JPH11310841A - Extruded aluminum alloy material excellent in fatigue strength and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide an aluminum alloy extruded material excellent in fatigue strength, which is optimal for use in a vehicle body structure, and a method for producing the same. SOLUTION: An aluminum alloy extruded shape for an automobile body structure excellent in fatigue strength, wherein a rate of a precipitate on a crystal grain boundary covering an interface of the crystal grain is 10% or less. Preferably, Mg: 0.3-1.5% by weight,
Si: 0.4 to 2.0%, Mg + Si: 2.4% or less,
Cu: contains less than 0.8%, the balance consists of Al and unavoidable impurities, and further contains Zn, Mn, Cr, Zr,
It is intended for a 6000 series aluminum alloy extruded material to which at least one of V, Fe and Ti is added. The production method is to reduce the cooling rate after hot extrusion or solution treatment to 1 ° C./sec or less,
Alternatively, the heat treatment after cooling is specified to be 160 ° C. or more × 1 hour or more.

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 material having excellent fatigue strength, which is mainly used for structural members of an automobile body.

[0002]

2. Description of the Related Art Conventionally, the body structure of an automobile has mainly been a monocoque structure made of a thin steel plate. However, the weight of the vehicle body is being reduced for the purpose of reducing CO ₂ emission in consideration of the global environment. As one of the weight reduction technologies, an aluminum space frame structure using an aluminum alloy extruded shape for a body frame has been proposed, and its practical use has begun. As the aluminum space frame material, a 6000 series alloy extruded material is mainly used in terms of various strength characteristics, weldability, corrosion resistance, recyclability, and the like. However, in order to further reduce the weight of the vehicle body, it is necessary to make these extruded profiles thinner and lighter, and the demand for improvement in the strength characteristics is becoming increasingly severe. Above all, although there is a great need to improve fatigue strength, existing 6N0
At present, it is becoming difficult to cope with the use of alloy 1 or 6063 alloy.

[0003]

SUMMARY OF THE INVENTION In view of the above problems, the present invention relates to a 6000 series alloy having many characteristics suitable as a material for a vehicle body structural member such as an aluminum space frame of an automobile. An object of the present invention is to provide an extruded aluminum alloy material having improved characteristics and more excellent fatigue strength than the conventional 6000 series alloy.

[0004]

Accordingly, in order to solve the above-mentioned problems, in the case of the extruded aluminum alloy material, the correlation between the fatigue characteristics and the microstructure of the material has been intensively studied and tested. As a result, precipitates are present on the grain boundaries of the alloy, and when the precipitates cover a certain percentage of the grain boundaries, they break brittlely at the grain boundaries,
It has been found that the fatigue strength is reduced. In addition, even if precipitates are present on the grain boundaries, it is possible to reduce the adverse effect on the fatigue properties by increasing the size of the precipitates by heat treatment and reducing the area of the precipitates covering the grain boundaries. Also found.

[0005] These grain boundary precipitates are mainly formed during the cooling process after hot extrusion or solution treatment. For example, in the case of a 6000 series alloy, Mg ₂ Si, Si, A
l-Cu-Mg-Si compound (when containing Cu)
Are deposited on the crystal grain boundaries. That is, in the extruded aluminum alloy, it has been clarified that by appropriately controlling the dispersion state of precipitates on the crystal grain boundaries, the fatigue strength can be improved, and the present invention has been accomplished.

That is, the gist of the present invention is as follows. (1) An extruded aluminum alloy material for an automobile body structure excellent in fatigue strength, wherein a ratio of a precipitate on a crystal grain boundary covering an interface of the crystal grain is 10% or less. (2) By weight%, Mg: 0.3-1.5%, Si: 0.
In an extruded aluminum alloy material containing 4 to 2.0%, Mg + Si: 2.4% or less, and Cu: less than 0.8%, and the balance being Al and unavoidable impurities, precipitates on the crystal grain boundaries are crystal grains. 10% covering the interface
An extruded aluminum alloy material for an automobile body structure having excellent fatigue strength, characterized in that:

(3) Zn: 0.03 to 1.5% by weight
%, Mn: 0.03 to 0.2%, Cr: 0.03 to 0.
2%, Zr: 0.03 to 0.2%, V: 0.03 to
0.1%, Fe: 0.03 to 0.3%, Ti: 0.00
The aluminum alloy extruded shape for automobile body structure excellent in fatigue strength according to the above (2), further comprising at least one of 5 to 0.1%.

