Forming-optimized aluminum alloy strip and method for its production

Description

[0001] The invention relates to an aluminum alloy strip made of an aluminum alloy, a method for producing the aluminum alloy strip and its use.

[0002] In particular, aluminum magnesium (AlMg) alloys of type AA 5xxx are used in sheet or strip form for the construction of welded or joined components in shipbuilding, automotive, and aircraft construction. These aluminum magnesium alloys are characterized by high strength with increasing magnesium content and, at magnesium contents above 3%, by increasing formability. Therefore, aluminum magnesium alloys of type AA 5xxx can increasingly replace steel materials in automotive construction, for example, and can thus contribute to further weight reduction in vehicles.

[0003] AlMg alloys of type AA 5xxx with Mg contents of more than 3%, especially more than 4%, are increasingly prone to intergranular corrosion when exposed to elevated temperatures. At temperatures of 70–200°C, base Al 5 Mg 3 phases precipitate along the grain boundaries. These phases are referred to as β-particles and can be selectively dissolved in the presence of a corrosive medium. As a result, the aluminum alloy of type AA 5182 (Al 4.5% Mg 0.4% Mn), which exhibits very good strength properties and excellent formability, is not used in heat-stressed areas where the presence of a corrosive medium, such as water in the form of moisture, is expected.This particularly applies to automotive components, which are typically subjected to cathodic dip painting (CDP) and subsequent baking, as this baking process alone can cause sensitization to intergranular corrosion in conventional aluminum alloy strips. Furthermore, for use in the automotive sector, the forming process during component production and the subsequent operational stress on the component must be considered.

[0004] International patent application WO 2014 / 0298531 A1 discloses an aluminum alloy strip with magnesium contents above four wt.% and suitable for automotive components. Despite providing high strength, it exhibits very good resistance to intergranular corrosion. However, it has been shown that the formability of this aluminum alloy strip made from an AA 5182 aluminum alloy, which is particularly resistant to intergranular corrosion, can be improved.

[0005] A further development of this aluminum alloy strip with regard to formability is therefore known from international patent application WO 2014 / 029856 A1. Here, the aluminum alloy strip was optimized with regard to formability using a nearly identical alloy concept. Both international patent applications concern aluminum strips with a maximum Mg content of 4.50 wt.% within the specification of the AA 5182 aluminum alloy.

[0006] From the Japanese patent application JP 2001 303164 A, an aluminum alloy of type AA5xxx is known, the secondary phase densities of which were determined for secondary phases with a maximum length of at least 3 µm and more.

[0007] The international patent application WO 2016 / 207274 A1 discloses reannealed aluminum alloy strips and processes for their production.

[0008] From the US patent application US 2020 / 0157668 A1, soft-annealed aluminum alloy strips are known whose secondary phase density has been maximized for copper-containing secondary phases with an equivalent circular diameter of 0.3 µm to 4 µm.

[0009] It has now been determined that there is further potential for improvement in formability within the specification of the AA 5182 aluminum alloy, without compromising other properties such as providing the necessary strength and corrosion resistance. It has been found that the usual indicators for formability, such as uniform elongation Ag or elongation at break A 80mm, are not sufficiently meaningful for the practical use of aluminum alloy sheets in forming processes.

[0010] The DIN EN ISO 12004-2:2021-07 standard specifies test conditions that allow for the determination of permissible major and minor deformations of aluminum sheet during forming processes, thus ensuring a safe forming process. The major and minor deformations determined according to this standard are derived from the forming limit curve, which characterizes the specific behavior of the sheet to be formed in the drawing test. The forming limit curve is obtained through a deformation analysis of defective drawn parts to determine the deformation diagrams dependent on the drawn part and the forming process.

[0011] A deterministic grid with precise dimensions or a stochastic pattern is applied or optically projected onto the undeformed surface of specimens with a specific geometry. The specifically cut specimen is then deformed, for example, using the Nakajima method, using a defined punch in a precisely defined matrix until it breaks, after which the test is terminated. All values for the principal strain ε 1 stated in this document refer to testing using the Nakajima method according to EN ISO 12004-2:2021-07. The principal strain ε 1 is determined here on specimens with a width of 100 mm. All values given are averages of three specimens.

[0012] All other mechanical properties are measured according to DIN EN ISO 6892. Since the grain size of a material always exists in the form of a distribution, all grain size specifications refer to the mean grain size. The mean grain size can be determined according to ASTM E1382.

[0013] The present invention has for its object to provide an aluminum alloy strip, in particular for the production of body parts of a motor vehicle, preferably interior body parts, which, in addition to the necessary resistance to intergranular corrosion, provides the required strength and at the same time improved forming properties.

