JP7634408B2 - Aluminum alloy extrusion material and its manufacturing method - Google Patents

Description

The present invention relates to an aluminum alloy extrusion material with excellent strength and bendability that is used for structural components of automobiles and other products, and to a method for manufacturing the same.

In recent years, there has been an increasing social demand for lighter automobile bodies due to considerations of the global environment. In order to meet this demand, aluminum alloy materials are being used in place of steel materials such as steel plates in parts of automobile bodies, such as panels (outer and inner panels such as hoods, doors, and roofs) and reinforcing materials such as bumper reinforcements and door beams.

To further reduce the weight of automobile bodies, it is necessary to expand the use of aluminum alloy materials to automobile structural components such as side members, frames, and pillars, which are particularly important components for reducing weight. However, compared to the above-mentioned automobile panel materials, these automobile structural components need to be endowed with new properties such as even higher strength plate strength and crush resistance (crush resistance, crush characteristics) that will help absorb shock during a vehicle collision and protect passengers.

Due to the recent increase in the level of automobile collision safety standards (stricter standards), in Europe and other regions, automobile structural components such as the frames and pillars are now required to meet the crushing characteristics (crushing resistance, shock absorption) during an automobile collision, as evaluated by the "VDA238-100 Plate bending test for metallic materials (hereinafter referred to as the VDA bending test)" standardized by the German Association of the Automotive Industry (VDA).

To date, evaluations of the crush resistance of 6000 series alloys using the VDA bending test have been carried out, such as the plate material described in Patent Document 1. Patent Document 1 satisfies certain required characteristics as a structural member, with a bending angle of 90° or more and a 0.2% yield strength of 250 MPa or more in the VDA bending test, but in recent years, there has been a demand for even higher strength and improved crush resistance in structural members for automobiles, etc.

Furthermore, structural components such as side sills have complex cross-sectional shapes and cannot be manufactured by press molding using sheet material. For this reason, extrusion materials that can be used to obtain long products with complex cross-sectional shapes are suitable, but there are few extrusion materials that satisfy the crushing properties required by the VDA standard. For example, Patent Document 2 proposes a 6000 series alloy extrusion material for use in automobile frame materials, but it is characterized by the absence of cracks under bending conditions and makes no mention of crushing resistance.

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

Extrusion materials that meet the demands for even higher strength and crush resistance are needed for structural components such as side sills.

The present invention was made in view of this background, and aims to provide a 6000 series aluminum alloy extrusion material that can be used as a structural component material with improved strength and crush resistance compared to conventional materials, and a method for manufacturing the same.

One aspect of the present invention is a steel sheet having a chemical composition comprising: Si: 0.6 to 1.3% (mass%, the same applies below), Mg: 0.3 to 0.6% , Mn: 0.4 to 0.6%, Cu: less than 0.05%, with the balance being Al and unavoidable impurities, and further comprising a Si content and a Mg content which satisfy the following formula 1 and formula 2:
The internal tissue is made of fibrous tissue,
The 0.2% yield strength is 240 MPa or more,
The 0.2% proof stress (MPa) and the bending angle (degrees) in the VDA bending test are in an aluminum alloy extrusion material 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% yield strength] + 365 (Equation 3)

The aluminum alloy extrusion material has the specific chemical composition described above, and the Mg and Si contents are controlled so as to satisfy formulas 1 and 2, and the range of Mn content is optimized to obtain a fibrous structure, thereby improving bendability (= crush resistance). This makes it possible to obtain an extrusion material suitable for structural members of automobiles, for example.

4 is an optical microscope photograph of the metal structure of Example 2. Metal structure photograph of Comparative Example 12. 3 is an optical microscope photograph of a cross section showing the corrosion test results of Example 22. 4 is an optical microscope photograph of a cross section showing the corrosion test results of Example 23. 4 is an optical microscope photograph of a cross section showing the corrosion test results of Comparative Example 24. 4 is an optical microscope photograph of a cross section showing the corrosion test results of Comparative Example 25.

First, we will explain the chemical composition.

