.2% proof stress of about 200 to 350 MPa, and a 7000 series aluminum alloy has a 0.2% proof stress of about 300 to 500 MPa, which can be obtained by T5, T6 or T7 tempering. In particular, the 7000 series aluminum alloy has high strength and can be expected to have a high weight reduction effect.
On the other hand, it is known that the 7000 series aluminum alloy has a risk of causing cracks called stress corrosion cracking (SCC) if it is continuously exposed to a corrosive environment in a state where tensile stress is applied. Since cracking due to SCC is very sensitive, it progresses quickly and there is a risk of sudden damage, and there is a strong concern that it will occur from the viewpoint of quality assurance. SCC of 7000 series aluminum alloy is more likely to occur as the material becomes higher strength, which is a hindrance to increasing the strength of 7000 series aluminum alloy.

The main cause of SCC generation is the presence of tensile residual stress above the critical value in a corrosive environment. Tension residual stress is generated by plastic working, cutting, heat treatment (quenching, etc.) in the manufacturing process.
In the case of an aluminum alloy extruded profile, it is in the state of a straight long material having a constant cross-sectional shape immediately after extrusion. In the process of manufacturing automobile members and the like using extruded aluminum alloy profiles, plastic working such as bending, variable cross-sectioning and shearing, and machining such as cutting are required to obtain the required shape. It is known that when such machining is performed cold, a high tensile residual stress is generated in the extruded aluminum alloy profile.

In the 7000 series aluminum alloy extruded profile, which has a risk of generating SCC, it is an issue to suppress the tensile residual stress generated when the above-mentioned machining is performed cold.
In Patent Document 1, when an end portion of a 7000 series aluminum alloy extruded profile is cut off diagonally and then plastic deformation is applied to the end portion to manufacture a door beam, the generation of tensile residual stress due to plastic deformation is suppressed. , It is described that the SCC resistance is improved.
In Patent Document 2, a 7000 series aluminum alloy extruded profile in a T1 tempered state is heat-treated under predetermined rapid heating and quenching conditions, and then plastic working is performed to reduce the tensile residual stress associated with the plastic working. It is described that the occurrence is suppressed and the SCC resistance is improved.

Japanese Unexamined Patent Publication No. 2002-362157 Japanese Patent No. 5671422

The strain due to bending depends on the cross-sectional height H of the extruded profile and the radius of curvature R of the bending neutral axis, and strain of approximately ± H / 2R occurs in the longitudinal direction (extrusion parallel direction) on the outside and inside of the bending. .. The strain distribution (before unloading) of the extruded profile is as shown in FIG. 1A, and the strain amount is the largest in the outermost layer of the extruded profile. The ratio R / H of the cross-sectional height H and the radius of curvature R of the extruded profile is generally about 10 to 100 in the case of an automobile member, for example, and the strain of the outermost layer is about 0.5 to 5%.
Further, the residual stress after bending (after unloading) occurs mainly in the longitudinal direction of the extruded profile, and generally has a residual stress distribution as shown in FIG. 1B. The position where the tensile residual stress that causes SCC generation shows the maximum value is not the outermost layer (bending outer side) where the strain amount is the largest, but the position near the center of the cross section (bending center), that is, it is added by bending. The distortion is relatively small.
In the case of tensile bending, bending is performed by applying a tension of about 0.3 to 0.7 times the product of yield strength σy and cross-sectional area S (σy × S) to the extruded profile, resulting in tensile residual stress. The position showing the maximum value approaches the position closer to the center of the cross section (the center of bending when the tension is zero). In this case as well, the position where the tensile residual stress shows the maximum value is the position where the strain applied by the tensile bending process is relatively small, as in the bending process with zero tension.

Even in the variable cross-section machining, high tensile residual stress occurs not in the region where a large strain is introduced but in the region where the strain is small. FIG. 2 shows a side view and a cross-sectional view when an aluminum alloy extruded profile composed of a pair of flanges 1 and 2 and a pair of webs 3 and 4 is subjected to a variable cross-sectional processing (crushing processing). The tensile residual stress after the variable cross-section processing is mainly generated in the direction perpendicular to the extrusion direction (see the up and down arrows in FIG. 2). The tensile residual stress generated in the webs 3 and 4 is the largest in the vicinity of the boundary between the region where the plastic strain is introduced and the region where the plastic strain is not introduced (near the CC cross section), that is, in the region where the strain is small (Japanese Patent Laid-Open No. 2014-145119). See Gazette).