[0014] According to a first teaching of the present invention, the stated object is achieved by providing an aluminum alloy strip which comprises an aluminum alloy with the following composition in wt.%: Si ≤ 0 , 10 % , Fe ≤ 0 , 25 % , 0 , 20 % ≤ Mn ≤ 0 , 30 % 4 , 72 % ≤ Mg ≤ 4 , 95 % , Cu ≤ 0 , 10 % , Cr ≤ 0 , 02 % , Ni ≤ 0 , 01 % , Zn ≤ 0 , 10 % , Ti ≤ 0 , 04 % ,

[0015] Remainder Al with unavoidable impurities individually ≤ 0.05%, in total ≤ 0.15%, whereby the aluminum alloy strip has an average secondary phase density of less than 250 per 1000 µm 2<.

[0016] Through intensive investigations, the inventors have discovered that, for aluminum alloy strips comprising the aforementioned aluminum alloy, by limiting the secondary phase density to less than 250 per 1000 µm², an advantageous increase in formability can be achieved while simultaneously maintaining the advantages of the aluminum alloy type AA 5182 with regard to the corrosion resistance and mechanical strength of the aluminum alloy strip. The secondary phases typically present are Al 6 Mn, alpha-Al(Fe,Mn)Si, and Mg 2 Si. According to the inventors' findings, a high number of secondary phases limits the formability and is particularly noticeable in the complex deep-drawing processes used to manufacture, for example, car body components.By selecting a specific alloy composition in conjunction with equally specific manufacturing processes, the secondary phase density could be reduced to values below 250 per 1000 µm 2<.

[0017] The secondary phase density, i.e., the (area) density of the dispersoids, is determined in this document using light microscopy as follows. A sample of the aluminum alloy strip to be examined is embedded in a longitudinal section and prepared using conventional metallographic techniques. After grinding and polishing the section, the sample is etched for one minute at room temperature in a dilute aqueous solution of sulfuric acid and hydrofluoric acid. For this purpose, a solution of 100 cm³ of 10% concentrated sulfuric acid is mixed with 100 cm³ of another solution consisting of 60 cm³ of water and 40 cm³ of 5% hydrofluoric acid. After etching, the section is rinsed with distilled water and dried for subsequent light microscopic examination. The etching marks the secondary phases in the microstructure, allowing their areal density to be determined with good accuracy under the light microscope.To ensure sufficient statistical relevance, a minimum of 10 statistically distributed image sections are analyzed as measurement fields at high magnification (1000:1) using an oil objective with a light microscope, so that a total of at least 2000 secondary phases are recorded. The total number of secondary phases determined, relative to the entire measurement area of all examined measurement fields, then yields the areal density of the secondary phases or secondary phase density (expressed as number per area, e.g., number per 1000µm 2< ).

[0018] For this purpose, the silicon content of the alloy was reduced to a maximum of 0.10 wt.%. In magnesium-containing aluminum alloys, silicon forms alpha-Al(Fe,Mn)Si and Mg2Si precipitates as secondary phases. As previously discussed, these impair the formability of the aluminum alloy strip. A preferred silicon content is therefore a maximum of 0.08 wt.%.

[0019] Iron is predominantly bound in the so-called casting phases, but also participates in the formation of secondary precipitates. Therefore, reducing the iron content to a maximum of 0.25 wt.%, preferably to a maximum of 0.20 wt.%, contributes to improved formability.

[0020] Manganese is a typical dispersoid former, with the dispersoid particles effectively preventing dislocation movements of atoms from the metal crystal structure. Thus, dispersoids contribute to a desired increase in yield strength. Furthermore, Mn-containing dispersoids help control the grain size of the aluminum alloy strip. However, dispersoid particles limit the forming behavior. The aluminum alloy strip therefore has a Mn content of 0.20 wt.% to 0.30 wt. Below a Mn content of 0.20 wt.%, the strength-enhancing effect of the dispersoids is reduced, and the aluminum alloy strip may exhibit undesirable grain enlargement during heat treatment. At a manganese content of more than 0.30 wt.%, the dispersion severely impedes the elongation of the material, resulting in suboptimal forming behavior. A Mn content optimized for the aspects of forming behavior can be 0.20 wt.% ≤ Mn ≤ 0.26 wt.-% will be made available.

[0021] Magnesium is present in the aluminum alloy according to the invention in a concentration of 4.72 wt.% to 4.95 wt.%, preferably 4.80 wt.% to 4.92 wt.%. It has been found that precisely these magnesium contents not only achieve high strengths despite reduced proportions of strength-enhancing dispersoid formers, but also simultaneously improve the forming behavior. However, as explained above, higher Mg contents lead to excessive susceptibility of the material to intergranular corrosion.