Si: 0.4-1.3%;
Si (silicon) coexists with Mg to form Mg ₂ Si, which is effective in improving strength. If the Si content is less than 0.4%, this effect cannot be obtained, and if it exceeds 1.3%, grain boundary precipitation of Si increases, and toughness and extrudability decrease. 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%, and 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, the effect of precipitation strengthening is not sufficiently obtained, and if the Mg content is too high, the deformation resistance during extrusion increases, resulting in poor extrudability. 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%, and more preferably in the range of 0.3 to 0.6%.

Mn: 0.4-0.6%;
Mn (manganese) crystallizes as an AlMnSi phase, and Mn that does not crystallize precipitates and has the effect of suppressing recrystallization during extrusion. This effect allows the structure after hot extrusion to be fibrous, and this makes it possible to achieve both high strength and improved bendability. If the Mn content is too low, the effect of suppressing recrystallization cannot be obtained, the recrystallized structure becomes coarse, and strength decreases, and since a fibrous structure cannot be obtained, bendability decreases. On the other hand, if the Mn content is too high, the hardenability decreases significantly, and the effect of improving strength cannot be obtained. Therefore, the Mn content is set to a range of 0.4 to 0.6%.

Cu: less than 0.05% (excluding the case of 0%);
Although Cu (copper) has the effect of improving strength through solid solution strengthening, it is regulated to less than 0.05% because it may reduce corrosion resistance. Cu has a higher natural potential than aluminum, and promotes local corrosion of aluminum. Note that Cu can be contained as an impurity, so it is difficult to reduce it to 0%.

Other elements:
Other elements such as Ti, B, Fe, Zn, and V are unavoidable impurities, and the inclusion of each of these elements is permitted for 6000 series alloys within the ranges specified by JIS standards, etc. For example, the inclusion of Ti is permitted to be up to 0.10%, B is permitted to be up to 0.10%, Fe is permitted to be up to 0.15%, Zn is permitted to be up to 0.10%, and V is permitted to be up to 0.10% as unavoidable impurities.

Mg, Si range;
Mg and Si must satisfy the following formulas 1 and 2. These formulas define Mg and Si in 6000 series aluminum alloys in order to keep the strength within a certain range. If formula 1 is not satisfied, the strength is low and the alloy is not suitable for use as a structural member. If formula 2 is not satisfied, the strength is too high and the bendability is poor. Therefore, Mg and Si must be within the above-mentioned component range and must satisfy formulas 1 and 2.
([Mg%]-0.15)×([Si%]-0.25)≧ 0.113 (Formula 1)
([Mg%]-0.45)×([Si%]-0.63)≦0.120 (Formula 2)

<Production Method>
The following method can be used to manufacture the aluminum alloy extrusion material.
That is, an ingot having the above-mentioned chemical composition is subjected to a homogenization treatment,
The casting is subjected to hot extrusion to produce an extruded material;
The extruded material is subjected to a quenching treatment and then a tempering treatment,
The quenching treatment is performed by quenching the extruded material immediately after the hot extrusion process, or by reheating the extruded material cooled after the hot extrusion process to a solution treatment temperature and then quenching the same,
The rapid cooling is a method for producing the aluminum alloy extrusion material, in which cooling is performed to at least 100° C. at a cooling rate of 10° C./sec or more. The method will be described in more detail below.

(casting)
In the melting and casting process, the molten aluminum alloy that has been melted and adjusted to have the above-mentioned chemical composition range is cast into a billet by appropriately selecting a normal melting and casting method such as semi-continuous casting (DC casting).

(Homogenization treatment)
The homogenization treatment is intended to homogenize the structure, that is, to eliminate segregation within the crystal grains in the ingot structure, and to stabilize the fibrous structure after hot working by finely precipitating Al-Mn-Si compounds. The homogenization treatment is performed at a temperature range of 450 to 590°C for a holding time of 5 to 24 hours. If the heating temperature of the homogenization treatment is lower than 450°C, the segregation layer in the ingot is not homogenized, and further, the precipitation of Al-Mn-Si compounds becomes insufficient, resulting in a non-uniform structure after hot working. On the other hand, if the heating temperature is higher than 590°C, there is a risk of the ingot melting locally, making it difficult to actually manufacture the product.

