JP2022156481A - Aluminum alloy extruded material and manufacturing method thereof - Google Patents

Abstract

To provide a 6,000 series aluminum alloy extruded material that is a raw material of a structural member and has improved strength and collapse resistance compared with conventional ones, and a manufacturing method of the 6,000 series aluminum alloy extruded material.SOLUTION: An aluminum alloy extruded material is provided, containing Si: 0.4-1.3% (mass%, same hereinafter), Mg: 0.3-0.9%, Mn: 0.4-0.6%, Cu: less than 0.05%, and the balance of Al and inevitable impurities, wherein the Si content and the Mg content satisfy following expressions (1) and (2), the material has a fibrous internal structure, and wherein a 0.2% proof stress (MPa) and a bending angle (degree) in a VDA bending test satisfy the following expression (3). ([Mg%]-0.15)×([Si%]-0.25)≥0.113 (expression 1), ([Mg%]-0.45)×([Si%]-0.63)≤0.120 (expression 2), [bending angle]≥-[0.2% proof stress]+365 (expression 3).SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to an aluminum alloy extruded material which is used for structural members of automobiles and the like and has excellent strength and bendability, and a method for producing the same.

In recent years, social demands for weight reduction of automobile bodies have been increasing more and more in consideration of the global environment. In order to meet these demands, conventional steel materials such as steel plates are used for parts such as panels (outer panels such as hoods, doors, and roofs, inner panels) and reinforcing materials such as bumper reinforcements and door beams in automobile bodies. Instead, aluminum alloy materials have been applied.

In order to further reduce the weight of automobile bodies, we will expand the application of aluminum alloy materials to members such as side members, frames, and automobile structural members such as pillars, which particularly contribute to weight reduction. It is necessary to However, compared to the above-mentioned automobile panel materials, these automobile structural members require higher strength of the material plate, shock absorption in the event of a car body collision, and crush resistance (crushing resistance, crushing characteristics) that leads to the protection of the occupants. ) as a new property.

Due to the recent upgrade (tightening) of automobile collision safety standards, in Europe, etc., the automotive structural members such as frames and pillars are standardized by the German Automobile Manufacturers Association (VDA) "VDA238-100 Plate It is now required to meet the crushing properties (crushing resistance, impact absorption) at the time of automobile collision, which are evaluated by "bending test for metallic materials (hereinafter referred to as "VDA bending test")".

6000-series alloys that have been evaluated for crush resistance by a VDA bending test include, for example, a sheet material described in Patent Document 1. Patent document 1 has a bending angle of 90° or more and a 0.2% yield strength of 250 MPa or more according to a VDA bending test, and satisfies certain required properties as a structural member. There is a demand for higher strength and improved crush resistance.

Moreover, among structural members, side sills, for example, have complicated cross-sectional shapes and cannot be manufactured by press molding or the like using plate materials as raw materials. Therefore, an extruded material is suitable for obtaining a long product with a complicated cross-sectional shape, but there are few extruded materials that satisfy the crushing property according to the VDA standard. For example, Patent Document 2 proposes a 6000 series alloy extruded material used for automobile frame materials, etc., but it is characterized by no cracks under bending conditions, and there is no description of crush resistance.

JP 2017-125240 A Japanese Patent Application No. 5-171328

Structural members such as side sills require an extruded material that satisfies such demands for further strength and crush resistance.

The present invention has been made in view of such a background, and aims to provide a 6000 series aluminum alloy extruded material that is a material for structural members with improved strength and crush resistance compared to conventional ones, and a method for producing the same. be.