[0022] To optimize forming behavior, the copper content has also been limited to a maximum of 0.10 wt.%. Copper increases the strength of the aluminum alloy strip even at low levels, but also leads to a deterioration in general corrosion behavior. Therefore, preferred copper contents are a maximum of 0.07 wt.%, particularly preferably at least 0.02 wt.% and less than 0.04 wt.%.

[0023] The alloying element chromium is a very effective dispersoid former and is therefore contained in the aluminum alloy at a maximum content of 0.02 wt.%, preferably 0.01 wt.% and particularly preferably a maximum of 0.008 wt.%.

[0024] The same applies to the nickel content due to the tendency to form dispersoid particles even at the lowest concentrations. The Ni content is therefore reduced to a maximum of 0.01 wt.%, preferably to 0.005 wt.%.

[0025] Zinc, which is contained in the aluminum alloy in a content of maximum 0.10 wt.%, preferably maximum 0.01 wt.%, particularly preferably maximum 0.008 wt.%, has a detrimental effect on the corrosion resistance of the aluminum alloy strip.

[0026] The titanium used for grain refinement in the melting process should be limited to a maximum of 0.04 wt.%, preferably a maximum of 0.02 wt.%, since titanium also forms dispersoids and is highly prone to segregation in larger concentrations. Since the titanium, which originates, for example, from grain refiners, supports the melting process and thus improves the casting of the rolled ingot, a titanium content of 0.005 wt.% to a maximum of 0.02 wt.% in the aluminum alloy is preferred. This titanium range allows a compromise between melting properties and the number of secondary precipitates to be achieved.

[0027] According to a first embodiment of the aluminum alloy strip, the aluminum alloy strip has a secondary phase density of less than 220 per 1000 µm², particularly preferably less than 200 per 1000 µm². It has been demonstrated that the selection of aluminum alloy elements in conjunction with the manufacturing process of the aluminum alloy strip can further reduce the secondary phase density in the aluminum alloy strip. These aluminum alloy strips exhibited a further improvement in forming behavior while simultaneously providing high mechanical strength and good corrosion resistance.

[0028] Aluminum alloy strip in the O or H111 microstructure exhibits excellent forming properties. The O microstructure is characterized by a recrystallized microstructure, which allows for maximum forming. In the H111 microstructure, the aluminum alloy strip has been slightly strengthened, for example, by stretching or straightening the aluminum alloy strip. The H111 microstructure is therefore preferred for processing aluminum alloy sheets, as the aluminum alloy sheets exhibit little distortion while still achieving particularly high forming values.

[0029] Further investigations have shown that the aluminum alloy strip according to a further embodiment has an average grain size of 15 µm to 30 µm. It has been found that the corrosion resistance of the aluminum alloy with the present alloy composition at grain sizes of 15 µm to 30 µm meets the requirements for car body applications. At the same time, the smaller grain sizes contribute to improved formability.

[0030] The aluminum alloy strip is preferably cold-rolled to provide the necessary dimensional accuracy and surface quality for the preferred application in automotive construction.

[0031] According to one embodiment, the final thickness of the cold-rolled aluminum alloy strip is 0.5 mm to a maximum of 4 mm, preferably 0.8 mm to 2.5 mm. Particularly in these thickness ranges, the aluminum alloy strip can provide significantly improved forming properties in conjunction with conventional forming processes and tools.

[0032] According to a further embodiment of the aluminum alloy strip, it has an Ae value transverse to the rolling direction of less than 1.0%, preferably less than 0.9%. The Ae value is also referred to as the yield point extensometer elongation. The Ae value is measured transverse to the rolling direction in accordance with DIN EN ISO 6892 and is given in %. The Ae value of an aluminum alloy strip is characteristic of the formation of Lüders lines during the forming of the aluminum alloy strip, which are undesirable, for example, in car body components. The smaller the Ae value, the fewer Lüders lines are created. With values of less than 1.0% or less than 0.9% transverse to the rolling direction, the aluminum alloy strip can be described as essentially free of Lüders lines.

