JP2000282162A - Aluminum alloy extruded material excellent in corrosion fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a 6000 series Al alloy high in corrosion fatigue strength even in a corrosive environment in which Cl- ions are present by allowing it to have a compsn. contg. specified weight ratios of Mg and Si, and the balance Al with inevitable impurities and forming the structure of the surface layer part of a recrystallized structure of a specified average grain size. SOLUTION: This material is the one having a compsn. contg., by weight, 0.4 to 1.2% Mg, 0.3 to 1.4% Si, and the balance Al with inevitable impurities, in which the average grain size in the surface layer part is formed of a recrystallized structure of <=300 μm average grain size. Preferably, it is an Al alloy extruded material having a compsn. contg., by weight, 0.4 to 1.2% Mg, 0.3 to 1.4% Si, and the balance Al with inevitable impurities, in which the surface layer part has a fibrous structure, and moreover contg. one or more kinds among, by weight, 0.05 to 1.0% Cu, 0.05 to 0.6% Zn, 0.03 to 0.5% Mn, 0.03 to 0.5% Cr, 0.03 to 0.3% V, 0.05 to 0.5% Fe and 0.005 to 0.3% Ti. It is widely applicable as an Al stock for a structural member used for an automotive body or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an extruded aluminum alloy used for a structural member of an automobile or the like.

[0002]

2. Description of the Related Art Steel is often used as a structural member for a vehicle body such as a truck, but the use of aluminum has been studied to reduce the weight. When manufacturing a 6000 series alloy with high strength among aluminum, especially a material with a diameter of 5 mm or more, by hot extrusion, if the manufacturing is performed under normal conditions, the center part will have a fiber due to the difference in cooling rate between the center part and the surface part. The composite structure has a shape-like structure and a surface layer composed of a recrystallized structure.

[0003] However, under such extrusion conditions that a fibrous structure is formed at the center, a structure composed of coarse crystal grains whose average grain size of the recrystallized structure in the surface layer exceeds 500 μm is obtained. Would. These organizations, since the area of the grain boundary is reduced, and the precipitated concentrate on aging precipitates smaller grain boundaries, there Cl ^- to become susceptible to such corrosion ions, Cl ^- ions In a corrosive environment that exists, for example, in a coastal area or a cold region where salt is sprayed to prevent freezing, the 6000 series aluminum alloy has a significantly reduced corrosion fatigue strength (durability against repeated stress such as vibration in a corrosive environment). There was a disadvantage.

[0004] In order to obtain higher strength,
It is remarkable when a large amount of additional elements (Mg, Si, etc.) are added to the 0-series aluminum alloy, and the incompatibility between strength and corrosion fatigue strength is required for the application of the 6000-series aluminum alloy to structural members such as automobile bodies. It is a major factor that hinders.

[0005]

SUMMARY OF THE INVENTION In view of the above situation, an object of the present invention is to provide a 6000 series aluminum alloy having high corrosion fatigue strength even in a corrosive environment where Cl ^- ions are present. It was done.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies on the relationship between the corrosion fatigue strength and the structure of a 6000 series aluminum alloy, and as a result, have found the structure to be provided in a 6000 series aluminum alloy having excellent corrosion fatigue strength. The present invention has been completed on the basis thereof, and the gist of the invention is as follows: (1) Mg: 0.4 to 1.2%;
An extruded aluminum alloy material having excellent corrosion fatigue strength, characterized by containing 3 to 1.4%, the balance being Al and unavoidable impurities, and a surface layer having a recrystallized structure having an average grain size of 300 µm or less. . (2) By weight%, Mg: 0.4-1.2%, Si: 0.
An extruded aluminum alloy material having an excellent corrosion fatigue strength, containing 3 to 1.4%, the balance being Al and unavoidable impurities, and a surface layer having a fibrous structure.

(3) Cu: 0.05-1.0% by weight
%, Zn: 0.05-0.6%, Mn: 0.03-0.
5%, Cr: 0.03 to 0.5%, V: 0.03 to 0.
3%, Fe: 0.05-0.5%, Ti: 0.005-
The extruded aluminum alloy material having excellent corrosion fatigue strength according to the above (1) or (2), further comprising at least one kind of 0.3%. (4) The above-mentioned (1)-
A structural member having excellent corrosion fatigue strength, comprising the aluminum alloy extruded material according to any one of (3).

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Corrosion fatigue of the 6000 series can be measured by the equilibrium phase (Mg ₂ Si) precipitated at the grain boundaries as described in, for example, Aluminum Handbook (5th edition), Light Metal Association of Japan (1994), page 53. And Si are preferentially dissolved compared to the base material. Corrosion due to repeated stress is accelerated, causing cracks on the member surface. Starting from the crack that occurred, the fatigue crack propagated,
The member breaks. It is said to occur by the mechanism.

The present inventors have paid attention to the fact that the precipitates, which are the starting points of corrosion, are concentrated on the grain boundaries having a particularly small area and are the primary cause of corrosion fatigue. As a result of a detailed investigation of the precipitation state such as the β phase, 6000
This has led to the identification of a structure in which a grain boundary area effective for corrosion fatigue is sufficiently large in a system aluminum alloy.

