JP2017214656A - Manufacturing method of 7000 series aluminum alloy member excellent in stress corrosion crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy member having high strength and excellent in stress corrosion crack resistance without generation of cracking by conducting an impregnation process on a 7000 series aluminum alloy extrusion material.SOLUTION: A 7000 series aluminum alloy extrusion material containing Zn:3.0 to 8.0 mass%, Mg:0.4 to 2.5 mass%, Cu:0.05 to 2.0 mass% and Ti:0.005 to 0.2 mass% and manufactured by press hardening is heated at temperature rise rate of 0.4°C/sec. held at a temperature range of 200 to 550°C for over 0 sec. to 60 sec. or less, then applied to a restoration treatment for cooling at cooling rate of 0.5°C/sec. or more and an impregnation treatment in 72 hours after the restoration treatment, and then an aging treatment is conducted on whole.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a 7000 series aluminum alloy member obtained by crushing at least a partial region along the longitudinal direction of a high-strength 7000 series aluminum alloy extruded shape, particularly stress corrosion cracking resistance. The present invention relates to a method for producing an excellent 7000 series aluminum alloy member.

  In Patent Documents 1 to 3, an end region of an aluminum alloy extruded shape member composed of a pair of opposed flanges and a plurality of webs connected to the flanges is crushed in a direction perpendicular to the flange surface, and a door beam And manufacturing a reinforcing member for automobiles such as bumper reinforcement. Such crushing processing is desirably performed after aging treatment from the viewpoint of processing accuracy and cost, and Patent Document 2 discloses that crushing processing is performed after aging treatment on a press-quenched 6000 series aluminum alloy extruded shape. Is described.

On the other hand, in the 7000 series aluminum alloy extruded shape that has a large amount of alloy elements such as Zn, Mg, Cu, etc., and is strengthened compared to other alloy systems when subjected to aging treatment, the formability after aging treatment is poor, When crushing is performed after the aging treatment, even if the crushing rate (reduction rate of the cross-sectional height) is small, a crack is generated in the web that is bent and deformed. This tendency is more remarkable on the high alloy side. For this reason, for example, Patent Document 4 describes that it is desirable to perform crushing in a T1 tempered state after extrusion, and then perform an aging treatment.
However, the 7000 series aluminum alloy extruded shape is cured by natural aging even after press quenching and before aging treatment (T1 tempering), and the formability is lowered. In order to improve the formability, for example, as described in Patent Documents 5 to 7, a restoration treatment for reducing the strength of a 7000 series aluminum alloy hardened by natural aging has been conventionally performed.

Japanese Patent No. 3465862 Japanese Patent No. 4111651 Japanese Patent Laid-Open No. 7-25296 JP 2003-118367 A Japanese Patent Laid-Open No. 7-305151 JP-A-10-168553 JP 2007-119853 A

  Certainly, when this restoration process is applied to a T1 tempered 7000 series aluminum alloy extruded shape, the shape of the same shape is reduced and the formability is improved. However, when a practical material having a web thickness of 1.5 to 4 mm is used and subjected to a crushing process, a crack is generated on the outside of the web depending on the size of the crushing rate. It cannot be resolved by the restoration process. At the same time, there is a problem in that a high tensile residual stress is imparted to the web after crushing and the stress corrosion cracking resistance is lowered.

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

The manufacturing method of the 7000 series aluminum alloy member excellent in stress corrosion cracking resistance according to the present invention is Zn: 3.0 to 8.0 mass%, Mg: 0.5 to 2.5 mass%, Cu: 0.00. 0.5 to 2.0 mass%, Ti: 0.005 to 0.2 mass%, Mn: 0.01 to 0.3 mass%, Cr: 0.01 to 0.3 mass%, Zr : 7000 series aluminum alloy extruded form containing one or more of 0.01 to 0.3% by mass, having a composition consisting of the balance Al and inevitable impurities, and composed of a plurality of plates and press-hardened At least part of the region along the longitudinal direction of the material is subjected to crushing in a direction perpendicular to the extrusion direction to form a member. At least the region of the 7000 series aluminum alloy extruded shape (region to be crushed) ) With a heating rate of 0.4 ° C / second or more In the temperature range of 200 to 550 ° C., a restoration process is performed in which the temperature is maintained for more than 0 seconds and not more than 60 seconds and then cooled at a cooling rate of 0.5 ° C./second or more. The crushing process is performed under the conditions of t ≦ 4.0 mm and 3t / 2 ≦ R ≦ 10t, and after the crushing process, the entire member is subjected to an aging treatment.
The 7000 series aluminum alloy extruded profile typically comprises a pair of opposed flanges and one or more webs connecting them. In that case, the web is usually a plate that undergoes the largest bending deformation by crushing.

