JP6795460B2 - Manufacturing method of 7000 series aluminum alloy member with excellent stress corrosion cracking resistance - Google Patents

Description

The present invention relates to a method for manufacturing a 7000 series aluminum alloy member in which at least a part of a high-strength 7000 series aluminum alloy extruded profile is crushed along the longitudinal direction to form a member, particularly stress corrosion cracking resistance. The present invention relates to a method for manufacturing an excellent 7000 series aluminum alloy member.

In Patent Documents 1 to 3, the end region of an aluminum alloy extruded profile composed of a pair of flanges arranged to face each other and a plurality of webs connected to them is crushed in a direction perpendicular to the flange surface, and a door beam is provided. It is described that it manufactures reinforcing members for automobiles such as bumper alloys. From the viewpoint of processing accuracy and cost, it is desirable to perform such crushing after aging, and Patent Document 2 states that press-quenched 6000 series aluminum alloy extruded profiles are crushed after aging. Is described.

On the other hand, the 7000 series aluminum alloy extruded profile, which has a large amount of alloying elements such as Zn, Mg, and Cu and has higher strength than other alloys when aging, has poor moldability after aging. When the crushing process is performed after the aging process, cracks occur in the web that is bent and deformed even if the crushing process rate (decrease rate of the cross-sectional height) is small. This tendency is more remarkable on the high alloy side. Therefore, for example, Patent Document 4 describes that it is desirable to perform a crushing process in a state of T1 tempering after extrusion, and then perform an aging process.
However, the 7000 series aluminum alloy extruded profile is hardened by natural aging even if it is a material (T1 tempered) after press quenching and before aging treatment, and the moldability is lowered. In order to improve the moldability, for example, as described in Patent Documents 5 to 7, a restoration treatment for reducing the strength of a 7000 series aluminum alloy cured by natural aging has been conventionally performed.

Japanese Patent No. 3465862 Japanese Patent No. 4111651 Japanese Unexamined Patent Publication No. 7-25296 Japanese Unexamined Patent Publication No. 2003-118367 Japanese Unexamined Patent Publication No. 7-305151 Japanese Unexamined Patent Publication No. 10-168553 Japanese Unexamined Patent Publication No. 2007-1198553

Certainly, when this restoration treatment is applied to a T1 tempered 7000 series aluminum alloy extruded shape material, the strength of the shape material is reduced and the moldability is improved. However, when a practical material with a web plate thickness of 1.5 to 4 mm is used and crushed, cracks occur on the bent outer side of the web depending on the crushing rate, which is the conventional method. It cannot be resolved by the restoration process. At the same time, there is also a problem that a high tensile residual stress is applied to the web after the crushing process, and the stress corrosion cracking resistance is lowered.

Since the present invention has been made in view of such a problem, at least a part of the region along the longitudinal direction of the 7000 series aluminum alloy extruded profile is crushed in the direction perpendicular to the extrusion direction to form a member. It is an object of the present invention to prevent the occurrence of cracks due to crushing in the 7000 series aluminum alloy member, and at the same time, to reduce the tensile residual stress and improve the stress corrosion cracking resistance.

The method for producing a 7000 series aluminum alloy member having excellent stress-resistant corrosion cracking resistance according to the present invention is Zn: 3.0 to 8.0% by mass, Mg: 0.5 to 2.5% by mass, Cu: 0. It contains 05 to 2.0% by mass, Ti: 0.005 to 0.2% by mass, and further, Mn: 0.01 to 0.3% by mass, Cr: 0.01 to 0.3% by mass, Zr. : 7000 series aluminum alloy extruded type containing 0.01 to 0.3% by mass of 1 type or 2 or more types, having a composition of the balance Al and unavoidable impurities, composed of a plurality of plates and press-hardened. At least a part of the region along the longitudinal direction of the material is crushed in the direction perpendicular to the extrusion direction to form a member, and at least the region (the region to be crushed) of the 7000 series aluminum alloy extruded profile is formed. ), Heated at a heating rate of 0.4 ° C./sec or higher, held in the temperature range of 200 to 550 ° C. for more than 0 seconds and 60 seconds or less, and then cooled at a cooling rate of 0.5 ° C./sec or higher. The restoration process is performed, and within 72 hours after the restoration process, the crushing process is performed under the condition that 1.5 mm ≦ t ≦ 4.0 mm, 3 t / 2 ≦ R ≦ 10 t, and after the crushed process, the entire member is aged. It is characterized by applying.
The 7000 series aluminum alloy extruded profile typically consists of a pair of opposed flanges and one or more webs connecting them. In that case, the web is usually the plate that undergoes the largest bending deformation due to the crushing process.