[0033] Finally, a design of the aluminum alloy strip according to the invention, with a sheet thickness of 1.2 mm and a sample width b) of 100 mm according to DIN EN ISO 120004-2, exhibits an average principal strain ε1 of more than 0.200 in the Nakajima test. This principal strain value was achieved with the aluminum alloy strip according to the invention by adjusting the reduced secondary phase density while taking into account a manufacturing process tailored to the material. The principal strain ε1 with a sample width b) of 100 mm according to Nakajima reflects the complex interplay of the microstructure of the aluminum alloy strip during the drawing process in a single parameter and shows a significant improvement over the principal strain ε1 achieved previously for form-optimized aluminum alloy strips of type AA 5182.For all values given here, the specimen width of 100 mm refers to the value b) of a specimen with an axially parallel recess length a) according to Figure 2 of DIN EN ISO 120004-2 (6.1.2 Specimen geometry).

[0034] At the same time, according to a further embodiment, the aluminum alloy strip provides a yield strength Rp 0.2 transverse to the rolling direction of at least 115 MPa, preferably at least 120 MPa in the microstructure state O or H111, so that the strength requirements in automotive construction are also met by the forming-optimized aluminum alloy strip.

[0035] The mass losses of the aluminum alloy strip due to intergranular corrosion range from 13 mg / cm² to 19 mg / cm² after a heat load of 195 °C for 45 minutes, measured according to ASTM G67. This heat load corresponds to the maximum heat load the component can experience during a cathodic dip-painting process and thus indicates that corrosion problems are not to be expected during subsequent use of the component.

[0036] According to a further teaching of the invention, the aluminum alloy strip according to the invention is produced by a process comprising the following steps: Casting a rolling ingot from an aluminum alloy with the following composition: Si ≤ 0.10%, preferably ≤ 0.08%, Fe ≤ 0.25%, preferably ≤ 0.20%, 0.20% ≤ Mn ≤ 0.30%, preferably 0.20% ≤ Mn ≤ 0.26%, 4.72% ≤ Mg ≤ 4.95%, preferably 4.80% ≤ Mg ≤ 4.92%, Cu ≤ 0.10%, preferably Cu ≤ 0.07%, particularly preferably Cu < 0.04%, Cr ≤ 0.02%, preferably Cr ≤ 0.01%, particularly preferably Cr ≤ 0.008%, Ni ≤ 0.01%, preferably Ni ≤ 0.005%, Zn ≤ 0.10%, preferably Zn ≤ 0.01%, particularly preferably Zn ≤ 0.008%, Ti ≤ 0.04%, preferably Ti ≤ 0.02%, remainder Al with unavoidable impurities individually ≤ 0.05%, in total ≤ 0.15%, homogenization of the rolling ingot at 480 °C to 550 °C for at least 0.5 h, hot rolling of the rolling ingot to a final hot strip thickness of 3 to 6 mm, cold rolling of the aluminum alloy strip to final thickness with a degree of reduction of 40% to 60%, preferably 50% to 60% and soft annealing of the finished rolled aluminum alloy strip at more than 500 °C,preferably 510°C to 540 °C in a continuous furnace.

[0037] In addition to the particularly critical selection of the alloying constituents of the aluminum alloy, which are responsible for the secondary phase density, it has been found that, in conjunction with the alloy composition, the process characteristics mentioned and the selection of the degree of cold rolling to final thickness of 40% to 60% in conjunction with the soft annealing of the finish-rolled aluminum alloy strip at more than 500 °C, preferably at 510 °C to 540 °C in a continuous furnace, represent characteristics which ensure the provision of a low secondary phase density per 1000 µm 2<.

[0038] According to a further variant of the method according to the invention for producing the aluminum alloy strip, the following process steps are carried out alternatively after hot rolling: Cold rolling of the hot-rolled aluminum alloy strip to an intermediate thickness which is determined such that the final cold rolling degree of final thickness is 40% to 60%, preferably 50% to 60%, intermediate annealing of the aluminum alloy strip at 300 °C to 500 °C, cold rolling of the aluminum alloy strip to final thickness with a rolling degree of 40% to 60%, preferably 50% to 60%, soft annealing of the finish-rolled aluminum alloy strip at more than 500 °C, preferably 510 °C to 540 °C in a continuous furnace.

[0039] Regardless of whether the aluminum alloy strip was produced with or without intermediate annealing, it has been found that the final cold rolling to final thickness, combined with the required soft annealing in a continuous furnace, surprisingly produces a unique combination of properties for the aluminum alloy strip. At the same time, the soft annealing in the continuous furnace at the temperatures mentioned above results in grain sizes of 15 µm to 30 µm, which not only contribute to the surprisingly good corrosion resistance of the aluminum alloy strip produced in this way but also enhance its forming properties.

