JP5204793B2 - High strength aluminum alloy extruded material with excellent stress corrosion cracking resistance - Google Patents

Description

  The present invention relates to a high-strength aluminum alloy extruded material excellent in stress corrosion cracking resistance, and more particularly to an aluminum alloy extruded material suitably used as a structural material for automobiles such as a bumper reinforcement and a door guard bar.

  In order to reduce the weight of automobiles, Al-Zn-Mg high-strength aluminum alloy extruded materials (see Patent Documents 1 and 2) are used as energy absorbing members such as bumper reinforcements and door guard bars. However, the Al-Zn-Mg-based aluminum alloy extruded material has a risk of causing stress corrosion cracking (hereinafter referred to as SCC). The characteristics as a high-strength alloy are weakened.

JP 2002-327229 A Japanese Patent Laid-Open No. 11-264044

In order to further reduce the weight of automobiles, it is required to increase the strength of Al—Zn—Mg alloy extruded materials used for automobile structural materials such as bumper reinforcement. However, when Zn and Mg are increased in concentration in order to achieve higher strength of this alloy, the grain boundary precipitate MgZn ₂ is distributed at a high density, so that the SCC resistance is reduced and applied as a structural material for automobiles. become unable. At the same time, the extrudability deteriorates and thin-wall molding becomes difficult, and as a result, the effect of reducing the weight cannot be exhibited.
The present invention has been made in view of such problems of the prior art, and provides an Al—Zn—Mg-based aluminum alloy extruded material having high strength, excellent SCC resistance, and excellent extrudability. Objective.

The Al—Zn—Mg-based aluminum alloy is an alloy that achieves high strength by distributing precipitates MgZn 2 composed of Zn and Mg at a high density. In the present invention, it was utilized that Zn added in excess of the amount of Zn and Mg (stoichiometric ratio of MgZn ₂ ) that forms MgZn ₂ without excess or deficiency contributes to high strength. By adding Mg to excess over stoichiometric ratio of MgZn _2, it is possible to increase the strength of suppressing the MgZn ₂ amount, thereby, the Al-Zn-Mg series aluminum alloy extruded material, the SCC resistance The strength can be increased without lowering. On the other hand, if the amount of excess Mg is too large, the extrudability deteriorates and the extrusion speed becomes slow, so that die quench air cooling (on-line cooling of the extruded material immediately after extrusion, also referred to as press quenching) cannot be performed. Further, as the excess Mg increases, the grain boundary precipitates become fine and continuous, eventually reducing the SCC resistance. Therefore, in the present invention, the limit amount of excess Mg that can be increased in strength without degrading extrudability and SCC resistance has been determined.

In the Al—Zn—Mg-based aluminum alloy extruded material according to the present invention, [Mg] and [Zn] are expressed by the following (1) to [Mg] when the Mg mass% is [Mg] and the Zn mass% is [Zn]. (3) is satisfied,
5.43 ≦ [Zn] ≦ 7.0 (1)
[Zn] /5.38+0.15≦ [Mg] ≦ [Zn] /5.38+0.7 (2)
[Zn] +4.7 [Mg] ≦ 12 (3)
Furthermore, Cu: 0.1-0.6 mass%, Ag: 0.01-0.15 mass% 1 type or 2 types, Ti: 0.005-0.05 mass%, Mn: 0.0. 1-0.3 mass%, Cr: 0.05-0.2 mass%, Zr: 0.05-0.2 mass% of 1 type (s) or 2 or more types are comprised, and it consists of remainder Al and an unavoidable impurity.
Since the stoichiometric ratio (mass ratio) of MgZn ₂ is [Mg]: [Zn] is 1: 5.38, the above formula (2) indicates that [Mg] is 0 from the stoichiometric ratio of MgZn _2. .15% by mass or more and excess [Mg] is 0.7% by mass or less.

  The Al—Zn—Mg-based aluminum alloy extruded material of the present invention has high strength and excellent SCC resistance. And since it is excellent in extrudability, the die-quenching air cooling can provide high strength almost comparable to T6 material (aging treatment after solution treatment), and thin-wall molding is possible. By applying the high-strength Al—Zn—Mg-based aluminum alloy extruded material of the present invention, it is possible to further reduce the weight of automobile structural members such as bumper reinforcements and door guard bars.