Plastic working on 7000 series aluminum alloy extruded profiles is often performed in the state of T1 tempering or T4 tempering. This is because 1) the extruded shape after artificial aging treatment is hardened and has poor ductility and cracks during processing, so 2) the residual stress to be introduced is reduced (the magnitude of the residual stress is the size of the extruded shape during processing). (It is almost proportional to the bearing capacity of). Therefore, after the plastic working, the extruded profile is subjected to artificial aging treatment. The artificial aging treatment conditions for the 7000 series aluminum alloy extruded profile are generally in the range of 120 to 180 ° C. × 5 to 20 hours.
Further, in the case of the automobile member described above, a process called baking coating is performed after the vehicle body is assembled (after artificial aging treatment). The heating condition of the baking coating is generally higher than that of the artificial aging treatment, and is generally 150 to 200 ° C. within 1 hour.

The residual tensile stress generated in the 7000 series aluminum alloy extruded profile by plastic working is reduced by heating in artificial aging treatment or baking finish. However, the reduction of residual tensile stress (stress relaxation) by heating tends to proceed in the region where the introduced strain is large, but does not proceed sufficiently in the region where the introduced strain is small. On the other hand, as described above, in the extruded profile after plastic working, a large residual tensile stress may exist in a region where the strain is small. In such a region, the reduction of the residual tensile stress due to heating does not proceed, and as a result, the residual tensile stress exists in the product (aluminum alloy member) without being reduced, which causes the generation of SCC.

The present invention promotes reduction of tensile residual stress by heat treatment in a method for manufacturing a 7000 series aluminum alloy member using a 7000 series aluminum alloy extruded profile as a material, and manufactures a 7000 series aluminum alloy member having a low tensile residual stress. The purpose is.

In the method for manufacturing a 7000 series aluminum alloy member according to the present invention, a 7000 series aluminum alloy billet is hotly extruded, an extruded profile having a hollow cross section is formed, and then cooled, and a part of the extruded profile in the longitudinal direction or a part thereof or It is characterized in that a prestrain of 2 to 5% is applied to the entire region, the region is machined with residual stress, and the extruded profile is heat-treated.
In the above invention, the machining includes bending, changing cross-section, shearing (punching, etc.) and other plastic working, and cutting. Further, the heat treatment includes artificial aging treatment, baking finish and the like.

According to the above manufacturing method, by applying a prestrain of 2 to 5% to an aluminum alloy extruded profile and then performing machining with residual stress, it is possible to promote reduction of tensile residual stress due to subsequent heating. , A 7000 series aluminum alloy member having a low tensile residual stress can be manufactured. By reducing the tensile residual stress, the SCC resistance of the 7000 series aluminum alloy member can be improved.

It is a figure (1A) which shows the distribution of the strain generated in an extruded profile by bending process, and the figure (1B) which shows the distribution of the residual stress generated in an extruded profile after unloading. It is a side view of the extruded profile which has been subjected to the variable cross-section processing (crushing processing), and is the AA cross-sectional view, the BB cross-sectional view, and the CC cross-sectional view of the same side view. It is a flow chart which shows the typical example of the process of this invention. FIG. (4A) shows the dimensions of the creep test piece, and FIG. 4A is an enlarged view of part A (4B9) of FIG. 4A. Example No. It is a figure which shows the influence of the pre-strain on the creep curve (creep strain-elapsed time) of the alloy of 1. Example No. It is a figure which shows the influence of the pre-strain on the creep curve (creep strain-elapsed time) of the alloy of 2. Example No. It is a figure which shows the influence of the pre-strain on the creep curve (creep strain-elapsed time) of the alloy of 3. It is a figure which shows the relationship between a steady creep rate and a prestrain.

The method for manufacturing a 7000 series aluminum alloy member according to the present invention includes a hot extrusion step, a cooling step immediately after extrusion, a step of applying prestrain, a machining step accompanied by generation of residual stress, and a heating step. Hereinafter, the aluminum alloy to which the present invention is applied and each of the above steps will be described with reference to the flow chart of FIG.

(Composition of aluminum alloy)
The aluminum alloy to which the present invention is applied is a 7000 series (Al—Mg—Zn (—Cu) series) aluminum alloy specified by JIS or AA, and the composition is not particularly limited. Preferred compositions include Zn: 3.0 to 8.0% by mass, Mg: 0.4 to 2.5% by mass, Cu: 0.05 to 2.0% by mass, and Ti: 0.005 to 0.2% by mass. %, Further, one or more of Mn: 0.01 to 0.3% by mass, Cr: 0.01 to 0.3% by mass, Zr: 0.01 to 0.3% by mass. The composition which contains and consists of the balance Al and impurities can be mentioned.

(Hot extrusion step S1)
In this step, a heated 7000 series aluminum alloy billet is extruded from an extruded die to form an extruded profile having a hollow cross section. The hollow cross section can take any shape depending on the application, and for example, in the case of an automobile, a substantially rectangular cross section and a cross section having one or more middle ribs inside the contour of the substantially rectangular shape are mentioned as typical examples. Can be done.
(Cooling step S2)
The hot-extruded extruded profile is cooled online immediately after extrusion, and is preferably press-quenched by air cooling or water cooling. If necessary, the extruded profile is once cooled, cut to a predetermined length, and solution-treated offline.