One aspect of the present invention is Si: 0.4 to 1.3% (% by mass, the same applies hereinafter), Mg: 0.3 to 0.9%, Mn: 0.4 to 0.6%, Cu: 0 containing less than 0.05%, the balance being Al and unavoidable impurities, and having a chemical component composition in which the Si content and Mg content satisfy the following formulas 1 and 2,
The internal tissue consists of fibrous tissue,
The aluminum alloy extruded material has a 0.2% yield strength (MPa) and a bending angle (degrees) in the VDA bending test that satisfies the following formula 3.
([Mg%]-0.15) × ([Si%]-0.25) ≥ 0.113 (Formula 1)
([Mg%]-0.45) × ([Si%]-0.63) ≤ 0.120 (Formula 2)
[Bending angle] ≥ - [0.2% proof stress] + 365 (Formula 3)

The aluminum alloy extruded material has the specific chemical composition, controls the contents of Mg and Si so as to satisfy the formulas 1 and 2, and optimizes the range of the Mn content to form a fibrous By obtaining a structure, bendability (= crush resistance) is improved. This makes it possible to obtain an extruded material suitable for structural members such as automobiles.

2 is an optical micrograph of the metallographic structure of Example 2. FIG. A metal structure photograph of Comparative Example 12. Optical micrograph of a cross section showing the corrosion test results of Example 22. Optical micrograph of a cross section showing the corrosion test results of Example 23. An optical micrograph of a cross section showing the corrosion test results of Comparative Example 24. FIG. An optical micrograph of a cross section showing the corrosion test results of Comparative Example 25. FIG.

First, the chemical composition will be described.

Si: 0.4-1.3%;
Si (silicon) coexists with Mg to form Mg ₂ Si and is effective in improving the strength. If the Si content is less than 0.4%, the effect cannot be obtained, and if it exceeds 1.3%, grain boundary precipitation of Si increases, toughness decreases, and extrudability decreases. Therefore, the Si content should be in the range of 0.4 to 1.3%, preferably in the range of 0.6 to 1.3%, more preferably in the range of 0.7 to 1.3%.

Mg: 0.3-0.9%;
Mg (magnesium) is effective in improving strength together with Si. If the Mg content is too low, a sufficient effect of precipitation strengthening cannot be obtained, and if the Mg content is too high, the deformation resistance during extrusion increases and the extrudability deteriorates. Therefore, the Mg content should be in the range of 0.3 to 0.9%, preferably in the range of 0.3 to 0.7%, more preferably in the range of 0.3 to 0.6%.

Mn: 0.4-0.6%;
Mn (manganese) crystallizes as an AlMnSi phase, and uncrystallized Mn precipitates and has the effect of suppressing recrystallization during extrusion. Due to this effect, the structure after hot extrusion processing can be made into a fibrous structure, and thus both high strength and improved bendability can be achieved. If the Mn content is too low, the effect of suppressing recrystallization cannot be obtained, and the recrystallized structure becomes coarse, resulting in a decrease in strength. On the other hand, if the Mn content is too high, the hardenability is significantly lowered, and the effect of improving the strength cannot be obtained. Therefore, the Mn content should be in the range of 0.4 to 0.6%.

Cu: less than 0.05% (not including 0%);
Cu (copper) has the effect of improving the strength by solid solution strengthening, but it may lower the corrosion resistance, so it is restricted to less than 0.05%. Cu has a higher natural potential than aluminum and promotes local corrosion of aluminum. Incidentally, since Cu may be contained as an impurity, it is difficult to completely reduce it to 0%.

other elements;
Other elements such as Ti, B, Fe, Zn, and V are unavoidable impurities, and are allowed to be contained within the range specified by JIS standards for 6000 series alloys. For example, Ti is 0.10% or less, B is 0.10% or less, Fe is 0.15% or less, Zn is 0.10% or less, and V is 0.10% or less as inevitable impurities. is allowed.

range of Mg, Si;
Mg and Si must satisfy Formulas 1 and 2 below. These formulas define Mg and Si in the 6000 series aluminum alloys in order to keep the strength within a certain range. If the formula 1 is not satisfied, the strength is low and it is not suitable as a structural member. Moreover, when the formula 2 is not satisfied, the strength is too high and the bendability is deteriorated. Therefore, Mg and Si are required to be within the above ranges of ingredients and to satisfy these formulas (1) and (2).
([Mg%]-0.15) × ([Si%]-0.25) ≥ 0.09 (Formula 1)
([Mg%]-0.45) × ([Si%]-0.63) ≤ 0.120 (Formula 2)