[0040] According to a further embodiment of the method according to the invention, the duration of the soft annealing of the finished aluminum alloy strip in the continuous furnace is between 5 seconds and 300 seconds, with an interval of 10 seconds to 60 seconds being preferred. At these times, complete recrystallization of the microstructure can already be achieved in the continuous furnace, with the duration also being adapted to the respective thickness of the strip.

[0041] According to a further embodiment of the process, the hot rolling of the rolling ingot consists of the steps of pre-rolling to a thickness of 30 mm to 40 mm at a starting temperature of at least 450 °C and finish rolling to the final hot strip thickness with a coiling temperature of 300 °C to 350 °C. It has been shown that by adhering to these parameters, hot rolling can be advantageously optimized with regard to providing a low secondary phase density and contributes to stable process control.

[0042] Finally, the aluminum alloy strip according to the invention is preferably used for the production of a body interior part, in particular a door interior part, a hood interior part, or a trunk lid interior part of a motor vehicle. Body interior parts are often subjected to complex forming processes in order to achieve specific strengths for the motor vehicle body structure. Therefore, body interior parts are also made from high-strength materials, such as the aluminum alloy in question. At the same time, however, they must also be capable of complex forming in order to produce the body interior parts from as few individual components as possible. This eliminates additional work steps related to joining technology, for example, joining or welding different components. At the same time, body interior parts are also exposed to corrosive conditions, so good corrosion resistance is also required.The aluminum alloy strip meets these conditions to a particularly high degree and is therefore predestined for this application.

[0043] Due to the optimized forming behavior of the aluminum alloy strip according to the invention without any loss in strength and corrosion resistance, the aluminum alloy strip is optimally suited for the production of complex-shaped interior body parts.

[0044] The invention will be described below using exemplary embodiments in conjunction with the drawings. The drawing shows in Fig. 1 is a schematic flow diagram of the manufacturing process for an aluminum alloy strip according to the invention, Fig. 2 is a diagram showing the secondary phase density / 1000µm 2< as a function of the measured main strain ε1 for a 100mm sample width according to Nakajima, Fig. 3 is a typical use of the aluminum alloy strip in the form of an inner door part, the so-called "body in white" of a motor vehicle, and Fig. 4 is an etched ground surface of an aluminum alloy strip according to the invention for evaluating the total number of secondary phases.

[0045] Fig. 1 schematically shows the process steps and the sequence of one exemplary embodiment of a method for producing aluminum alloy strip. In step 1, a rolling ingot made of an aluminum alloy with the following alloying components is cast, for example, in DC continuous casting: Si ≤ 0.10%, preferably ≤ 0.08%, Fe ≤ 0.25%, preferably ≤ 0.20%, 0.20% ≤ Mn ≤ 0.30%, preferably 0.20% ≤ Mn ≤ 0.26%, 4.72% ≤ Mg ≤ 4.95%, preferably 4.80% ≤ Mg ≤ 4.92%, Cu ≤ 0.10%, preferably Cu ≤ 0.07%, particularly preferably Cu < 0.04%, Cr ≤ 0.02%, preferably Cr ≤ 0.01%, particularly preferably Cr ≤ 0.008%, Ni ≤ 0.01%, preferably Ni ≤ 0.005%, Zn ≤ 0.10%, preferably Zn ≤ 0.01%, particularly preferably Zn ≤ 0.008%, Ti ≤ 0.04%, preferably Ti ≤ 0.02%, remainder Al with unavoidable impurities individually ≤ 0.05%, in total ≤ 0.15%.

[0046] The rolling ingot is then subjected to homogenization in process step 2, which can be carried out in one or more stages. During homogenization, the rolling ingot reaches temperatures of 480 to 550 °C for at least 0.5 h. In process step 3, the rolling ingot is then hot rolled. The final thickness of the hot strip is, for example, 3 to 6 mm. The final hot strip thickness can be selected so that after hot rolling, only one cold rolling step 4 follows, in which the hot strip is reduced in thickness to the final thickness with a reduction ratio of 40% to 60%, preferably 50% to 60%. The aluminum alloy strip cold-rolled to the final thickness is then subjected to soft annealing. Soft annealing is carried out in a continuous furnace at temperatures of more than 500 °C, preferably at 510 °C to 540 °C.