  According to the present invention, when forming a member by crushing at least a partial region along the longitudinal direction of the press-quenched 7000 series aluminum alloy extruded shape, high strength and generation of cracks are caused. Therefore, it is possible to provide a 7000 series aluminum alloy member in which the tensile residual stress is reduced and the stress corrosion cracking resistance is improved.

It is a graph which shows the relationship between Y (= (sigma) rs / (sigma) _0.2 ) and X (= [Mg] + [Zn]) in a 7000 series aluminum alloy extrusion shape material. It is sectional drawing (2A) of 7000 series aluminum alloy extrusion shape material produced in the Example, and a 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.
(Aluminum alloy composition)
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: 3.0-8.0 mass%
Mg: 0.4 to 2.5% by mass
Zn and Mg are elements that improve the strength of the 7000 series aluminum alloy by forming MgZn2 which is an intermetallic compound. When the Zn content is less than 3.0 mass% or the Mg content is less than 0.4 mass%, the proof stress of 200 MPa or more necessary as a practical material cannot be obtained. On the other hand, if the Zn content exceeds 8.0% by mass or the Mg content exceeds 2.5% by mass, cracks due to the crushing process may occur even if the extruded shape is subjected to a predetermined restoration process before crushing. Generation | occurrence | production cannot be prevented, but the tensile residual stress provided by crushing cannot be reduced simultaneously, and stress corrosion cracking resistance falls remarkably. From the viewpoint of increasing the strength and reducing the weight, the Zn content and the Mg content are higher on the alloy side, for example, 5.0 to 8.0 mass% and 1.0 to 2.5 mass%, respectively, for a total of 6. 0-10.5 mass% is desirable.

Cu: 0.05-2.0 mass%
Cu is an element that improves the strength of the 7000 series aluminum alloy. When the Cu content is less than 0.05% by mass, there is no sufficient strength improvement effect, while when it exceeds 2.0% by mass, the extrusion processability is lowered. The Cu content is desirably 0.5 to 1.5 mass%.
Ti: 0.005 to 0.2% by mass
Ti has the effect of refining crystal grains during casting of a 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, when the content exceeds 0.2% by mass, the action is saturated and a coarse intermetallic compound is crystallized, which deteriorates the formability.

Mn: 0.01 to 0.3% by mass
Cr: 0.01-0.3 mass%
Zr: 0.01 to 0.3% by mass
Mn, Cr, Zr has the effect of suppressing the recrystallization of the 7000 series aluminum alloy extruded shape and making the crystal structure a fine recrystallized or fibrous structure and improving the stress corrosion cracking resistance. Two or more kinds are added within the above range.
Inevitable impurities Fe and Si are the main inevitable impurities of the 7000 series aluminum alloy. In order not to deteriorate various properties of the 7000 series aluminum alloy, the content is limited to Fe: 0.35 mass% or less and Si: 0.3 mass% or less.

(Method for producing aluminum alloy member)
The 7000 series aluminum alloy member according to the present invention has the above composition and is manufactured by press-quenching a 7000 series aluminum alloy extruded section composed of a plurality of plates (usually storage for several tens of days to several months). The whole or part of the region along the longitudinal direction of the isomorphic material is heated at a temperature rising rate of 0.4 ° C./second or more, and the temperature range of 200 to 550 ° C. exceeds 0 second and exceeds 60 seconds. A restoration process is performed in which the sheet is held for 2 seconds or less and then cooled at a cooling rate of 0.5 ° C./second or more. Within 72 hours after the restoration process, the areas are crushed in a direction perpendicular to the extrusion direction. Where t is the thickness of the plate that has undergone the largest bending deformation, and R is the minimum value of the inner radius of bending, with the conditions of 1.5 mm ≦ t ≦ 4.0 mm and 3t / 2 ≦ R ≦ 10 t Made by applying aging treatment to the entire component It can be.

The extruded shape material is typically composed of a pair of opposed flanges and one or more webs connecting them, including, for example, a cross-sectional mouth shape, a date shape, an eye shape, and the like. This includes flanges protruding to the left and right of the web. The flange or web is plate-like, but this includes one that is somewhat curved. When this extruded profile is crushed in a direction in which the pair of flanges approach each other, the web is the plate that is subjected to the largest bending deformation (with a large curvature). Hereinafter, a plate that undergoes the largest bending deformation by crushing is referred to as a web.
In the present invention, the sheet thickness t of the extruded profile web is defined to be relatively thick as 1.5 mm ≦ t ≦ 4.0 mm because the application of the 7000 series aluminum alloy member according to the present invention mainly includes door beams and bumpers. This is because an automobile reinforcing member such as reinforcement is assumed.