According to the present invention, when at least a part of a press-quenched 7000 series aluminum alloy extruded profile is crushed along the longitudinal direction to form a member, cracks are generated with high strength. It is possible to provide a 7000 series aluminum alloy member having reduced tensile residual stress and improved stress corrosion cracking resistance.

It is a graph which shows the relationship between Y (= σrs / σ _0.2 ) and X (= [Mg] + [Zn]) in the 7000 series aluminum alloy extruded profile. It is sectional drawing (2A) of the 7000 series aluminum alloy extruded profile produced in an Example, and the side view (2B) explaining the test method of a crushing process.

Hereinafter, the 7000 series aluminum alloy member and the manufacturing method thereof according to the present invention will be specifically described.
(Composition of aluminum alloy)
First, the composition of the 7000 series aluminum alloy according to the present invention will be described. However, this composition itself is known as a 7000 series aluminum alloy.
Zn: 3.0 to 8.0% by mass
Mg: 0.4 to 2.5% by mass
Zn and Mg are elements that form MgZn2, which is an intermetallic compound, to improve the strength of the 7000 series aluminum alloy. If the Zn content is less than 3.0% by mass or the Mg content is less than 0.4% by mass, the proof stress of 200 MPa or more required for a practical material cannot be obtained. On the other hand, when the Zn content exceeds 8.0% by mass or the Mg content exceeds 2.5% by mass, cracks due to the crushing process are cracked even if the extruded profile is subjected to a predetermined restoration treatment before the crushing process. The occurrence cannot be prevented, and at the same time, the tensile residual stress applied by the crushing process cannot be reduced, and the stress corrosion cracking resistance is significantly reduced. From the viewpoint of high strength and light weight, the Zn content and Mg content are higher alloy side, for example, 5.0 to 8.0% by mass and 1.0 to 2.5% by mass, respectively, for a total of 6. 0 to 10.5% by mass is desirable.

Cu: 0.05 to 2.0% by mass
Cu is an element that improves the strength of 7000 series aluminum alloys. If the Cu content is less than 0.05% by mass, there is no sufficient effect of improving the strength, while if it exceeds 2.0% by mass, the extrusion processability is lowered. The Cu content is preferably 0.5 to 1.5% by mass.
Ti: 0.005 to 0.2% by mass
Ti has the effect of refining the crystal grains during casting of the 7000 series aluminum alloy to improve the formability (crushing workability) of the extruded profile, and is added in an amount of 0.005% by mass or more. On the other hand, if it exceeds 0.2% by mass, the action is saturated and a coarse intermetallic compound crystallizes, which rather lowers the moldability.

Mn: 0.01 to 0.3% by mass
Cr: 0.01 to 0.3% by mass
Zr: 0.01 to 0.3% by mass
Mn, Cr, and Zr have the effect of suppressing recrystallization of the 7000 series aluminum alloy extruded profile, making the crystal structure fine recrystallization or fibrous structure, and improving stress corrosion cracking resistance. Two or more are added within the above range.
Inevitable Impurities Fe and Si are mentioned as the main unavoidable impurities of the 7000 series aluminum alloy. In order not to deteriorate various properties of the 7000 series aluminum alloy, Fe: 0.35% by mass or less and Si: 0.3% by mass or less are limited.