<Manufacturing method>
As a method for producing the aluminum alloy extruded material, the following method can be adopted.
That is, the ingot having the chemical composition is homogenized,
The casting is subjected to hot extrusion processing to produce an extruded material,
After subjecting the extruded material to quenching treatment, tempering treatment is performed,
The quenching treatment is performed by quenching the extruded material immediately after the hot extrusion, or by reheating the extruded material cooled after the hot extrusion to a solution treatment temperature and then quenching,
The quenching is the method for producing the aluminum alloy extruded material, wherein cooling to at least 100° C. is performed at a cooling rate of 10° C./second or more. Further details are given below.

(casting)
In the melting and casting process, the aluminum alloy molten metal melted and adjusted within the range of the chemical composition is cast by appropriately selecting a normal melting and casting method such as semi-continuous casting (DC casting), and cast into a billet. to make.

(homogenization treatment)
The homogenization process homogenizes the structure, that is, eliminates segregation within the crystal grains in the ingot structure, and finely precipitates the Al-Mn-Si compound, thereby stabilizing the fibrous structure after hot working. intended to The conditions for this homogenization treatment are a temperature range of 450-590° C. and a holding time of 5-24 hours. If the heating temperature of the homogenization treatment is lower than 450 ° C., the segregation layer in the ingot will not be homogenized, and the precipitation of the Al-Mn-Si-based compound will be insufficient, so that after hot working structure becomes heterogeneous. On the other hand, if the temperature is higher than 590° C., the ingot may melt locally, making substantial production difficult.

(Hot extrusion processing)
The homogenized ingot is heated to a temperature of 450 to 550° C., and hot extrusion is performed while the temperature is maintained. When the ingot temperature is lower than 450°C, the extrusion load becomes high, making extrusion difficult. On the other hand, if the temperature is higher than 550°C, there is a possibility that hot cracking occurs and crystallized substances and precipitates become coarse.

(Quenching treatment)
After extrusion, the extruded material is quenched. Specifically, this quenching treatment is either a method of quenching the extruded material immediately after hot extrusion or a method of reheating the extruded material cooled after hot extrusion to the solution treatment temperature and then quenching it. can be done by Regardless of which method is employed, the rapid cooling must be performed at a cooling rate of 10°C/second or more to at least 100°C.

The method of quenching the extruded material immediately after hot extrusion is applicable, for example, when the temperature of the extruded material immediately after hot extrusion is within the range of 500 to 560°C. As a method of quenching, there is a method of water cooling or forced air cooling of the extruded material on the delivery side of the extruder (press quenching).

Moreover, regardless of the temperature of the extruded material after hot extrusion processing, a method of heating the extruded material cooled by an arbitrary method to 500 to 560° C. again and then rapidly cooling it can be selected. Cooling methods include water cooling and forced air cooling.

In any of the above methods, water quenching is suitable for quenching. This makes it possible to ensure that the cooling rate in cooling to at least 100° C. is at least 10° C./sec. If the cooling rate is less than 10°C/sec, precipitation of solid solution elements progresses during cooling, and the subsequent age hardening ability decreases, making it difficult to ensure sufficient strength and crush resistance. becomes.

The temperature of the solution treatment is preferably 500 to 560° C. as described above. If the temperature is 500° C. or less, re-dissolution of compounds such as Mg—Si compounds generated before the solution treatment becomes insufficient, and the amount of dissolved Mg and the amount of dissolved Si decrease, resulting in a decrease in strength. . A setting exceeding 560° C. is not preferable because fine precipitates containing transition metals may become coarse, and a fibrous structure may not be obtained.