As a result of the investigation by the inventor, a structure satisfying the above conditions can be realized by (1) a recrystallized structure having an average particle size of 300 μm or less and (2) a fibrous structure in a 6000 series aluminum alloy. It has been found that when the surface layer portion of 500 μm or less from the surface has any of the above structures, the corrosion fatigue strength is remarkably improved as compared with a conventional alloy having the same composition.

A 6000 series aluminum alloy has a fibrous structure when cooling is slow, and a recrystallized structure when cooling is fast. Therefore, when a 6000 series aluminum alloy, particularly a material having a size of 5 mm or more, is manufactured by hot extrusion, if the manufacturing is performed under normal conditions, the central portion has a fibrous structure due to a difference in cooling rate between the central portion and the surface portion. Then, the surface layer becomes a composite structure composed of a recrystallized structure. However, under such ordinary extrusion conditions that a fibrous structure is formed at the center, the average grain size of the recrystallized structure in the surface layer is 5%.
Since the structure is made of coarse crystal grains exceeding 00 μm, the corrosion fatigue strength is not high.

If the extrusion rate is increased while ensuring a sufficient degree of processing and the surface layer is rapidly cooled, the grain size of the recrystallized structure becomes smaller. The inventors set the extrusion speed to 15 m / mi.
When n is faster than n, it has been found that a recrystallized structure having an average particle size of 300 μm or less is formed in the surface layer portion, and the corrosion fatigue strength is significantly improved. Since the corrosion fatigue occurs and proceeds from the surface layer of the material, it is sufficient that the structure of the surface layer is defined in the present invention. However, when manufactured under the above conditions, the entire surface of the material is recrystallized, depending on the material thickness. Become an organization.

The average particle size in the surface layer may be determined according to the method used for ordinary metal materials.
At any position on the tissue photograph of about 0 to 50 times, for example, 1
It is relatively simple to draw a straight line having a specific length of 00 mm and obtain the number from the number of particles intersecting the straight line. The inventors determined the particle size of one sample by drawing five straight lines in the thickness direction and in a direction perpendicular to the thickness direction on the surface layer of 500 μm or less from the surface to obtain the average particle size. Diameter.

Contrary to the recrystallized structure, the extrusion speed is 1 m
If the rate is not more than / min, the cooling rate is sufficiently reduced even in the surface layer portion, and a fibrous structure is formed up to the surface layer portion. In this structure, the grain size itself cannot be defined, but the inventors have found that this structure has the same effect on corrosion fatigue as a recrystallized structure having an average particle size of 300 μm or less. . The present invention defines only the structure of the surface layer that has an effect on corrosion fatigue, but in the case of this fibrous structure,
Since the cooling rate of the central portion is lower than that of the surface portion, the entire surface has a fibrous structure. Also, depending on the specifications of the hot extrusion equipment, a slight recrystallized structure may remain in the outermost layer, but this structure naturally has an extremely large average particle size, and it is difficult to prevent corrosion fatigue. Therefore, it is necessary to grind the recrystallized structure to make only the fibrous structure in the surface layer.

The thickness of the sheet capable of realizing the surface layer structure is 15 m for the recrystallized structure specified in the above (1).
It is preferably applied to a material having a thickness of not more than m. In hot extrusion, recrystallization occurs due to elevated temperatures during extrusion and processing. In general, an extruded material having a thickness of more than 15 mm does not have sufficient workability, and it is very difficult to increase the extrusion speed to obtain the structure specified in the present invention. On the other hand, in the fibrous structure specified in the above (2), it is possible to cope with any plate thickness as long as attention is paid to coarse recrystallized grains in the outermost layer.

The member described in the above (4) is characterized by having the aluminum alloy extruded material of the present invention, but the aluminum alloy extruded material of the present invention is preferably used in a portion where corrosion fatigue becomes a problem. . For example, in a material having a C-shaped or H-shaped cross section, it is preferable that at least the flange portion is the aluminum alloy extruded material of the present invention. When using the aluminum alloy extruded material of the present invention as a constituent material of a member, special attention must be paid to the surface of the member exposed to corrosion fatigue. That is, even if the surface structure of the present invention is provided at the time of manufacturing, it is necessary to pay sufficient attention so that a surface not belonging to the present invention is not exposed to corrosion fatigue by cutting or grinding.

Next, the reasons for limiting the components will be described. Mg is a basic alloy element in the alloy of the system targeted in the present invention, and forms a compound with Si to contribute to an increase in strength. When the amount of Mg is less than 0.4%, the amount of the β ″ phase that contributes to the increase in strength due to the precipitation effect decreases, so that sufficient strength cannot be obtained. At the same time, even if the solution treatment is performed, the segregation of the grain boundary precipitates cannot be sufficiently suppressed, so that the intergranular corrosion property is reduced, so that the Mg content is 0.4 to 1.2%.
Range.