(Manufacturing method of aluminum alloy member)
The 7000 series aluminum alloy member according to the present invention has the above composition, and after producing a 7000 series aluminum alloy extruded profile composed of a plurality of plates by press quenching (usually, storage for several tens of days to several months). (There is a period), all or part of the region along the longitudinal direction of the same shape material is heated at a heating rate of 0.4 ° C./sec or more, and exceeds 0 seconds in the temperature range of 200 to 550 ° C. 60. The plurality of plates are held for a second or less, and then cooled at a cooling rate of 0.5 ° C./sec or more, and within 72 hours after the restoration treatment, the region is crushed in a direction perpendicular to the extrusion direction. When the thickness of the plate that has undergone the largest bending deformation is t and the minimum value of the bending inner radius is R, then 1.5 mm ≤ t ≤ 4.0 mm, 3 t / 2 ≤ R ≤ 10 t. It can be manufactured by applying it and further applying an aging treatment to the entire member.

The extruded profile, which is the material, typically consists of a pair of flanges that are opposed to each other and one or more webs that connect them, such as cross-section mouth-shaped, sun-shaped, eye-shaped, and more. Includes flanges that protrude to the left and right of the web. Flange or web is plate-shaped, including slightly curved ones. When this extruded profile is crushed in a direction in which a pair of flanges approach each other, the web becomes the plate that has undergone the largest bending deformation (with a large curvature). Hereinafter, the plate that undergoes the largest bending deformation due to the crushing process will be referred to as a web.
In the present invention, the thickness t of the web of the extruded profile is specified to be relatively thick as 1.5 mm ≦ t ≦ 4.0 mm, because the 7000 series aluminum alloy member according to the present invention is mainly used for door beams and bumpers. This is because it is assumed to be a reinforcing member for automobiles such as reinforce.

In the extruded profile produced by press quenching, the intermetallic compound is precipitated and hardened by natural aging, but the intermetallic compound is re-solidified and extruded by undergoing the restoration treatment before the crushing process. The shape material is softened and the moldability (crushing workability) is improved. As a result, when the extruded profile is crushed, it is possible to prevent cracks from being generated on the bent outer side of the bent and deformed web, and at the same time, it is possible to reduce the tensile residual stress generated on the web.

In the restoration process, if the rate of temperature rise is less than 0.4 ° C./sec, precipitation of the work between metals is promoted in the process of temperature rise, and the effect of the restoration process cannot be obtained. When the holding temperature (substantive temperature) is less than 200 ° C., the intermetallic compound precipitated by natural aging does not re-solidify, but rather the precipitation is promoted and coarsened, while when the holding temperature exceeds 550 ° C., the extruded shape material However, the required strength cannot be obtained after the aging treatment. The retention time should exceed at least 0 seconds. In short, after the extruded profile reaches the holding temperature, it may be held at the same temperature for a predetermined time and then cooled, or it may be cooled immediately. The upper limit of the holding time is not particularly limited, but for example, a short time of 60 seconds or less is desirable from the viewpoint of production efficiency, and a shorter time of 10 seconds or less and 5 seconds or less is preferable. As the heating means, for example, a high frequency induction heating device or a saltpeter furnace can be used.
Further, if the cooling rate from the holding temperature is less than 0.5 ° C./sec, the intermetallic compound is precipitated again in the cooling process, and the effect of this restoration treatment is weakened or lost. In the conventional restoration process, the cooling rate in the cooling process was not particularly considered.

After the restoration treatment, the extruded shape is crushed before being re-hardened by natural aging. Specifically, it is desirable to perform the crushing process within 72 hours after the restoration process. When the minimum value of the bending inner radius of the web after crushing is R, if the crushing ratio satisfies 1.5t ≦ R, it is possible to prevent cracks from occurring on the bending outer side of the bent and deformed web, and at the same time. It is possible to prevent an increase in tensile residual stress generated on the web. However, when R <1.5t, even if the restoration treatment is performed before the extruded profile is crushed, it is not possible to prevent cracks from occurring on the bent outer side of the web. At the same time, it is not possible to prevent the tensile residual stress generated in the web from increasing, and the stress corrosion cracking resistance of the member is reduced. On the other hand, when R> 10t, cracks do not occur even if the restoration treatment is not performed before the extruded shape is crushed (even if the extruded shape is in the T1 state).