[0047] As also in Fig. 1As shown, an alternative production route can also be used in which the hot-rolled aluminum alloy strip is first cold-rolled to an intermediate thickness in step 4a. The intermediate thickness is determined such that the final reduction ratio of the cold rolling to the final thickness is 40% to 60%, preferably 50% to 60%. The intermediate annealing of the aluminum alloy strip is preferably carried out at 300°C to 500°C, for example in a batch furnace for at least 1.5 hours or in a continuous furnace for a maximum of 300 s. The intermediate annealing in step 4b can preferably be carried out either in a continuous furnace at 400°C to 500°C or in a batch furnace at 330°C to 450°C. The cold rolling of the aluminum alloy strip to the final thickness takes place in step 4c with a reduction ratio of 40% to 60%, preferably 50% to 60%.Subsequently, the finish-rolled aluminum alloy strip is soft annealed in step 5 at more than 500 °C, preferably at 510 °C to 540 °C in a continuous furnace.

[0048] Various aluminum alloy strips were produced using the alternative manufacturing method with intermediate annealing, with the final thickness of the working examples and comparative examples being 1.2 mm to ensure comparability in the forming behavior tests.

[0049] The various alloy compositions in wt.% are shown in Table 1, with all compositions containing aluminium as the remainder and unavoidable impurities with a maximum of 0.05 wt.% individually and a maximum of 0.15 wt.% in total.

[0050] Comparative Examples 1, 2 and 7, like Working Examples 3 to 6, comprise an aluminum alloy composition according to the invention.

[0051] The manufacturing parameters of working examples 1 to 7 are given in Table 2. The homogenization of the rolling ingot was identical for all aluminum alloy strips produced and was 480°C to 550°C for at least 0.5 h. Rough rolling of the rolling ingot with a starting temperature of at least 450°C was completed in Comparative Examples 1 and 7 at a blank thickness of 32 mm. Working examples 3 to 6 of the invention were rough rolled to a blank thickness of 36 mm. Hot rolling ended in Comparative Examples 1, 2, and 7, as well as in Working Examples 3 to 6, at a coiling temperature of 300 to 350°C and a final hot strip thickness of 3 to 6 mm.

[0052] Comparative Example 7 was cold-rolled from an intermediate annealing thickness of 1.5 mm with a final reduction of 20%, while Comparative Example 1 was cold-rolled with a reduction of 14.3% of the final thickness. Comparative Example 2 was manufactured with a reduction of 50% of the final thickness and soft-annealed in a continuous furnace at 400 °C for 300 s. Comparative Example 1 underwent an identical annealing process with a duration of 60 s.

[0053] Examples 3 to 6 were annealed at more than 500 °C, here at 530 °C for 60 s in a continuous furnace and, like all other examples, subsequently quenched in air.

[0054] The test results are shown in Table 3. No Ae values were determined for Comparative Example 7. A comparison of the typical mechanical parameters for forming, in this case the uniform elongation Ag and elongation at break A 80mm, reveals no clear differences between the comparative examples and the exemplary embodiments according to the invention. Nevertheless, the forming behavior of the comparative examples and the exemplary embodiments in the manufacturing process of complex-shaped components is fundamentally different, which is attributed to the differences in the microstructure. This is clearly demonstrated by the investigations regarding the principal strain ε1 at a 100mm specimen width according to Nakajima, measured in accordance with DIN EN ISO 120004-2.

[0055] The inventive examples 3 to 6 achieve values between 9% and almost 20% higher than the comparative examples. The result of the test for the main strain ε1 with a 100 mm sample width was reflected in the material with a significant decrease in the secondary phase density to below 250 per 1000 µm². The secondary phase density was determined using the method described above. Fig. 2 shows the determined values for comparison in the diagram.

[0056] Fig. 4shows an etched longitudinal section of an embodiment according to the invention. After grinding and polishing the section, the sample was etched by etching for one minute at room temperature in a dilute aqueous solution of sulfuric acid and hydrofluoric acid. The solution consisted of 100 cm³ of 10% concentrated sulfuric acid and 100 cm³ of another solution consisting of 60 cm³ of water and 40 cm³ of 5% hydrofluoric acid. After etching, the longitudinal section was rinsed with distilled water and dried for subsequent light microscopic examination. The etching marks the secondary phases.

[0057] At a magnification of 1000:1, the secondary phases were analyzed using a light microscope with an oil objective. This method allows objects with a diameter of at least 0.39 µm to be detected and counted. The etching process used dissolves the actual secondary phases, leaving etched pits whose size is significantly larger than the dissolved secondary phases. Thus, this method allows secondary phases to be detected with significantly less than the optical resolution of 0.39 µm. A comparison of the light-optical method used with scanning electron microscope investigations has shown that phases from approximately 50 nm can be determined statistically reliably. The total area of all examined measurement fields was 20331 µm². Fig. 4 One of the measuring fields is shown as an example.

[0058] The yield strength values of the examples, at 120 MPa transverse to the rolling direction, also demonstrate a good suitability for the preferred application of the aluminum alloy strips for automotive body interior parts. This also applies to the measured Ae values transverse to the rolling direction, which, at 0.7% and 0.6%, enable strain-free forming.