(Hot extrusion processing)
The ingot after homogenization is heated to a temperature of 450 to 550°C, and hot extrusion is carried out while maintaining the temperature. If the ingot temperature is lower than 450°C, the extrusion load becomes high, making extrusion difficult. If the temperature is higher than 550°C, hot cracks may occur and the crystallized particles and precipitates may become coarse.

(Quenching treatment)
After the extrusion process, the extruded material is subjected to a quenching process. Specifically, the quenching process can be performed by either a method of quenching the extruded material immediately after the hot extrusion process, or a method of reheating the extruded material cooled after the hot extrusion process to a solution treatment temperature and then quenching it. In either case, the quenching is required to be performed at least to 100°C at a cooling rate of 10°C/sec or more.

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. Rapid cooling methods include water cooling or forced air cooling of the extruded material at the exit of the extruder (press quenching).

In addition, regardless of the temperature of the extruded material after hot extrusion, it is possible to select a method in which the extruded material, which has been cooled by any method, is heated again to 500 to 560°C and then rapidly cooled. Cooling methods include water cooling and forced air cooling.

In any of the above methods, rapid cooling is preferably performed by water quenching. This ensures that the cooling rate for 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 will progress during cooling, causing a decrease in the subsequent age hardening ability, making it difficult to ensure sufficient strength and crush resistance.

As mentioned above, the solution treatment temperature is preferably 500 to 560°C. If the temperature is below 500°C, the re-dissolution of compounds such as Mg-Si compounds that were formed before the solution treatment will be insufficient, resulting in a decrease in the amount of dissolved Mg and Si, and a decrease in strength. Setting the temperature above 560°C is not preferable, as it may cause the fine precipitates containing transition metals to become coarse, making it impossible to obtain a fibrous structure.

(Tempering treatment (artificial aging treatment))
Next, the extruded material after quenching is subjected to a tempering treatment (artificial aging treatment) in which the material is heated and held at 150 to 200°C for 5 to 24 hours. The temperature and holding time of the aging treatment are selected according to the desired strength, etc. If the aging temperature is low or the aging time is short, sufficient strength cannot be obtained. On the other hand, if the aging temperature is high or the aging time is long, the precipitates grow coarsely and the strength decreases.

(Microstructure)
By employing the manufacturing method including the above heat treatment, the aluminum alloy extrusion material can have a fibrous structure with an average aspect ratio of crystal grains of 3.5 or more due to the effect of suppressing recrystallization by the addition of Mn. If the Mn content is outside the above-mentioned appropriate range, the metal structure of the obtained extrusion material will not be a fibrous structure but a recrystallized structure.

When measuring the aspect ratio of crystal grains to determine the fibrous structure, first use a sample with a cross section parallel to the extrusion direction of the aluminum alloy material (L-ST cross section), and obtain an image of the cross section by observing the cross section etched with hydrofluoric acid in a bright field, or by observing the cross section anodized with 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. If the crystal grain length in the direction parallel to the extrusion direction is X and the crystal grain length in the direction perpendicular to the extrusion direction (thickness direction) is Y, then X/Y is the aspect ratio. When measuring the aspect ratio, it is preferable to observe about three fields of view for one sample. In this case, it is preferable to analyze an image that includes alloy material with a length of at least 500 μm as one field of view.

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

In addition, for some test materials, the crystal grains are observed to be linear, making it difficult to measure the aspect ratio by image analysis. For such test materials, whether or not they have a fibrous structure is determined based on direct measurement by visual judgment using a crystal grain sample with an aspect ratio of 3.5.

To explain visual judgment in detail, a rectangular parallelepiped with an aspect ratio of 3.5 is created and judged. If the crystal grains being measured are longer than a rectangular parallelepiped with an aspect ratio of 3.5, they are judged to have an aspect ratio of 3.5 or more. Then, for the crystal grains in the field of view, if the maximum value of the crystal grain length Y in the thickness direction is 100 μm or less and there are 10 or more crystal grains per field of view with an aspect ratio of 3.5 or more, it is judged to have a fibrous structure.