  Extruded shapes produced by press quenching have intermetallic compounds precipitated and hardened by natural aging, but the intermetallic compounds are re-dissolved by undergoing the restoration process before crushing, and extruded. The shape material is softened and the moldability (crushing workability) is improved. As a result, when the extruded shape member is crushed, it is possible to prevent cracks from occurring on the outer side of the bent web, and at the same time, to reduce the tensile residual stress generated in the web.

In the restoration process, if the rate of temperature rise is less than 0.4 ° C./second, precipitation of the intermetallic workpiece is promoted during the temperature raising process, and the effect of the restoration process cannot be obtained. When the holding temperature (substance temperature) is less than 200 ° C., the intermetallic compound precipitated by natural aging does not re-dissolve, but rather the precipitation is accelerated and coarsened, whereas when the holding temperature exceeds 550 ° C., the extruded shape However, in any case, the required strength cannot be obtained after the aging treatment. The holding time needs to exceed at least 0 seconds. In short, after the extruded profile reaches the holding temperature, it may be cooled after being held at the same temperature for a predetermined time, or may be cooled immediately. Although the upper limit of the holding time is not particularly limited, for example, it is preferable in terms of production efficiency that a short time of 60 seconds or less is desirable, and a shorter time of 10 seconds or less and 5 seconds or less is more desirable. As the heating means, for example, a high frequency induction heating device or a glass furnace can be used.
Further, when the cooling rate from the holding temperature is less than 0.5 ° C./second, the intermetallic compound is precipitated again in the cooling process, and the effect of the restoration treatment is weakened or lost. In the conventional restoration process, the cooling rate in the cooling process is not particularly considered.

  After the restoration process, the extruded profile is crushed before it is re-cured by natural aging. Specifically, it is desirable to perform crushing 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 rate satisfies 1.5 t ≦ R, it is possible to prevent cracks from occurring on the outer side of the bent web, and at the same time An increase in the tensile residual stress generated in the web can be prevented. However, if R <1.5t, cracks cannot be prevented from occurring on the outer side of the web even if the restoration process is performed before the extruded profile is crushed. At the same time, it is impossible to prevent an increase in the tensile residual stress generated in the web, and the stress corrosion cracking resistance of the member is lowered. On the other hand, when R> 10t, cracks do not occur even if the restoration process is not performed before the extruded profile is crushed (even if the extruded profile is in the T1 state).

The aging treatment after the crushing process may be performed under the well-known conditions performed with a normal 7000 series aluminum alloy. By this aging treatment, a strength (0.2% proof stress value) of 200 MPa or more is ensured in the product 7000 series aluminum alloy member.
Although the 7000 series aluminum alloy member manufactured by the above-described manufacturing method is a high-strength material, the web in the area subjected to the crushing process does not generate cracks, and the tensile residual stress σsr of the web is reduced to 0. The ratio Y (σsr / σ _0.2 ) of the 2% proof stress value σ _0.2 is the sum X of Mg content [Mg] and Zn content [Zn] of the 7000 series aluminum alloy (= [Mg] + [Zn ]), The following formula (1) is satisfied, and excellent stress corrosion cracking resistance is exhibited.
Y ≦ −0.1X + 1.4 (1)

The graph shown in FIG. 1, the total content of Zn and Mg X (= [Zn] + [Mg]), tensile residual stresses Shigumars and 0.2% proof stress sigma _0.2 ratio Y (σrs / σ _{0. 2} ) are plotted (Δ, □) of data of examples described later on XY coordinates, and the line in the figure is a straight line represented by Y = −0.1X + 1.4. In FIG. 1 to 6 and these all fall within the range of Y ≦ −0.1X + 1.4 and are excellent in stress corrosion cracking resistance as shown in Table 2. On the other hand, □ indicates no. It corresponds to 7 to 14, all enter the region of Y> −0.1X + 1.4, and as shown in Table 2, the stress corrosion cracking resistance is inferior. In addition, as shown in Table 2, No. 1 entering the region of Y ≦ −0.1X + 1.4. Nos. 1 to 6 have no cracks in the web and No. 1 in the range of Y> −0.1X + 1.4. In all of Nos. 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 natural aging of the extruded material produced by press quenching, as shown in the examples described later. It is 0.2% proof stress of the part age-treated without performing processing and pipe expansion processing.