(4) The method according to any of (1) to (3), wherein the cooling rate in a temperature range from immediately after hot extrusion or solution treatment to 200 ° C. is 1 ° C./sec or more. A method for producing an extruded aluminum alloy material for an automobile body structure having excellent fatigue strength. (5) The fatigue according to any one of (1) to (3), wherein after cooling to room temperature after hot extrusion or solution treatment, heat treatment is performed at a temperature of 160 ° C. or more for 1 hour or more. Manufacturing method of extruded aluminum alloy for automobile body structure with excellent strength.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reason for defining the existence of grain boundary precipitates will be described. The effect of the grain boundary precipitates on the fatigue properties varies depending on the rate of covering the grain boundary surface. 10% coverage of grain boundary precipitates
In the case where the temperature exceeds the above range, cracks are likely to be generated on the crystal grain boundaries, and the crack propagation is also promoted, so that the grain boundary fracture is likely to occur, and the fatigue strength tends to decrease. In a corrosive environment, intergranular corrosion is likely to be promoted.

Therefore, the condition of grain boundary precipitation is defined as 10% or less as a ratio of the precipitate covering the crystal grain interface. The presence of the grain boundary precipitates can be evaluated using a transmission electron microscope as schematically shown in FIG. The ratio of covering the grain boundary is defined as the ratio of the total length of the grain boundary precipitates traversed by the grain boundary to the total length of the grain boundary of one crystal grain. In these evaluations, if the above measurement is performed on a plurality of crystals in any five or more visual fields and the average value is obtained,
In the normal case, it may be regarded as a representative value.

[0011] In the general extruded aluminum alloy material,
As described above, by controlling the rate at which the grain boundary precipitate covers the crystal grain boundary, it is possible to obtain excellent fatigue characteristics. In the present invention, the embodiment will be described focusing on the 6000 series among the aluminum alloy extruded shapes. Next, the reasons for limiting the alloy composition of the present invention will be described.

[0012] Mg and Si are basic alloy components essential for the present invention which contribute to the improvement of strength.
%, Si: 0.4 to 2.0%. In addition, Mg
If + Si exceeds 2.4%, it becomes impossible to dissolve Mg and Si under ordinary hot extrusion temperature and solution treatment conditions, and crystal grains may be formed in a cooling process after hot extrusion or solution treatment. Mg +
Si was set to 2.4% or less.

[0013] Cu is an element that promotes aging precipitation during the artificial aging treatment and increases the strength of the alloy. However, if added in excess of 0.8%, the crystal is formed in the cooling process after hot extrusion or solution treatment. The amount of Cu added was set to less than 0.8% because precipitation on the grain boundaries is likely to occur and corrosion resistance is reduced. In the present invention, Zn, Mn, Cr, Z
r, V, Fe, and Ti are added, and the reasons for limiting each element are as follows.

One, two or more of Zn, Mn, Cr, Zr and V are added for improving strength and refining crystal grains. Among them, Zn is an element that contributes to strength improvement through aging of the alloy, and its content is 0.03.
If the amount is less than 1.5%, the above effect is insufficient, and if it exceeds 1.5%, the corrosion resistance is reduced. Therefore, the amount of Zn added is set in the range of 0.03 to 1.5%.

Further, Mn, Cr, Zr, and V are elements that are effective in improving strength, miniaturizing crystal grains, and stabilizing the structure. It is also effective in suppressing the generation of coarse particles on the surface. If the content of any of the elements is less than 0.03%, the above effects cannot be sufficiently obtained. Mn, Cr, Zr
When V exceeds 0.1% and V exceeds 0.1%, not only the above effects are saturated, but also a coarse intermetallic compound is formed, the deformation resistance increases, and the extrudability deteriorates. I will. Therefore, Mn is in the range of 0.03-0.2%, Cr is in the range of 0.03-0.2%, Zr is in the range of 0.03-0.2%, and V is in the range of 0.03-0.1%. .