(Strength and crush resistance)
The 0.2% yield strength is used as a guideline for the strength required for structural members. The 0.2% yield strength is measured by conducting a tensile test on the extruded material after aging treatment. The bending angle obtained by the VDA bending test is used as a guideline for the crush resistance. A material is deemed to pass if the 0.2% yield strength and the bending angle in the VDA bending test satisfy the following relational expression 3.
[Bending angle] ≧-[0.2% yield strength] + 365 (Equation 3)

Next, we will explain in detail examples of the above aluminum alloy extrusion material and its manufacturing method, but the present invention is not limited to these.

[Experimental Example 1]
In this example, aluminum alloy molten metal obtained by heating aluminum alloys having the chemical composition shown in Table 1 was formed into an ingot billet having a diameter of 90 mm, which was homogenized at 500 to 590°C and extruded at a billet heating temperature of 520°C and an extrusion speed 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. Next, quenching was performed at the quenching speed shown in Table 1, and artificial aging treatment was performed at 185°C for 6 hours.

(Microstructure observation)
For the test material after aging treatment, a cross section parallel to the extrusion direction (L-ST cross section) was cut out, the cross section was mirror-polished, and etched by anodizing treatment, and then the cross section was observed with polarized light using an optical microscope. The aspect ratio was measured based on the observed image, and it was determined that the average aspect ratio was 3.5 or more was a fibrous structure, and that the average aspect ratio was less than 3.5 was a recrystallized structure. For reference, an image of Example 2 is shown in FIG. 1 as an example of a fibrous structure, and an image of Comparative Example 12 is shown in FIG. 2 as an example of a recrystallized structure that does not fall under the category of a fibrous structure.

(Tensile test)
From the test material after the aging treatment, a JIS Z 2241:2011 No. 5 test piece was taken, and a tensile test was carried out at room temperature. The tensile direction of the test piece was parallel to the extrusion direction. The test method was as follows:
In accordance with Z 2241:2011, the gauge length was 50 mm, and the tensile speed was initially set to 2 mm/min, and after the 0.2% proof stress measurement, it was set to 20 mm/min.

(VDA bending test)
The bending test for evaluating the impact absorption was carried out in accordance with the German Association of the Automotive Industry (VDA) standard "VDA238-100 Plate bending test for metallic materials". The bending angle was calculated from the stroke at the maximum load based on the formula described in the aforementioned standard.

A specimen in which the 0.2% proof stress obtained by the above tensile test and the bending angle obtained by the VDA bending test satisfied the following formula 3 was deemed to be acceptable, that is, to satisfy the requirements for a structural member.
[Bending angle] ≧-[0.2% yield strength] + 365 (Equation 3)

The results of each test are shown in Table 1.

In Examples 1 to 6, aluminum alloys whose chemical compositions are within the appropriate ranges shown in Table 1 are used, and quenching is performed at an appropriate quenching rate, so that the internal structure is a fibrous structure with an average aspect ratio of 3.5 or more, as shown in Figure 1. This allows for high strength and good bendability, and as shown in Table 2, the relationship between the 0.2% proof stress and the bending angle in the VDA bending test satisfies Equation 3.

In Comparative Examples 7 to 13, the contents of Mg, Si, and Cu are within the appropriate range, but since Mn is not added, the structure does not become fibrous, and the structure is a recrystallized structure with an average aspect ratio of less than 3.5 as shown in Figure 2. The 0.2% yield strength is relatively high at 250 to 270 MPa, but because it is a recrystallized structure, the bending angle value is low and formula 3 is not satisfied.

In Comparative Examples 14 to 17, the Mg and Si contents are within the appropriate range, but the Cu content is greater than the appropriate range, and the Mn content is less than the appropriate range. Because Cu is added, the 0.2% yield strength is 260 to 290 MPa, which is higher than Comparative Examples 7 to 13, but because the amount of Mn added is small, a recrystallized structure with an average aspect ratio of less than 3.5 is presented, the bending angle value is low, and formula 3 is not satisfied.