(Pre-strain applying step S3)
A prestrain (plastic strain) of 2 to 5% is applied to the cooled extruded profile in the cold. The extruded profile to which prestrain is applied is preferably in a T1 tempered state or a T4 tempered state. In the present embodiment, the T1 tempered state is a state in which artificial tempering other than natural aging is not performed after press quenching, and the T4 tempered state is a state after solution heat treatment. It means that no artificial tempering other than natural aging has been performed. The region to which the pre-strain is applied is a part or all of the region in the longitudinal direction of the extruded profile, and the region where the machining described later is performed needs to be included in the region.
The reason why the pre-strain amount applied in this step is 2% or more is that, as shown in Examples described later, by setting the pre-strain amount to 2% or more, the tensile residual stress is reduced in the heat treatment step described later. This is because the effect becomes remarkable. On the other hand, the reason why the pre-strain amount is 5% or less is that the effect of reducing the tensile residual stress does not change significantly even if it exceeds 5%, and the cross-sectional area reduction and the wall thickness reduction of the extruded profile become remarkable. Is.

When the extruded profile is press-quenched and in a T1 tempered state, the prestraining step can be performed as part of the tensile straightening normally performed on the long extruded profile. In this case, there is an advantage that uniform pre-strain can be applied to the entire length (excluding the chuck portion) of the long extruded profile. However, the amount of strain applied by normal tensile straightening is 1% or less, but in the present invention, the amount of strain is 2 to 5%. The extruded profile after the prestraining step (after tensile straightening) is cut to a length corresponding to the length of the target product (7000 series aluminum alloy member). Before the pre-straining step, the long extruded profile is cut to a length corresponding to the length of the product (7000 series aluminum alloy member), and the extruded profile after cutting is cut. , This pre-straining step can also be performed.
When the extruded profile is solution-treated and in a T4 tempered state, the extruded profile is usually cut to a length corresponding to the length of the target product (7000 series aluminum alloy member). Also in this case, it is preferable that the prestrain is applied to the entire length (excluding the chuck portion) of the extruded profile by tension (stretching).

(Machining process S4)
After the pre-straining process, the machining process is performed cold. Machining as used in the present invention includes plastic working such as bending (see FIG. 1), variable cross-section machining (see FIG. 2), shearing (punching, etc.), and cutting. This machining creates residual tensile stress in the extruded profile. When the bending process is a tensile bending process, the pre-strain applying step (stretching) can be performed by a chuck holding both ends of the extruded profile, and then the process can be directly shifted to the tensile bending process. In this case, the tension T applied to the extruded profile by the chuck is set to be more than 1 times the product of the proof stress σy and the cross-sectional area S (σy × S) in the prestraining step, and the extruded profile is prestrained by 2 to 5%. In the tensile bending process, the amount is reduced to less than 1 times (for example, about 0.3 to 0.7 times) of (σy × S).

(Heat treatment step S5)
Following the machining process, a heat treatment process is performed. As the heat treatment step, for example, one or both of artificial aging treatment and baking finish is performed. Note that baking coating means that after coating a baking-curable paint, heat is applied to forcibly dry the paint.
By this heat treatment step, the tensile residual stress existing in the extruded profile is reduced, and in particular, by performing the prestraining step, the tensile residual stress existing in the place where a large strain was not introduced in the machining process is effective. Reduced. The heat treatment conditions are appropriately selected within the range of, for example, 120 to 200 ° C. × 20 minutes to 20 hours.
The conditions for the artificial aging treatment may be those of a normal 7000 series aluminum alloy, and may be performed within the range of, for example, 120 to 180 ° C. × 5 to 20 hours. The baking coating conditions may be normal, and may be performed at 150 to 200 ° C. for 20 minutes or more and 1 hour or less. When baking coating is performed as a heat treatment step, the artificial aging treatment can be performed before the machining step (after the prestraining step), and in this case, the artificial aging treatment is the heat treatment step referred to in the present invention. Not equivalent.

The heat treatment is preferably performed under the condition that the 0.2% proof stress of the extruded profile (member made of 7000 series aluminum alloy) is 400 MPa or more. The higher the strength of the 7000 series aluminum alloy, the more easily SCC is generated in a corrosive environment, and when the 7000 series aluminum alloy has a high 0.2% proof stress of 400 MPa or more, the SCC suppressing effect of the present invention is remarkably exhibited. Is.