Further, Fe is also an element effective for improving the strength and refining the crystal grains. When the content is 0.03%, a sufficient effect cannot be obtained. It may decrease. Therefore, the added amount of Fe is set in the range of 0.03 to 0.3%. Regarding Ti,
It is an element effective for improving the strength and refining the cast structure of the billet. If the addition amount is less than 0.005%, a sufficient effect cannot be obtained. If the addition amount exceeds 0.1%, not only the above effect is saturated, Since there is a possibility that coarse crystals are formed, the amount of addition is set to 0.005 to 0.1%.

Incidentally, these Zn, Mn, Cr, Zr,
The component ranges of V, Fe, and Ti are shown for the case where these elements are contained as aggressive additive elements, and it is particularly troublesome that any of them contains an amount smaller than the lower limit as an impurity. There is no. Other than the above elements, Al and unavoidable impurities may be basically used.

However, in general, a small amount of Be may be added to the Mg-containing alloy to prevent the oxidation of the molten metal. However, even in the case of the alloy of the present invention, Be is added during melting and casting of the billet for extrusion. Even 0.0001-0.05
% Is acceptable. Next, the manufacturing conditions of the present invention for realizing the above-described grain boundary precipitation will be described.

The process up to the hot extrusion may be carried out according to a conventional method for an aluminum alloy. In the present invention, the cooling rate after hot extrusion or after solution treatment is important. In the case of the T5 material subjected to the artificial aging treatment after cooling from the hot extrusion, the cooling after the hot extrusion is controlled so that grain boundary precipitation that covers the crystal grain boundary by 10% or more does not occur.

T6 which is subjected to an artificial aging treatment after the solution treatment
In the case of a material, the cooling after the solution treatment may be controlled so that grain boundary precipitation that covers 10% or more of the crystal grain boundary does not occur, similarly to the T5 material. Although the cooling rate that can be suppressed to the grain boundary precipitation state of the present invention varies depending on the alloy component, in a commonly known aluminum alloy, a temperature range from the solution treatment temperature to 200 ° C. or from the hot extrusion outlet temperature to 200 ° C. It can be realized by cooling at a cooling rate of 1 ° C./sec or more in a temperature range of ° C. The higher the cooling rate, the greater the effect of suppressing grain boundary precipitation,
It is also excellent in securing the degree of supersaturation of solute elements, and is advantageous in improving the strength of extruded profiles. However, depending on the cross-sectional shape of the extruded profile, if the quenching is performed at a high cooling rate, the profile may be bent after extrusion, which may cause a problem in shape. Therefore, the cooling rate is preferably about 30 ° C./sec or less.

Further, even when the above-mentioned cooling rate conditions cannot be ensured, grain boundary precipitates that once cover the grain boundaries by 10% or more are formed during the cooling process.
By performing the heat treatment at a temperature of 60 ° C. or more for 1 hour or more, the grain boundary precipitates are coarsened, and the ratio of the grain boundary precipitates covering the crystal grain boundaries can be reduced to 10% or less. However, a long-time heat treatment at a high temperature may result in an excessive decrease in strength. In addition, the heat treatment at a low temperature has a relatively small effect of coarsening the grain boundary precipitates, and it takes a long time to reach the grain boundary precipitation state within a specified range, which may be undesirable in terms of manufacturing cost. Therefore, the upper limit of the heat treatment temperature and time is preferably 210 ° C. or less and 100 hours or less, respectively. An appropriate heat treatment may be performed within this range according to the alloy components and the state of grain boundary precipitation. This heat treatment can also serve as an artificial aging treatment in the production of T5 and T6 materials. In this case, the heat treatment may be performed for a longer time than a normal artificial aging treatment time.