The aging treatment after the crushing process may be performed under well-known conditions that are carried out with ordinary 7000 series aluminum alloys. By this aging treatment, the strength (0.2% proof stress value) of 200 MPa or more is secured in the 7000 series aluminum alloy member which is a product.
Although the 7000 series aluminum alloy member manufactured by the above manufacturing method is a high-strength material, there is no cracking in the web in the crushed region, and the tensile residual stress σsr of the web and 0. The ratio Y (σsr / σ _0.2 ) of the 2% proof stress value σ _0.2 is the total X (= [Mg] + [Zn] of the Mg content [Mg] and the Zn content [Zn] of the 7000 series aluminum alloy. ]) Satisfies the following formula (1) and exhibits excellent stress corrosion cracking resistance.
Y ≦ −0.1X + 1.4 ・ ・ ・ (1)

The graph shown in FIG. 1 shows the ratio Y (σrs / σ _0. ) of the total content X (= [Zn] + [Mg]) of Zn and Mg, the tensile residual stress σrs, and the 0.2% proof stress σ _{0.2 .} The data of the examples described later are plotted (Δ, □) on the XY coordinates consisting of ₂ ), and the line in the figure is a straight line represented by Y = −0.1X + 1.4. In FIG. 1, Δ is No. 1 of the embodiment. Corresponding to 1 to 6, all of them fall into the region of Y ≦ −0.1X + 1.4, and as shown in Table 2, they are excellent in stress corrosion cracking resistance. On the other hand, □ is No. Corresponding to 7 to 14, all of them are in the region of Y> -0.1X + 1.4, and as shown in Table 2, the stress corrosion cracking resistance is inferior. Further, as shown in Table 2, No. 1 in the region of Y ≦ −0.1X + 1.4. Nos. 1 to 6 have no cracks in the web and fall into the region of Y> -0.1X + 1.4. In each of 7 to 14, the web is cracked.
In FIG. 1, the 0.2% proof stress (σ _0.2 ), which is the denominator of Y, is restored after the extruded material produced by press quenching is naturally aged, as shown in Examples described later. It has a 0.2% proof stress of the portion that has been aged without being treated or expanded.

The 7000 series aluminum alloy shown in Table 1 is hot-extruded, immediately after extrusion, fan air-cooled (press-quenched) online, and as shown in FIG. 2A, a pair of flanges (inner flange 1, outer flange) arranged opposite to each other. 2) and an extruded shape member having a substantially open cross section (with a protruding flange) composed of two webs 3 and 4 for vertically connecting them were manufactured. This extruded profile assumes a door beam, height 30.0 mm, outer flange 1 plate thickness 4.0 mm, width 40.0 mm, inner flange 2 plate thickness 4.0 mm, width 50.0 mm, both webs. The plate thickness of 3 and 4 is 2.0 mm or 4.0 mm, the outer flange 1 protrudes 5 mm to the left and right from both webs 3 and 4, and the inner flange 2 protrudes 10 mm to the left and right from both webs 3 and 4.

No. after press quenching The extruded profiles 1 to 14 were cut to a predetermined length to obtain No. Two test materials (extruded shapes) were collected for each of 1 to 14, left at room temperature for 20 days to allow natural aging, and then using a high-frequency induction heating device, various heating rates shown in Table 1 were used. , The restoration process was performed at the reached temperature (body temperature), the holding time, and the cooling rate (not only No. 11). The restoration treatment was applied only to a part of the test material along the longitudinal direction (end region).