It is a figure which shows the range of Zn and Mg amount of the Al-Zn-Mg alloy of this invention. It is a figure which shows the cross-sectional shape of the extrusion material of an Example. It is a microscope picture of the cross-sectional structure | tissue of the extrusion material of an Example.

Hereinafter, the composition of the Al—Zn—Mg-based aluminum alloy extruded material according to the present invention will be described in detail.
Zn;
If the Zn content is less than 5.0% by mass, the strength is insufficient, and if it exceeds 7.0% by mass, the grain boundary precipitate MgZn ₂ increases and the SCC sensitivity becomes sharp. Therefore, Zn content shall be 5.0-7.0 mass%. When SCC resistance is particularly important, the Zn content is relatively low, specifically 5.0 to 6.3% by mass, more preferably less than 6.0% by mass, and further 5.8% by mass. The following is more desirable. On the other hand, when Zn content exceeds 6.3 mass%, in order to suppress that SCC sensitivity becomes sharp, it is desirable to add both Cu and Ag mentioned later.

Mg;
Mg forms MgZn ₂ together with Zn to improve the strength of the Al—Zn—Mg alloy. The content is limited as shown in the above formulas (2) and (3) in relation to the Zn content.
In a region where the Mg content is less than the stoichiometric ratio of MgZn ₂ ([Zn] /5.38), the amount of MgZn ₂ decreases and the strength is insufficient. When Mg content is the (2) the lower limit value or more regions (stoichiometric ratio +0.15 wt% or more excess Mg region of MgZn ₂₎ of, for excess Mg contributes to higher strength, MgZn ₂ It is possible to increase the strength while suppressing the amount. However, when the excess Mg amount exceeds 0.7% by mass, the extrudability decreases, and die quench air cooling does not provide high strength (compared to T6 material). In addition, productivity is reduced and thin wall molding becomes difficult. Desirably, the excess Mg amount is 0.6% by mass or less.
Moreover, when Zn and Mg content exceeds the prescription | regulation of said (3) Formula, a grain-boundary precipitate will be formed finely and continuously, and SCC resistance will fall.

FIG. 1 illustrates the range of Zn and Mg content of an Al—Zn—Mg alloy according to the present invention. Plot ◯ in the figure indicates No. in Table 1 described later. 1 to 12 and plot ● are No. 1 in Table 1. 13-18. In FIG. 1, the ranges defined by the equations (1) to (3) are substantially [Zn] = 5.43, [Mg] = [Zn] /5.38+0.15, and [Zn]. ] +4.7 [Mg] = 12.

Cu, Ag;
Cu and Ag have the effect of improving the SCC resistance of the Al—Zn—Mg alloy, and either one or both are added.
When the Cu content is less than 0.1% by mass and the Ag content is less than 0.01% by mass, the effect of improving the SCC resistance is small. On the other hand, when Cu content exceeds 0.6 mass%, extrudability and weldability will be reduced. Moreover, since quenching sensitivity becomes sharp, it becomes impossible to quench by air cooling. Even if the Ag content exceeds 0.15%, the effect is saturated. Therefore, the Cu content is 0.1 to 0.6 mass%, and the Ag content is 0.01 to 0.15 mass%.
When the Zn content is less than 6.3% by mass, either Cu or Ag may be added. However, when the Zn content exceeds 6.3% by mass, Cu and Ag are used to suppress a decrease in SCC resistance. Both additions of Ag are desirable.

Ti;
Ti has the effect of forming Al ₃ Ti in the molten metal and refining the crystal grains of the ingot. When the Ti content is less than 0.005% by mass, the effect of crystal grain refinement is small. On the other hand, if the Ti content exceeds 0.05% by mass, coarse crystals are formed in the ingot and the elongation is lowered. Therefore, Ti content shall be 0.005-0.05 mass%.