  7000 series aluminum alloy shown in Table 1 is hot-extrusion molded, and fan air cooling (press quenching) is performed on-line immediately after extrusion, and a pair of opposed flanges (inner flange 1, outer flange) as shown in FIG. 2) and an extruded profile having a substantially mouth-shaped cross section (with a protruding flange) composed of two webs 3 and 4 that connect them vertically. This extruded shape is assumed to be a door beam. Height 30.0mm, outer flange 1 thickness 4.0mm, width 40.0mm, inner flange 2 thickness 4.0mm, width 50.0mm, both webs 3 and 4 have a thickness of 2.0 mm or 4.0 mm, the outer flange 1 protrudes from both webs 3 and 4 by 5 mm to the left and right, and the inner flange 2 protrudes from both webs 3 and 4 by 10 mm to the left and right.

  No. after press quenching Nos. 1 to 14 are cut into a predetermined length, Two test materials (extruded shapes) for each of 1 to 14 were collected, allowed to stand at room temperature for 20 days and allowed to naturally age, and then various heating rates shown in Table 1 using a high-frequency induction heating device. Then, the restoration process was performed at the ultimate temperature (substance temperature), the holding time, and the cooling rate (only No. 11 was applied). The restoration process was performed only on a partial region (end region) along the longitudinal direction of the test material.

After the restoration process, after the time shown in Table 1 has elapsed, as shown in FIG. 2B, the test material 5 is placed on the horizontal base 6 and vertically pressed by the crushing processing jig 7 disposed above the horizontal base 6. At the same time, an inward load is applied to the webs 3 and 4 with a horizontal processing jig (not shown) (see horizontal load jig 9 described in Patent Document 3), and the end region (No. ..., Only the .11 region that has not been restored is crushed in the vertical direction by the inclined surface 7 a of the crushing jig 7. The webs 3 and 4 of the test material 5 were bent and deformed by the crushing process, and protruded inside the hollow portion. In this crushing process, the stroke of the crushing jig 7 is made constant, and the position in the longitudinal direction (left and right direction in FIG. 2B) of the test material 5 is adjusted on the horizontal table 6. The crushing rate of 1 to 14 test materials 5 (two each), that is, the bending radius of the webs 3 and 4 was adjusted.
After crushing, no. Aging treatment of 130 ° C. × 8 hours was performed on the whole of 1 to 14 test materials (two each).
After aging treatment, no. Using one of the test materials 1 to 14, a tensile test, an inspection for the presence or absence of cracks on the outer side of the web, the measurement of the inner radius of bending (minimum value R), and the measurement of the tensile residual stress of the web Went. No. Using another test material of 1 to 14, a stress corrosion cracking resistance test was conducted. The results are shown in Table 2.

(Tensile test)
A JIS No. 5 test piece was collected from a region of the test material 5 not subjected to the restoration treatment, and subjected to a tensile test according to the metal material test method specified in JISZ2241, and 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 outside of the webs 3 and 4 was inspected. Cracks occurred mainly in the vicinity of the crushed end face of the test material 5.
(Minimum bending radius R)
Since the bending inner radius of the webs 3 and 4 is the smallest at the crushed end surface of the test material 5, the bending inner radius of the webs 3 and 4 was measured at the same end surface.

(Tensile residual stress of web)
The residual stress was measured by the following procedure using a cutting method. As the measurement target positions, the crushing start position A, the end position B, and the intermediate position C shown in FIG. 2 are selected (all at the height center position), and these measurement target position surfaces are polished with sandpaper and then washed with acetone. Then, a strain gauge is bonded to this polished part with a momentary adhesive, left at room temperature for 24 hours, the strain gauge lead wire is connected to a strain gauge, a zero point is set, and the periphery of the strain gauge is 10 mm square with a metal saw. 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 equation. σrs = −E × ε (E; Young's modulus), where E = 68894 N / mm ² .
In addition, No. In all the test materials 1 to 14, the tensile residual stress value measured at the crushing start position A was the maximum value. This is presumably because the material restraint is the largest at the crushing start position A, while the material restraint is relatively small at the end position B and the intermediate position C, and the distortion due to the crushing process is released. Therefore, the residual stress value σrs shown in Table 2 is a value measured at the crushing processing start position A.

(Stress corrosion cracking resistance)
The stress corrosion cracking test by the chromic acid acceleration method was conducted. Using the crushed test material, it was immersed in a test solution at 90 ° C. for a maximum of 16 hours, and stress corrosion cracking was visually observed. The test solution was prepared by adding 36 g of chromium added to distilled water and 30 g of potassium dichromate and 3 g of sodium chloride (per liter). Tests were taken out of the solution every hour to check for cracks, and those with no cracks or cracks that took 12 hours or more were evaluated as having excellent stress corrosion cracking resistance (◯). Then, the case where the time until the occurrence of cracking was less than 12 hours was evaluated as inferior (x). All stress corrosion cracking occurred near the crushing start position A (see FIG. 2B).