[0059] Not shown in Table 3 are the results of the grain size measurement, which showed an average grain size of 20 µm to 29 µm for the inventive embodiments according to ASTME 1382. Also not shown in Table 3 are the results of the corrosion tests, which showed a mass loss of 13.8 mg / cm 2< to 18.8 mg / cm 2< after a heat treatment of 45 minutes at 195 °C measured according to ASTM G67.

[0060] Finally, Fig. 3schematically shows a preferred use of the aluminum alloy strip, in which sheets were cut from the aluminum alloy strip and, by forming, for example, drawing, an interior part of a motor vehicle body in the form of an inner door part 6 was produced. These are usually made of steel. The aluminum alloy strips according to the invention are therefore preferably used for the production of interior body parts due to their improved forming behavior while maintaining consistent strength and corrosion resistance. Table 1 Nr Si Fe Cu Mn Mg Cr Ni Zn Ti See 1 0,08% 0,18% 0,0270% 0,23% 4,84% 0,0016% 0,0049% 0,0040% 0,0120% See 2 0,08% 0,22% 0,0801% 0,28% 4,75% 0,0078% 0,0045% 0,0081% 0,0170% Experience 3 0,06% 0,16% 0,0302% 0,26% 4,86% 0,0029% 0,0045% 0,0099% 0,0129% Experience 4 0,08% 0,17% 0,0227% 0,24% 4,90% 0,0036% 0,0045% 0,0062% 0,0130% Experience 5 0,07% 0,17% 0,0233% 0,23% 4,80% 0,0043% 0,0049% 0,0051% 0,0145% Experience 6 0,07% 0,17% 0,0245% 0,25% 4,82% 0,0022% 0,0043% 0,0047% 0,0145% See 7 0,07% 0,22% 0,0754% 0,29% 4,73% 0,0052% 0,0000% 0,0071% 0,0135% Table 2 Nr Homogenization Final thickness pre-rolling [mm] Hot strip thickness [mm] Intermediate annealing thickness [mm] Degree of rolling at final thickness [%] Annealing temperature [°C] Time [sec] deterrence See 1 480-550 °C > 0.5h 32 3,5 1,4 14,3 400 60 Air See 2 " 36 4,7 2,4 50,0 400 300 Air Experience 3 " 36 4,7 2,4 50,0 530 60 Air Experience 4 " 36 4,7 2,4 50,0 530 60 Air Experience 5 " 36 4,7 2,4 50,0 530 60 Air Experience 6 " 36 4,7 2,4 50,0 530 60 Air See 7 " 32 3,4 1,5 20,0 530 300 Air Table 3 Nr Rp0.2 Q [MPa] Ae Q [%] Ag Q [%] A80 [%] ε1@100mm Secondary phase density [number / 1000µm 2< ] See 1 115 0,4 22,2 24,9 0,189 266 See 2 130 0,9 23,5 26,1 0,181 260 Experience 3 120 0,7 23,8 27,2 0,206 202 Experience 4 120 0,7 23,5 27,6 0,216 178 Experience 5 120 0,6 23,2 26,9 0,215 192 Experience 6 120 0,6 23,9 27,3 0,207 171 See 7 120 - 21,9 26,1 0,188 259

Claims

1. Aluminium alloy strip comprising an aluminium alloy with the following composition in % by weight Si ≤ 0.10 % , Fe ≤ 0.25 % , 0.20 % ≤ Mn ≤ 0.30 % 4.72 % ≤ Mg ≤ 4.95 % , Cu ≤ 0.10 % , Cr ≤ 0.02 % , Ni ≤ 0.01 % , Zn ≤ 0.10 % , Ti ≤ 0.04 % , Remaining Al with unavoidable impurities individually ≤ 0.05 %, in total ≤ 0.15 %, whereby the aluminium alloy strip has an average secondary phase density of less than 250 / 1000 µm2,whereby the average secondary phase density results from the total number of secondary phases determined in at least 10 measurement fields in relation to the total measurement area of all measurement fields examined.

2. Aluminium alloy strip according to claim 1 characterised in that one or more alloying constituents of the aluminium alloy of the aluminium alloy strip have the following contents in % by weight: Si ≤ 0.08 % , Fe ≤ 20 % , 0.20 % ≤ Mn ≤ 0.26 % , 4.80 % ≤ Mg ≤ 4.92 % , Cu ≤ 0.07 , preferably < 0.04 % , Cr ≤ 0.01 , preferably ≤ 0.008 % , Ni ≤ 0.005 % , Zn ≤ 0.01 % , preferably ≤ 0.008 % , 0.005 % ≤ Ti ≤ 0.02 % .