(Tempering treatment (artificial aging treatment))
Next, the extruded material after quenching is subjected to a tempering treatment (artificial aging treatment) by heating and holding at, for example, 150 to 200° C. for 5 to 24 hours. Appropriate conditions such as the desired strength and the like are selected for the temperature and holding time of the aging treatment. Sufficient strength cannot be obtained when the aging temperature is low or the aging time is short. On the other hand, when the aging temperature is high or the aging time is long, the precipitates grow coarsely and the strength decreases.

(Microstructure)
By adopting the manufacturing method including the heat treatment, the aluminum alloy extruded material has a fibrous structure in which the average aspect ratio of the crystal grains is 3.5 or more due to the effect of suppressing recrystallization due to the addition of Mn. becomes possible. If the Mn content is out of the proper range, the resulting extruded material will not have a fibrous structure but will have a recrystallized structure.

To determine the fibrous structure, when measuring the aspect ratio of the crystal grains, first, using a sample provided with a cross section (L-ST cross section) parallel to the extrusion direction of the aluminum alloy material, hydrofluoric acid etching treatment is performed. An image of the cross section is obtained by observing the cut cross section in a bright field or by observing the anodized cross section in polarized light.

Next, in the obtained image, the aspect ratio is measured for crystal grains in the range of 1/4t to 3/4t, where t is the thickness of the material. When the crystal grain length in the direction parallel to the extrusion direction is X, and the crystal grain length in the direction (thickness direction) perpendicular to the extrusion direction is Y, the aspect ratio is X/Y. When measuring the aspect ratio, it is preferable to observe about three fields of view for one sample. At this time, it is preferable to analyze an image including an alloy material having a length of at least 500 μm or more as one field of view.

The aspect ratio can be measured by using image analysis software or by directly measuring the image. When the image is directly measured, it is preferable to measure the aspect ratios of 10 or more crystal grains per field of view and take the average aspect ratio. Regarding the crystal grains in the field of view, those with a maximum value of the crystal grain length Y in the thickness direction of 100 μm or less and an average aspect ratio of 3.5 or more are called a fibrous structure, and those of less than 3.5 are called a recrystallized structure. to decide.

In some test materials, linear grains are observed, making it difficult to measure the aspect ratio by image analysis. For such test materials, the fibrous structure is judged based on direct measurement by visual judgment using grain samples with an aspect ratio of 3.5.

Specifically, visual judgment will be made by creating a rectangular parallelepiped figure with an aspect ratio of 3.5. If the crystal grain to be measured is longer than the rectangular parallelepiped with an aspect ratio of 3.5, it is determined that the aspect ratio is 3.5 or more. Then, for the crystal grains in the field of view, when the maximum value of the crystal grain length Y in the thickness direction is 100 μm or less and the number of crystal grains with an aspect ratio of 3.5 or more is 10 or more per field, it is fibrous. Organization and judge.

(strength and crush resistance)
A 0.2% proof stress is used as a guideline for the strength required for structural members. The 0.2% yield strength is measured by performing a tensile test on the extruded material after aging treatment. In addition, the bending angle obtained by the VDA bending test is used as an index of crush resistance. If the 0.2% proof stress and the bending angle in the VDA bending test satisfies the following relational expression 3, it is judged to pass.
[Bending angle] ≥ - [0.2% proof stress] + 365 (Formula 3)

Examples of the aluminum alloy extruded material and the method for producing the same will now be described in detail, but the present invention is not limited to these examples.

[Experimental example 1]
In this example, molten aluminum alloys obtained by heating aluminum alloys having a plurality of chemical compositions shown in Table 1 were used as ingot billets with a diameter of 90 mm, homogenized at 500 to 590 ° C., and the billet heating temperature was 520. C. and an extrusion rate of 6 m/min to obtain an extruded flat plate material (test material) having a thickness of 1.5 mm and a width of 70 mm. Then, quenching was performed at the quenching rate shown in Table 1, and artificial aging treatment was performed at 185°C for 6 hours.