In Comparative Example 18, the contents of Mg, Si, and Cu are within the appropriate range, but a large amount of Mn is added. If too much Mn is added, the hardenability is significantly reduced, resulting in low strength with a 0.2% yield strength of 137 MPa, which does not satisfy formula 3.

Comparative examples 19 to 21 use aluminum alloys whose chemical composition is within the appropriate range, but the quenching speed is slow. As a result, the 0.2% yield strength is low at 150 to 170 MPa, and formula 3 is not satisfied.

[Experimental Example 2]
In this example, aluminum alloys having a plurality of chemical composition compositions shown in Table 2 (Examples 22 and 23 and Comparative Examples 24 and 25) were used, and the homogenization and artificial aging treatment conditions shown in Table 2 were adopted, with the other conditions being the same as those of Experimental Example 1, to obtain extruded flat plate materials (test materials). In Examples 22 and 23, all of the main chemical components were within the suitable range, while in Comparative Examples 24 and 25, the Cu content exceeded 0.05%. In this example, in addition to the evaluation items in Experimental Example 1, the following corrosion test was performed to examine the relationship between the Cu content and corrosion resistance.

(Corrosion Test)
The corrosion test was carried out by evaluating the intergranular corrosion resistance. The evaluation test of the intergranular corrosion resistance was in accordance with ISO11846 Method B. The test material was each test material plate after solution treatment and artificial aging treatment, and after degreasing with acetone, a cleaning solution in which 70% HNO ₃ and 48% HF were added to pure water was heated to 95 ° C., and immersed for 1 minute, washed with 70% HNO ₃ , rinsed with water, and dried at room temperature. As a corrosive solution, 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, and the above test material was immersed in 5 ml of the corrosive solution per 1 cm ² of the surface area of the material at 25 ° C. for 24 hours. Next, the corrosion products were removed by immersion in 70% HNO ₃ and brushing with a plastic brush, and then dried at room temperature after rinsing with water. Subsequently, each cross section was observed with an optical microscope. When the maximum corrosion depth from the surface was 250 μm or less, the specimen was judged to have excellent corrosion resistance and pass (○), and when the maximum corrosion depth was more than 250 μm, the specimen was judged to have poor corrosion resistance and fail (×). The evaluation results are shown in Table 2 together with other evaluation items.

In addition, optical microscope photographs of the cross sections in which the corrosion test results were observed for all examples are shown in Figures 3 to 6.

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

As shown in Figures 3 and 4, in Example 22, in which the Cu content was less than 0.01%, and Example 23, in which the Cu content was 0.03%, no corrosion occurred at all after the corrosion test, and the corrosion resistance was excellent.

On the other hand, as shown in Figures 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%, after the corrosion test, corrosion occurred to a maximum corrosion depth from the surface of more than 250 μm, resulting in low corrosion resistance.

The results of Experimental Example 2 show that limiting the Cu content to 0.05% or less is extremely important in improving corrosion resistance.

Claims (3)

The alloy has a chemical composition comprising: Si: 0.6 to 1.3% (mass%, the same applies below); Mg: 0.3 to 0.6% ; Mn: 0.4 to 0.6%; Cu: less than 0.05%; and the balance being Al and unavoidable impurities, and further comprising a Si content and a Mg content which satisfy the following formula 1 and formula 2,
The internal tissue is made of fibrous tissue,
The 0.2% yield strength is 240 MPa or more,
An aluminum alloy extrusion material in which the 0.2% proof stress (MPa) and the bending angle (degrees) in a 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% yield strength] + 365 (Equation 3) The ingot having the above chemical composition is subjected to homogenization treatment,
The casting is subjected to hot extrusion to produce an extruded material;
The extruded material is subjected to a quenching treatment and then a tempering treatment,
The quenching treatment is performed by quenching the extruded material immediately after the hot extrusion process, or by reheating the extruded material cooled after the hot extrusion process to a solution treatment temperature and then quenching the same,
The method for producing an aluminum alloy extrusion material according to claim 1, wherein the rapid cooling is performed to at least 100°C at a cooling rate of 10°C/sec or more. The method for producing an aluminum alloy extrusion material according to claim 2, wherein the rapid cooling is performed by water quenching.