After the time shown in Table 1 has elapsed after the restoration process, as shown in FIG. 2B, the test material 5 is placed on the horizontal table 6 and pressed vertically with the crushing jig 7 arranged above the horizontal table 6. At the same time, an inward load was applied to the webs 3 and 4 with a horizontal processing jig (see the horizontal load jig 9 described in Patent Document 3) (see Patent Document 3), and the end region (No.) of the test material 5 was restored. The region (area not restored only in .11) was crushed in the vertical direction on the inclined surface 7a of the crushing jig 7. The webs 3 and 4 of the test material 5 were bent and deformed by the crushing process, and overhanged inside the hollow portion. In this crushing process, the stroke of the crushing jig 7 is made constant, and the position of the test material 5 in the longitudinal direction (horizontal direction in FIG. 2B) is adjusted on the horizontal table 6 to obtain No. The crushing rate of the test materials 5 (2 each) of 1 to 14 was adjusted, that is, the bending radii of the webs 3 and 4 were adjusted.
After crushing, No. The entire test materials (2 each) from 1 to 14 were aged at 130 ° C. for 8 hours.
After the aging process, No. Using one of the test materials 1 to 14, a tensile test, inspection of the presence or absence of cracks on the outside of the bending of the web, measurement of the inner radius of bending of the web (minimum value R), and measurement of the tensile residual stress of the web are performed as follows. Was done. In addition, No. A stress corrosion cracking resistance test was performed using another test material from 1 to 14. The results are shown in Table 2.

(Tensile test)
A JIS No. 5 test piece was taken from the region of the test material 5 that had not been restored, and a tensile test was performed according to the metal material test method specified in JIS Z2241, and a 0.2% proof stress (σ _0.2 ) was measured.
(Presence or absence of cracks)
The webs 3 and 4 in the crushed region of the test material 5 were visually observed, and the presence or absence of cracks on the bent outer side of the webs 3 and 4 was inspected. The cracks were mainly generated near the crushed end face of the test material 5.
(Minimum value R of bending inner radius)
Since the bending inner radius of the webs 3 and 4 is the smallest on the crushed end face of the test material 5, the bending inner radius of the webs 3 and 4 was measured on the same end face.

(Tensile residual stress of web)
The method for measuring the residual stress was the following procedure by the cutting method. The crushing start position A, the end position B, and the intermediate position C shown in FIG. 2 are selected as the measurement target positions (all are at the center of the height), and the surface of these measurement target positions is sanded and then washed with acetone. Then, attach the strain gauge to this polished part with instant adhesive, leave it at room temperature for 24 hours, connect the lead wire of the strain gauge to the strain gauge, set the zero point, and use a metal saw around the strain gauge to set a 10 mm square. The stress was released by cutting, the strain amount ε after cutting was measured, and the residual stress value σrs was calculated by the following formula. σrs = −E × ε (E; Young's modulus), where E = 68894N / mm ² .
In addition, No. In all of the test materials 1 to 14, the tensile residual stress value measured at the crushing start position A was the maximum value. It is presumed that this is because the material constraint is the largest at the crushing start position A, while the material constraint is relatively small at the end position B and the intermediate position C, and the strain due to the crushing process is released. Therefore, the residual stress value σrs shown in Table 2 is a value measured at the crushing start position A.

(Stress corrosion cracking resistance)
A stress corrosion cracking resistance test was conducted by the chromic acid acceleration method. Using the crushed test material, the test material was immersed in a test solution at 90 ° C. for up to 16 hours, and stress corrosion cracking was visually observed. The test solution was prepared by adding 36 g of participating chromium, 30 g of potassium dichromate and 3 g of salt (per liter) to distilled water. In the test, the test material is taken out from the solution every hour, the presence or absence of cracking is confirmed, and those with no cracking or the time until cracking occurs are evaluated as having excellent stress corrosion cracking resistance (○). However, those in which the time until cracking occurred was less than 12 hours were evaluated as inferior (x). All stress corrosion cracking occurred near the crushing start position A (see FIG. 2B).

From the residual stress value (σrs) and the 0.2% proof stress value (σ _0.2 ), the ratio Y (= σrs / σ _0.2 ) of the two was calculated. Further, from the Zn content [Zn] and the Mg content [Mg], the total content X (= [Zn] + [Mg]) of Zn and Mg, and the right side (-0.1X + 1) of the above formula (1). The value of 4) was calculated. Based on the above calculation results, the case where the values of X and Y satisfy the above relational expression (1) is determined as ◯, and the case where the values do not satisfy is determined as x. Table 2 shows the above calculation results and judgment results.
From Tables 1 and 2, No. 1 having the alloy composition specified in the present invention and undergoing restoration treatment and crushing under the conditions specified in the present invention. The test materials 1 to 6 have no cracks in the web after crushing, have a proof stress value of 200 MPa or more after aging treatment, and Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]. ]) Satisfies the above formula (1), and all have excellent stress corrosion cracking resistance.