[0022]

EXAMPLE An aluminum alloy having an alloy composition shown in Table 1 was melted by a conventional method to cast a 200 mm diameter billet for hot extrusion. In Comparative Examples B6, B11, and B12, which were out of the component range of the present invention, cracks occurred during casting. After performing a homogenization treatment at 500 ° C. for each billet that could be cast soundly, extrusion molding was performed to a flat plate shape having a width of 40 mm and a thickness of 3 mm at an extrusion temperature of 500 ° C. and an extrusion speed of 10 m / min. went.

Although all of the alloys of the present invention could be extruded, the alloys B3, B5, B8, B9, and B10 of the comparative examples, which are out of the component range of the present invention, have high deformation resistance and hot extrusion. Each extruded material that could not be processed and was extruded was subjected to a solution treatment at 530 ° C. for 1 hour, and then the temperature range from 530 ° C. to 200 ° C. is shown in Table 2. Cool at the cooling rate. The sample cooled at a cooling rate of 1 ° C./sec or more was subsequently subjected to an artificial aging treatment at 175 ° C. for 8 hours.

The samples cooled at a cooling rate of less than 1 ° C./s were subjected to only the artificial aging treatment at 175 ° C. for 8 hours and the heat treatment for coarsening the grain boundary precipitates is shown in Table 2. The following conditions were evaluated by producing the samples performed under the conditions shown. Each specimen was subjected to a tensile test, and the fatigue strength (MPa) was measured by a rotating bending fatigue test.
The evaluation was performed 108 times under the condition of 10,000 rpm. The rate at which the grain boundary precipitates cover the grain boundaries is determined by using a transmission electron microscope to photograph five fields of bright field images of about 5000 times in any field and observing the grain boundaries observed in that range. It was calculated by measuring the number and size of the precipitates.

As is clear from Table 2, all of the alloys of the present invention have a fatigue strength of 90 MPa or more.
It can be seen that the alloy has better fatigue properties than the alloy of the comparative example.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

As is apparent from the above, according to the present invention, it is possible to obtain an extruded aluminum alloy material for an automobile body structure having excellent fatigue strength, which greatly contributes to weight reduction of an automobile. Therefore, it can be said that the industrial value of the present invention is extremely high.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a transmission electron micrograph of a grain boundary precipitate.

[Explanation of symbols]

 1: Crystal grains of aluminum alloy 2: Crystal grain boundaries 3: Grain boundary precipitates

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C22F 1/00 630 C22F 1/00 630G 631 631Z 683 683 691 691B 691C 692 692A 692B

Claims (5)

[Claims] 1. An extruded aluminum alloy material for an automobile body structure having excellent fatigue strength, wherein a ratio of a precipitate on a crystal grain boundary covering an interface of the crystal grain is 10% or less. 2. In% by weight, Mg: 0.3-1.5%, Si: 0.4-2.0%, Mg + Si: 2.4% or less Cu: Less than 0.8%, balance: In the extruded aluminum alloy material composed of Al and unavoidable impurities, the ratio of the precipitate on the crystal grain boundary covering the crystal grain interface is 1
An extruded aluminum alloy material for an automobile body structure having excellent fatigue strength characterized by being 0% or less. 3. Weight%: Zn: 0.03 to 1.5%, Mn: 0.03 to 0.2%, Cr: 0.03 to 0.2%, Zr: 0.03 to 0.3%. 2%, V: 0.03 to 0.1%, Fe: 0.03 to 0.3%, and Ti: 0.005 to 0.1%. Item 4. An extruded aluminum alloy material for an automobile body structure having excellent fatigue strength according to item 2. 4. Immediately after hot extrusion or solution treatment,
4. The method for producing an aluminum alloy extruded member having excellent fatigue strength according to claim 1, wherein a cooling rate in a temperature range up to 0 ° C. is 1 ° C./second or more. . 5. The fatigue strength according to claim 1, wherein after cooling to room temperature after hot extrusion or solution treatment, heat treatment is performed at a temperature of 160 ° C. or more for 1 hour or more. Of extruded aluminum alloy for automobile body structure excellent in quality.