In this example, the effect of prestrain on the creep characteristics of the 7000 series aluminum alloy was investigated. Creep is a strain change under a constant stress, and stress relaxation (reduction of residual stress) is a stress change under a constant strain, but the phenomenon occurring in the material is the same.
Three types of 7000 series aluminum alloy billets were hotly extruded to form a flat plate-shaped extruded material (length 500 mm × width 110 mm × thickness 3 mm), which was air-cooled online immediately after extrusion. The composition of each extruded material (No. 1 to 3) is shown in Table 1.

No. JIS No. 13B test pieces were collected from 1 to 3 extruded materials (all of which were T1 tempered) and subjected to a tensile test in accordance with the regulations of JIS Z2241. The results are shown in Table 2.

In addition, the above-mentioned No. A plate creep test piece having the shape shown in FIG. 4 was collected from the extruded materials 1 to 3 (all of which were T1 tempered) so that the extrusion orthogonal direction (LT direction) was the longitudinal direction.
The creep test piece was cold-stretched to give 1%, 2%, 5%, and 10% prestrain (plastic strain). A creep test was performed using a creep test piece to which a pre-strain was applied and a creep test piece having a pre-strain of 0% (no stretch). The test temperature was equivalent to the aging temperature of each alloy (No. 1: 130 ° C., No. 2: 140 ° C., No. 3: 140 ° C.), and the load stress was fixed at 220 MPa.
The results of the creep test (relationship between creep strain and elapsed time) are shown in FIGS. 5 to 7. FIG. 5 shows No. The test result of No. 1 and FIG. 6 show No. As a result of the test of No. 2, FIG. 7 shows No. It is a test result of 3. The values shown on the right side of FIGS. 5 to 7 are the magnitudes of pre-strain.
Table 3 shows the steady creep rate (strain rate in the steady creep region) for each pre-strain obtained from each diagram of FIGS. 5 to 7. Further, based on the data in Table 3, FIG. 8 shows the relationship between the steady creep rate and the prestrain, which is arranged with the steady creep speed of 0% prestrain as 1.

As shown in FIG. 8, by applying the prestrain, the steady creep rate was increased in all the alloys (Nos. 1 to 3). In particular, when a pre-strain of 2% or more is applied, the steady creep rate increases significantly as compared with the case where no pre-strain is applied. That is, by applying prestrain, stress relaxation of the extruded material due to heating is promoted.
From this result, stress relaxation is promoted in the heat treatment process such as artificial aging and baking coating by giving prestrain to the extruded profile by intentionally increasing the stretch amount, and the 7000 series aluminum alloy member. It can be seen that the tensile residual stress (which is a factor that causes SCC) can be significantly reduced.

Claims (9)

A 7000 series aluminum alloy billet is hotly extruded to form an extruded profile having a hollow cross section and then press-baked to apply 2-5% prestrain to a part or all of the longitudinal direction of the extruded profile. After applying, the extruded profile is cut to a predetermined length, and the extruded profile is machined with residual stress in the region, and this machining is bending, variable cross-sectioning, and shearing. A method for manufacturing a member made of a 7000 series aluminum alloy , which means machining and cutting, and is characterized in that the extruded profile after machining is heat-treated. A 7000 series aluminum alloy billet is hotly extruded to form an extruded profile having a hollow cross section, which is then press-baked, the extruded profile is cut to a predetermined length, and the extruded profile after cutting is formed in the longitudinal direction. After applying 2-5% prestrain to a part or all of the area, the area is machined with residual stress, which means bending, variable cross section, shearing and cutting. A method for manufacturing a 7000 series aluminum alloy member, which comprises heat-treating the extruded profile after machining . The 7000 series aluminum alloy billet is hotly extruded to form an extruded profile having a hollow cross section and then cooled, the extruded profile is cut to a predetermined length, and the extruded profile after cutting is solution-treated. After applying 2-5% prestrain to a part or all of the longitudinal region, the region is machined with residual stress, which is bending, profile change, shearing and cutting. A method for manufacturing a member made of a 7000 series aluminum alloy , which means that the extruded profile after machining is heat-treated. The method for manufacturing a 7000 series aluminum alloy member according to any one of claims 1 to 3, wherein the extruded profile is subjected to tensile straightening to impart prestrain to the extruded profile. 2 . A method for manufacturing a member made of a 7000 series aluminum alloy. The method for manufacturing a 7000 series aluminum alloy member according to any one of claims 1 to 5, wherein the extruded profile after the heat treatment has a 0.2% proof stress of 400 MPa or more. The method for manufacturing a 7000 series aluminum alloy member according to any one of claims 1 to 6, wherein an artificial aging treatment is performed as the heat treatment. The method for manufacturing a 7000 series aluminum alloy member according to any one of claims 1 to 6, wherein the heat treatment includes an artificial aging treatment and a baking finish. The method for manufacturing a 7000 series aluminum alloy member according to any one of claims 1 to 6 , wherein an artificial aging treatment is performed before the machining, and a baking finish is performed as the heat treatment.

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