Mn, Cr, Zr;
Mn, Cr and Zr are precipitated as finely dispersed particles in aluminum by homogenization treatment, and have the effect of suppressing recrystallization, and SCC resistance can be improved by suppressing recrystallization. 1 type (s) or 2 or more types are added. If Mn, Cr, and Zr are all less than 0.1% by mass, 0.05% by mass, and 0.05% by mass, surface recrystallization occurs thickly during extrusion and SCC resistance decreases. On the other hand, when Mn, Cr, and Zr exceed 0.3% by mass, 0.2% by mass, and 0.2% by mass, respectively, the quenching sensitivity becomes sharp, and further, a coarse crystallized product is formed, resulting in a decrease in elongation. Therefore, the contents of Mn, Cr, and Zr are 0.1 to 0.3 mass%, 0.05 to 0.2 mass%, and 0.05 to 0.2 mass%, respectively. Since Zr has a relatively small effect of sharpening quenching sensitivity, it is desirable to add Zr alone, or one or both of Zr and Mn or Cr.

Production method;
The Al—Zn—Mg-based aluminum alloy extruded material according to the present invention is melted to cast a billet, homogenized, and then extruded, and the extruded material immediately after extrusion is air-cooled and die-quenched, followed by aging treatment Thus, it can be manufactured. In addition, in order to quench by air cooling die quenching, it is necessary that the extrusion speed is sufficiently high (extrudability is excellent). Quenching requires quenching from a high temperature state (for example, 450 ° C. or higher), but if the extrusion speed is slow, the temperature of the extruded material will drop before it is air-cooled online, and it will not quench sufficiently. . For this reason, even if an aging treatment is performed, no high strength is obtained, and the strength is greatly inferior to that of T6 material.
On the other hand, the Al—Zn—Mg-based aluminum alloy extruded material according to the present invention can be subjected to solution treatment and aging treatment (T6 material) instead of die quenching. In any case, each process of the heat treatment may be performed under normal conditions. The aging conditions may be selected from a range of 65 to 95 ° C. for 2 to 6 hours and 125 to 165 ° C. for 7 to 13 hours (including an overaging region).

Al-Zn-Mg based alloys having chemical components shown in Table 1 were melted by a conventional method, and billets each having a diameter of 155 mm were cast. The billet was homogenized at 470 ° C. × 6 h, then air cooled by a fan, heated again to 450 ° C. and extruded into a hollow cross-sectional shape shown in FIG. The thickness of the cross section of the extruded material is 1.5 mm. The die was quenched from the high temperature state (450 ° C. or higher) during the extrusion process by fan air cooling, and the average cooling rate up to 200 ° C. was about 160 ° C./min.
Subsequently, two short materials were cut and collected from each extruded material, and one short material was subjected to a two-stage aging treatment of 90 ° C. × 3 hours and 140 ° C. × 8 hours, and a test material (T5 material) was obtained. Obtained. In addition, for the evaluation of extrudability, the other short material was subjected to a solution treatment (after heating at 450 ° C. × 1 hour, followed by water cooling), and then subjected to a two-stage aging treatment of 90 ° C. × 3 hours and 140 ° C. × 8 hours. T6 material used as the standard of property evaluation was obtained.

The following tests were conducted using the above-mentioned test materials and T6 materials. The results are shown in Table 2.
Tensile test;
JIS No. 13 B test specimens were collected from the test material (T5 material) and T6 material, and the tensile strength, proof stress and elongation were measured according to the tensile test method of JIS-Z2241. The mechanical properties shown in Table 2 are those of the test material (T5 material). The tensile strength and proof stress of the test material (T5 material) were evaluated as extrudability of 90% or more of T6 material, extrudability of 80% or more and less than 90%, extrudability x of less than 80%, and proof strength of 380N. / Mm ² or more and extrudability Δ or more was considered acceptable. Moreover, about elongation, 12% or more was set as the pass.

SCC test;
The stress corrosion cracking test by the chromic acid acceleration method was conducted. A plate-shaped test piece was collected from each specimen in parallel to the extrusion direction while avoiding the welded portion, and in the state where a tensile stress corresponding to a yield ratio of 95% was loaded in the extrusion direction according to JIS-H8711, It was immersed in the test solution for a maximum of 10 hours, and SCC was visually observed. In addition, the stress load generated tensile stress on the outer surface of the test piece by tightening the bolt and nut of the jig, and the stress value was measured with a strain gauge adhered to the outer surface. A test solution was prepared by adding 36 g of chromium oxide, 30 g of potassium dichromate and 3 g of sodium chloride (per liter) to distilled water. Observe the presence or absence of SCC every 0.5 hours, ○ if no SCC occurred for 10 hours, △ if SCC occurred in 6 hours or more but less than 10 hours, SCC occurred by 6 hours Was evaluated as x, and Δ or more was regarded as acceptable.