3. Aluminium alloy strip according to claim 1 or 2, characterised in that the aluminium alloy strip has an average secondary phase density of less than 220 / 1000µm2, particularly preferably less than 200 / 1000µm2.

4. Aluminium alloy strip according to any one of claims 1 to 3, characterised in that the aluminium alloy strip has a temper state O or H111.

5. Aluminium alloy strip according to any one of claims 1 to 4, characterised in that the aluminium alloy strip has an average grain size of 15 µm to 30 µm measured according to ASTM E1382.

6. Aluminium alloy strip according to any one of claims 1 to 5, characterised in that the aluminium alloy strip is cold-rolled and optionally has a thickness of 0.5 mm to 4 mm.

7. Aluminium alloy strip according to any one of claims 1 to 6, characterised in that the aluminium alloy strip has an Ae value according to DIN EN ISO 6892 transverse to the rolling direction of less than 1.0 %, preferably less than 0.9 %.

8. Aluminium alloy strip according to any one of claims 1 to 7, characterised in that the aluminium alloy strip with a sheet thickness of 1.2 mm has an average principal deformation ε1 with a sample width of 100 mm according to DIN EN ISO 120004-2 in the Nakajima test of more than 0.200.

9. Aluminium alloy strip according to any one of claims 1 to 8, characterised in that the aluminium alloy strip has a yield strength Rp0.2 transverse to the rolling direction of at least 115 MPa, preferably at least 120 MPa, in accordance with DIN EN ISO 6892.

10. A method of manufacturing an aluminium alloy strip according to claims 1 to 9, wherein the method comprises the following steps: - Casting a rolling ingot of an aluminium alloy having the following composition: Si ≤ 0.10%, preferably ≤ 0.08%, Fe ≤ 0.25 %, preferably ≤ 0.20 %, 0.20 % ≤ Mn ≤ 0.30 %, preferably 0.20 % ≤ Mn ≤ 0.26 %, 4.72 % < Mg ≤ 4.95 %, preferably 4.80 % < Mg ≤ 4.92 %, Cu ≤ 0.10 %, preferably Cu ≤ 0.07 %, particularly preferably Cu < 0.04 %, Cr ≤ 0.02 %, preferably Cr ≤ 0.01 %, particularly preferably Cr ≤ 0.008 %, Ni ≤ 0.01 %, preferably Ni ≤ 0.005 %, Zn ≤ 0.10 %, preferably Zn ≤ 0.01 %, particularly preferably Zn ≤ 0.008 %, Ti ≤ 0.04 %, preferably Ti ≤ 0.02 %, Remaining Al with unavoidable impurities individually ≤ 0.05 %, in total ≤ 0.15 %, - homogenisation of the rolling slab at 480 °C to 550 °C for at least 0.5 h, - hot rolling of the rolling slab to a hot strip thickness of 3 to 6 mm, - cold rolling the aluminium alloy strip to the final thickness with a degree of rolling of 40% to 60%, preferably 50% to 60%, and - soft annealing of the finished rolled aluminium alloy strip at more than 500 °C, preferably 510 °C to 540 °C in a continuous furnace.

11. The process according to claim 10, wherein the following process steps are alternatively carried out after hot rolling: - cold rolling of the hot-rolled aluminium alloy strip to an intermediate thickness, which is determined in such a way that the final cold rolling degree to final thickness is 40 % to 60 %, preferably 50 % to 60 %, - intermediate annealing of the aluminium alloy strip at 300 °C to 500 °C, - cold rolling of the aluminium alloy strip to final thickness with a degree of rolling of 40% to 60%, preferably 50% to 60%, - soft annealing of the finished rolled aluminium alloy strip at more than 500 °C, preferably 510 °C to 540 °C in a continuous furnace.

12. Method according to claim 10 or 11, characterised in that the duration of the soft annealing of the finished aluminium alloy strip in the continuous furnace is between 5 s and 300 s.

13. Method according to one of claims 10 to 12, characterised in that the hot rolling of the rolling slab consists of the steps of pre-rolling to a thickness of 30 mm to 40 mm at a starting temperature of at least 450 °C and finish hot rolling to hot strip thickness at a coiling temperature of 300 °C to 350 °C.

14. Use of an aluminium alloy strip according to any one of claims 1 to 9 for the manufacture of an interior body part, in particular an interior door part, an interior bonnet part or an interior boot lid part of a motor vehicle.