(microstructure observation)
After cutting out a cross section (L-ST cross section) parallel to the extrusion direction of the test material after aging treatment, the cross section was mirror-polished, etched by anodizing treatment, and then polarized light observation of the cross section with an optical microscope. Carried out. Aspect ratios were measured based on the observed images, and a fibrous structure was determined when the average aspect ratio was 3.5 or more, and a recrystallized structure was determined when the average aspect ratio was less than 3.5. For reference, the image of Example 2 is shown in FIG. 1 as an example of a fibrous structure, and the image of Comparative Example 12 is shown in FIG. 2 as an example of a recrystallized structure that does not correspond to a fibrous structure. shown as

(Tensile test)
A No. 5 test piece of JIS Z 2241:2011 was taken from the test material after the aging treatment, and a tensile test was performed at room temperature. The tensile direction of the test piece was parallel to the extrusion direction. The test method is JIS
Based on Z 2241:2011, the gauge length was set to 50 mm, the tensile speed was set to 2 mm/min at first, and set to 20 mm/min after the 0.2% yield strength measurement.

(VDA bending test)
A bending test for evaluating impact absorption was carried out according to the standard "VDA238-100 Plate bending test for metallic materials" of the German Automobile Manufacturers Association (VDA). The bending angle was calculated from the stroke at maximum load based on the formula described in the above standard.

If the 0.2% proof stress obtained by the above tensile test and the bending angle obtained by the VDA bending test satisfy the following formula 3, the product was judged to pass, that is, to satisfy the requirements as a structural member.
[Bending angle] ≥ - [0.2% proof stress] + 365 (Formula 3)

The results of each test are shown in Table 1.

Examples 1 to 6 use an aluminum alloy whose chemical composition is within the appropriate range shown in Table 1 and are quenched at an appropriate quenching rate. It has a fibrous structure with a ratio of 3.5 or more. Therefore, high strength and good bendability can be obtained.

In Comparative Examples 7 to 13, the contents of Mg, Si, and Cu are within appropriate ranges, but since Mn is not added, a fibrous structure is not formed, and the average aspect ratio is 3.5 as shown in FIG. It has a recrystallized structure of less than Although the 0.2% proof stress is relatively high at 250 to 270 MPa, the bending angle value is low due to the recrystallized structure, and Formula 3 is not satisfied.

In Comparative Examples 14 to 17, the contents of Mg and Si are within the proper range, but the amount of Cu added is larger than the proper range, and the content of Mn is less than the proper range. Since Cu is added, the 0.2% yield strength is 260 to 290 MPa, which is higher than that of Comparative Examples 7 to 13, but since the amount of Mn added is small, a recrystallized structure with an average aspect ratio of less than 3.5 is formed. The value of the bending angle is low and does not satisfy Equation 3.

In Comparative Example 18, the contents of Mg, Si, and Cu are within appropriate ranges, but a large amount of Mn is added. If the amount of Mn added is too large, the hardenability is remarkably lowered, so the 0.2% yield strength is as low as 137 MPa, and the formula 3 is not satisfied.

Comparative Examples 19 to 21 use aluminum alloys whose chemical components are within the proper range, but the quenching speed is slow. Therefore, the 0.2% proof stress is as low as 150 to 170 MPa, which does not satisfy Equation 3.

[Experimental example 2]
In this example, aluminum alloys (Examples 22 and 23 and Comparative Examples 24 and 25) having a plurality of chemical compositions shown in Table 2 were used, and the conditions shown in Table 2 were adopted for homogenization treatment and artificial aging treatment. , and other conditions were the same as in Experimental Example 1 to obtain an extruded flat plate material (test material). Examples 22 and 23 are in the preferred ranges for all major chemical components, while comparative examples 24 and 25 are examples with Cu content greater than 0.05%. In this example, in addition to the evaluation items in Experimental Example 1, the following corrosion test was conducted to examine the relationship between the Cu content and the corrosion resistance.