Si is also a basic alloying element in the alloys of the present invention, and forms a β ″ phase together with Mg, contributing to the improvement of strength. Since the amount of the contributing β ″ phase is small, sufficient strength cannot be obtained. On the other hand, if added over 1.4%,
In addition to lowering the intergranular corrosion characteristics, coarse Si is crystallized during solidification and lowers the extrusion speed. Therefore, the content of Si is set to 0.3 to 1.4%.

Cu is an element that promotes age hardening and increases the strength of the alloy. However, if the content is less than 0.05%, the effect is not seen, and if it exceeds 1.0%, the β phase or S
A grain boundary precipitate containing Cu other than the i-phase is formed, and the intergranular corrosion resistance is reduced. Therefore, the Cu content is 0.05 to
1.0%. Zn is an element effective for improving the strength, but if the content is less than 0.05%, the effect is small,
If it exceeds 0.6%, the intergranular corrosion resistance and the weldability decrease. Therefore, the content of Zn is set to 0.05 to 0.6%.

Each of Cr, Mn, Zr and V is an element having the effect of refining and stabilizing crystal grains and improving the strength, and one or more of them are added as necessary. If any of the elements is less than 0.03%, the above effects cannot be obtained. On the other hand, if the contents of Cr and Mn exceed 0.5% and the contents of Zr and V exceed 0.3%, not only the above effects are saturated, but also the extrudability is reduced. : 0.03 to 0.5%, M
n: 0.03-0.5%, Zr: 0.03-0.3%,
V: 0.03 to 0.3%.

Fe is an unavoidable impurity for Al, but has the effect of refining and stabilizing the crystal grains and improving the strength as in the case of Cr, Mn, Zr and V described above. You may make it contain according to it. In the case of adding Fe, if the addition amount is less than 0.05, this effect cannot be obtained, and if it exceeds 0.5%, not only the effect is saturated but also the Al—Fe—Si based intermetallic compound Produces and reduces extrudability. Therefore, the content of Fe is 0.05 to
0.5%.

Ti and B are generally added singly or in combination to refine the crystal grains of the ingot. Ti:
If the content is less than 0.05% and B: less than 0.0005%, this effect cannot be obtained. If the content of Ti exceeds 0.3% and the content of B exceeds 0.03%, the effect is saturated. Therefore, the content of each element is as follows: Ti: 0.05 to 0.3%, B: 0.0005 to
0.03%.

[0023]

Embodiments of the present invention will be described below. An Al alloy having the composition shown in Table 1 was melted, cast,
A homogenization treatment was performed to obtain a material for hot extrusion. This material was preheated at 500 ° C. for 5 minutes, and hot extrusion was performed.
The shape of the extruded material is a square plate having a thickness of 8 mm and a side of 100 mm. As shown in Table 2, the extrusion speed was changed,
Test materials belonging to Examples and Comparative Examples were separately manufactured. After extrusion 5
The solution was subjected to a solution treatment at 50 ° C. for 30 minutes, poured into ice water from this temperature, rapidly cooled and quenched, and then subjected to an aging treatment at 170 ° C. for 8 hours. The extrusion speed, tensile strength, proof stress, surface layer structure and particle size of each test material are as shown in Table 2.

[0024]

[Table 1]

[0025]

[Table 2]

A fatigue test piece was cut out from each of the test materials thus obtained and the results of a corrosion fatigue test in a 3.5% aqueous NaCl solution are shown in Table 2. The fatigue test conditions were a stress amplitude of 240 MPa, a stress ratio of R = -1, and a test speed of 1 Hz. According to the results shown in Table 2, the surface layer structure belonging to the present invention has excellent corrosion fatigue properties as compared with the comparative example.

[0027]

As described above, since the extruded aluminum alloy of the present invention has excellent corrosion fatigue strength, it can be sufficiently fatigued even in an environment susceptible to corrosion, such as in coastal areas or in cold regions where salt is sprayed to prevent freezing. Because of its strength, it can be widely applied as an aluminum material for structural members used in trucks and other automobile bodies. Therefore, it can be said that the present invention is an invention having extremely high industrial value.

Claims (4)

[Claims] 1. An alloy containing 0.4 to 1.2% by weight of Mg and 0.3 to 1.4% by weight of Si, the balance being Al and unavoidable impurities, and the surface layer having an average particle size of 1% by weight. An extruded aluminum alloy material having excellent corrosion fatigue strength, having a recrystallized structure of 300 μm or less. 2. The composition contains Mg: 0.4 to 1.2% and Si: 0.3 to 1.4% by weight, the balance being Al and unavoidable impurities, and the surface layer having a fibrous structure. An extruded aluminum alloy material having excellent corrosion fatigue strength. 3. Weight%: Cu: 0.05 to 1.0%, Zn: 0.05 to 0.6%, Mn: 0.03 to 0.5%, Cr: 0.03 to 0.5%. 5%, V: 0.03 to 0.3%, Fe: 0.05 to 0.5%, and Ti: 0.005 to 0.3%. Item 4. An extruded aluminum alloy material having excellent corrosion fatigue strength according to item 1 or 2. 4. The method according to claim 1, wherein the portion requiring corrosion fatigue strength is provided.
4. A structural member having excellent corrosion fatigue strength, comprising the aluminum alloy extruded material according to any one of items 1 to 3.