On the other hand, No. In the test material of No. 7, the contents of Zn and Mg were excessive, the web was cracked by crushing, and Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) were described above. It does not satisfy the formula (1) and is inferior in stress corrosion cracking resistance.
No. In the test material No. 8, the effect of the restoration process was lost due to the slow cooling rate of the restoration process, the web was cracked by the crushing process, and Y (= σrs / σ _0.2 ) and X (= [Zn] +). [Mg]) does not satisfy the above formula (1), and the stress corrosion cracking resistance is inferior.
No. The test material of No. 9 had no effect of the restoration treatment because the temperature reached by the restoration treatment was low, the yield strength was not improved by the aging treatment, and although the Zn and Mg were relatively low, the web was cracked by the crushing process. I couldn't prevent it from entering. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the above formula (1), and the stress corrosion cracking resistance is also inferior.

No. Since the R / t of the test material 10 was too small (the crushing rate was too high), the conditions for the restoration process were appropriate, but despite the relatively low Zn and Mg, cracks due to the crushing process. Could not be prevented. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the above formula (1), and the stress corrosion cracking resistance is also inferior.
No. Since the test material of No. 11 was not subjected to the restoration treatment, cracks were generated in the web due to the crushing process. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the above formula (1), and the stress corrosion cracking resistance is also inferior.
No. Since the test material of No. 12 has a long time from the restoration process to the crushing process, the effect of the restoration process is lost, the web is cracked by the crushing process, and Y (= σrs / σ _0.2 ) and X. (= [Zn] + [Mg]) does not satisfy the above formula (1), and the stress corrosion cracking resistance is inferior.

No. In the test material of No. 13, the effect of the restoration treatment could not be obtained because the temperature rise rate of the restoration treatment was small, the web was cracked by the crushing process, and Y (= σrs / σ _0.2 ) and X (= [Zn). ] + [Mg]) does not satisfy the above formula (1), and the stress corrosion cracking resistance is inferior.
No. In the test material No. 14, the effect of the restoration process was lost due to the low cooling rate of the restoration process, the web was cracked by the crushing process, and Y (= σrs / σ _0.2 ) and X (= [Zn] +). [Mg]) does not satisfy the above formula (1), and the stress corrosion cracking resistance is inferior.

1, 2, Flange 3, 4 Web 5 Test material (extruded profile)
7 Jig for crushing

Claims (1)

It contains 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, Mn: 0.01 to 0.3% by mass, Cr: 0.01 to 0.3% by mass, Zr: 0.01 to 0.3% by mass containing one or more kinds, and the balance. A 7000 series aluminum alloy having a composition consisting of Al and unavoidable impurities, composed of a pair of plate-shaped flanges arranged opposite to each other and one or more plate-shaped webs connecting them, and manufactured by press extrusion. In a method for manufacturing a 7000 series aluminum alloy member, in which the end region of an extruded profile is crushed in a direction perpendicular to the extrusion direction and in a direction in which the pair of flanges approach each other to bend and deform the web to form a member. Prior to the crushing process, at least the region of the extruded profile is heated at a heating rate of 0.4 ° C./sec or more and held in a temperature range of 200 to 550 ° C. for more than 0 seconds and 60 seconds or less. Then, a restoration process of cooling at a cooling rate of 0.5 ° C./sec or more is performed, and within 72 hours after the restoration process, the bending inner radius of the web becomes the smallest at the end face of the extruded profile, and the web When the plate thickness is t and the bending inner radius of the web on the end face is R, the crushing process is performed under the condition that 1.5 mm ≤ t ≤ 4.0 mm, 3 t / 2 ≤ R ≤ 10 t. A method for manufacturing a 7000 series aluminum alloy member having excellent stress-resistant corrosion cracking resistance, which is characterized by subjecting the entire member to an aging treatment.

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