(Corrosion test)
A corrosion test was performed by evaluating intergranular corrosion resistance. The intergranular corrosion resistance evaluation test complied with ISO11846 Method B. The test materials are _each test material plate after solution treatment and artificial aging treatment. After being immersed for a minute, it was washed with 70% HNO ₃ and then with water, and dried at room temperature. An aqueous solution containing HCl and NaCl (containing 30 g/L of NaCl and 10±1 mL/L of 36% concentrated hydrochloric acid) was prepared as a corrosive solution, and 5 ml per cm ² of surface area of the material was corroded at 25° C. for 24 hours. The test material was immersed in the liquid. Next, corrosion products were removed by immersion in 70% HNO ₃ and brushing with a plastic brush, washed with water, and dried at room temperature. Subsequently, each cross section was observed with an optical microscope. By cross-sectional observation, when the maximum corrosion depth from the surface was 250 μm or less, the corrosion resistance was excellent and judged to be acceptable (○), and when the maximum corrosion depth exceeded 250 μm, the corrosion resistance was low and judged to be unacceptable (x). The evaluation results are shown in Table 2 together with other evaluation items.

3 to 6 show cross-sectional optical micrographs obtained by observing the results of the corrosion test for all the examples.

As can be seen from Table 2, all examples showed good results except for the corrosion test, but Examples 22 and 23 passed and Comparative Examples 24 and 25 failed in the corrosion test.

As shown in FIGS. 3 and 4, in Example 22 with a Cu content of less than 0.01% and Example 23 with a Cu content of 0.03%, no corrosion occurred after the corrosion test. The corrosion resistance was excellent.

On the other hand, as shown in FIGS. 5 and 6, in Comparative Example 24 with a Cu content of 0.07% and Comparative Example 25 with a Cu content of 0.10%, both Corrosion occurred with a maximum corrosion depth exceeding 250 μm from the surface, resulting in low corrosion resistance.

From the results of Experimental Example 2, it can be understood that limiting the Cu content to 0.05% or less is very important for improving corrosion resistance.

range of Mg, Si;
Mg and Si must satisfy Formulas 1 and 2 below. These formulas define Mg and Si in the 6000 series aluminum alloy so as to keep the strength within a certain range. If the formula 1 is not satisfied, the strength is low and it is not suitable as a structural member. Moreover, when the formula 2 is not satisfied, the strength is too high and the bendability is deteriorated. Therefore, Mg and Si are required to be within the above ranges of ingredients and to satisfy these formulas (1) and (2).
([Mg%]-0.15) × ([Si%]-0.25) ≥ 0.113 (Formula 1)
([Mg%]-0.45) × ([Si%]-0.63) ≤ 0.120 (Formula 2)

Claims (3)

Si: 0.4 to 1.3% (% by mass, same below), Mg: 0.3 to 0.9%, Mn: 0.4 to 0.6%, Cu: less than 0.05% , the balance being Al and unavoidable impurities, and having a chemical component composition in which the Si content and Mg content satisfy the following formulas 1 and 2,
The internal tissue consists of fibrous tissue,
An aluminum alloy extruded material in which the 0.2% proof stress (MPa) and the bending angle (degrees) in the VDA bending test satisfy the following formula 3.
([Mg%]-0.15) × ([Si%]-0.25) ≥ 0.113 (Formula 1)
([Mg%]-0.45) × ([Si%]-0.63) ≤ 0.120 (Formula 2)
[Bending angle] ≥ - [0.2% proof stress] + 365 (Formula 3) Homogenizing the ingot having the above chemical composition,
The casting is subjected to hot extrusion processing to produce an extruded material,
After subjecting the extruded material to quenching treatment, tempering treatment is performed,
The quenching treatment is performed by quenching the extruded material immediately after the hot extrusion, or by reheating the extruded material cooled after the hot extrusion to a solution treatment temperature and then quenching,
The method for producing an aluminum alloy extruded material according to claim 1, wherein the rapid cooling is performed at a cooling rate of 10°C/second or more to at least 100°C. 3. The method for producing an aluminum alloy extruded material according to claim 2, wherein the rapid cooling is performed by water quenching.

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