Microstructure;
About the test material evaluated as Δ or × in the SCC test, a test material having a length of 20 mm was collected in parallel to the extrusion direction, and the extruded parallel cross section of the non-welded portion was etched with a Keller solution, and then the outer surface (hollow material). The microstructure of the part corresponding to the outer surface of was observed. The test material having a surface recrystallized layer thickness of 20 μm or more was judged to have reduced SCC resistance due to the thick surface recrystallized layer, and “x” was given in the microstructure column of Table 2. When the surface recrystallized layer thickness was less than 20 μm, it was judged that there was no problem in the microstructure itself, and a circle was given in the column of microstructure in Table 2. Note that FIG. 21 is a microstructure (micrograph) of the specimen 21. The thickness of the surface recrystallized layer is indicated by a double arrow, and coarsened surface recrystallized grains are observed.

As shown in Table 2, no . Nos. 1 to 12 (of which No. 2, 7 and 8 have compositions within the specified range of the present invention) have high yield strength and elongation, and at the same time are excellent in extrudability and SCC resistance. No. No. 3 has a Zn content exceeding 6.3% by mass, but is excellent in SCC resistance due to the addition of both Cu and Ag. No. No. 10 has a Zn content exceeding 6.3% by mass and no addition of Ag, so that the SCC resistance is slightly inferior to other examples. No. 11 has no addition of Cu (0.01% or less), but has excellent SCC resistance because Ag is added.

  In contrast, no. Since the amount of Zn is less than the lower limit, the amount of MgZn2 is small and the strength is low. No. Since the amount of Zn exceeds 7.0%, both Cu and Ag are added, but the SCC resistance is low. No. 15 is an excess Zn side (Mg content is lower than or equal to the lower limit of the formula (2)), so the amount of MgZn2 is small and the strength is low. No. No. 16 has too much excess Mg (Mg content exceeds the upper limit of the formula (2)), so the extrudability is low, and further Zn + 4.7 Mg exceeds the upper limit of the formula (3), so the SCC resistance is low. . No. No. 17 has an excessive amount of excess Mg (Mg content exceeds the upper limit of the formula (2)), so the extrudability is low. No. No. 18 has low SCC resistance because Zn + 4.7Mg exceeds the upper limit of the formula (3).

  No. No. 19 has low SCC resistance because the amount of Cu and the amount of Ag are less than the lower limit. No. No. 20 has a low extrudability due to the Cu amount exceeding the upper limit, and further has a sharp quenching sensitivity, which cannot be quenched by air cooling and has a low strength. No. Since Mn, Cr, and Zr are all below the lower limit, surface recrystallized grains are coarsened (see FIG. 3), and SCC resistance is low. No. In No. 19, since Zr exceeds the upper limit, coarse crystals are formed and elongation is low.

Claims (2)

When the mass% of Mg is [Mg] and the mass% of Zn is [Zn], [Mg] and [Zn] satisfy the following three formulas,
5.43 ≦ [Zn] ≦ 7.0
[Zn] /5.38+0.15≦ [Mg] ≦ [Zn] /5.38+0.7
[Zn] +4.7 [Mg] ≦ 12
Furthermore, Cu: 0.1-0.6 mass%, Ag: 0.01-0.15 mass% 1 type or 2 types, Ti: 0.005-0.05 mass%, Mn: 0.0. 1 to 0.3 wt%, Cr: 0.05 to 0.2 mass%, Zr: include one or more of 0.05 to 0.2 wt%, Ri Do the balance Al and inevitable impurities, A high-strength aluminum alloy extruded material for automobile energy absorbing members excellent in stress corrosion cracking resistance, characterized by performing die quench air cooling and aging treatment . The high-strength aluminum alloy extruded material for automobile energy absorbing members excellent in stress corrosion cracking resistance according to claim 1, wherein the extruded material is used for bumper reinforcement.