Residual stress value from (σrs) and 0.2% proof stress (sigma _0.2), was calculated both the ratio _{Y (= σrs / σ 0.2)} . 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. The value of 4) was calculated. Based on the above calculation results, a case where the values of X and Y satisfy the relational expression (1) was determined as ◯, and a case where the values did not satisfy the relationship was determined as x. Table 2 shows the above calculation results and determination results.
From Tables 1 and 2, No. 1 having an alloy composition defined in the present invention and subjected to restoration treatment and crushing under the conditions defined in the present invention. The test materials 1 to 6 have no cracks in the web after crushing, the proof stress after aging treatment is 200 MPa or more, and Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg ]) Satisfy | fills said Formula (1), and all are excellent in stress corrosion cracking resistance.

On the other hand, no. In the test material 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 The formula (1) is not satisfied, and the stress corrosion cracking resistance is inferior.
No. 8 of the test material, restoration effects for restoration process the cooling rate is slow processing is lost, the web to be cracked by crushing processing, and Y (= σrs / σ _0.2) and X (= [Zn] + [Mg] does not satisfy the formula (1), and the stress corrosion cracking resistance is poor.
No. The test material of No. 9 has no effect of the restoration process due to the low arrival temperature of the restoration process, and the aging process does not improve the yield strength, and the web is cracked by the crushing process despite being relatively low in Zn and Mg. I couldn't prevent it from entering. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the formula (1), and the stress corrosion cracking resistance is inferior.

No. Since the test material of No. 10 had R / t too small (the crushing rate was too high), the conditions for the restoration treatment were appropriate, but despite the relatively low Zn and Mg, cracks caused by crushing Could not be prevented. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the formula (1), and the stress corrosion cracking resistance is inferior.
No. Since the test material of No. 11 was not restored, cracks occurred in the web by crushing. Further, Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg]) do not satisfy the formula (1), and the stress corrosion cracking resistance is inferior.
No. Since the test material No. 12 takes 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 formula (1), and the stress corrosion cracking resistance is poor.

No. In the test material No. 13, since the temperature increase rate of the restoration process is small, the effect of the restoration process is not obtained, the web is cracked by crushing, and Y (= σrs / σ _0.2 ) and X (= [Zn) ] + [Mg]) does not satisfy the formula (1), and the stress corrosion cracking resistance is poor.
No. The test material No. 14 loses the effect of the restoration process due to the low cooling rate of the restoration process, the web is cracked by crushing, and Y (= σrs / σ _0.2 ) and X (= [Zn] + [Mg] does not satisfy the formula (1), and the stress corrosion cracking resistance is poor.

1, 2 Flange 3, 4 Web 5 Test material (extrusion profile)
7 Crushing jig

Claims (2)

Zn: 3.0-8.0 mass%, Mg: 0.4-2.5 mass%, Cu: 0.05-2.0 mass%, Ti: 0.005-0.2 mass% Furthermore, Mn: 0.01 to 0.3% by mass, Cr: 0.01 to 0.3% by mass, Zr: 0.01 to 0.3% by mass, or one or more, and the balance A composition composed of Al and inevitable impurities, composed of a plurality of plates, and perpendicular to the extrusion direction at least in a region along the longitudinal direction of a 7000 series aluminum alloy extruded profile manufactured by press quenching In a manufacturing method of a 7000 series aluminum alloy member that is crushed in a direction to form a member, before crushing, at least the region of the extruded shape member is heated at a temperature increase rate of 0.4 ° C./second or more. And hold in the temperature range of 200 to 550 ° C. for more than 0 seconds and not more than 60 seconds. The sheet is subjected to a restoration process of cooling at a cooling rate of 0.5 ° C./second or more, and within 72 hours after the restoration process, the thickness of the plate that has undergone the largest bending deformation among the plurality of plates is defined as t, When the minimum value of the radius is R, the crushing process is performed under the conditions of 1.5 mm ≦ t ≦ 4.0 mm and 3t / 2 ≦ R ≦ 10 t, and after the crushing process, the entire member is subjected to an aging treatment. A method for producing a 7000 series aluminum alloy member having excellent stress corrosion cracking resistance. The 7000 series aluminum alloy extruded profile consists of a pair of flanges arranged opposite to each other and one or more webs connecting them, and the plate that has undergone the greatest bending deformation is the web. The manufacturing method of the 7000 series aluminum alloy member excellent in the stress corrosion cracking resistance described in Claim 1.

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