Aluminium alloy strip which is resistant to intercrystalline corrosion and method for producing same

Abstract

The invention relates to an aluminum alloy strip composed of an AA 5xxx-type aluminum alloy containing at least 4 wt.% of Mg in addition to Al and inevitable impurities. The aim of the invention is to design an AlMg aluminum alloy strip which is resistant to intergranular corrosion also at high strength and even when the alloy contains at least 4 wt.% of Mg. According to a first teaching of the present invention, said aim is achieved by an aluminum alloy strip that has a recrystallized structure, the grain size (KG) of which in μm has the following relation to the Mg content (c_Mg) in wt.%: KG ≥ 22 + 2*c_Mg, the aluminum alloy of the aluminum alloy strip having the following composition in wt.%: Si ≤ 0.2 %, Fe ≤ 0.35 %, 0.04 % ≤ Cu ≤ 0.08 %, 0.2 % ≤ Mn ≤ 0.5 %, 4.35 % ≤ Mg ≤ 4.8 %, Cr ≤ 0.1 %, Zn < 0.25 %, Ti ≤ 0.1 %, the remainder consisting of Al and inevitable impurities, each of which amounts to no more than 0.05 wt.% and the total amount of which amounts to no more than 0.15 wt.%.

Description

Beschreibung

[0001] The invention relates to an aluminum alloy strip consisting of an aluminum alloy of type AA 5xxx, which, in addition to Al and unavoidable impurities, has a Mg content of at least 4 wt.%. Furthermore, the invention relates to a method for producing the aluminum alloy strip according to the invention and to a component made from an aluminum alloy strip according to the invention.

[0002] Aluminum-magnesium (AlMg) alloys of type AA 5xxx are used in the form of sheets, plates, or strips for the construction of welded or joined structures in shipbuilding, automotive, and aircraft construction. They are characterized in particular by their high strength, which increases with increasing magnesium content.

[0003] For example, the essay Development of twin-belt cast AA5XXX series aluminum alloy materials for automotive sheet applications Zhao et al. have developed an aluminum strip consisting of an AA5182 alloy with a Mg content of 4.65 wt.%, which is suitable for use in automotive engineering.

[0004] Aluminum alloy strips of type AA5182 with a Mg content of at least 4 wt.% are also available from the article Semi-Solid Processing of Alloys and Composites by Kang et al. and from the article Comparison of recrystallization textures in cold-rolled DC and CC AA 5182 aluminum alloys by Liu et al., as well as from US 2003 / 0150587 A1. The article Hot-Tear Susceptibility of Aluminum Wrought Alloys and the Effect of Grain Refining by Lin et al. concerns round bars made of its AA5182 alloy.

[0005] DE 102 31 437 A1 relates to corrosion-resistant aluminum alloy sheets, whereby sufficient resistance to intergranular corrosion is achieved by the addition of Zn in a content of more than 0.4 wt.%.

[0006] In addition, the document GB 2 027 621 A discloses a method for producing an aluminum strip.

[0007] 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, β-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.

[0008] Susceptibility to intergranular corrosion is typically tested using a standard test according to ASTM G67, in which samples are exposed to nitric acid and the mass loss due to the release of β-particles is measured. According to ASTM G67, the mass loss for materials not resistant to intergranular corrosion is greater than 15 mg / cm².

[0009] Corresponding materials and aluminum strips are therefore not suitable for use in heat-stressed areas.

[0010] Based on this, the present invention seeks to propose an aluminum alloy strip made of an AlMg alloy that, despite high strength and a Mg content of more than 4 wt. %, is resistant to intergranular corrosion, particularly even after forming and subsequent exposure to high temperatures. Furthermore, a manufacturing process is to be specified by which aluminum strips resistant to intergranular corrosion can be produced. Finally, intergranular corrosion-resistant components of a motor vehicle, for example, body components or body attachments, such as doors, hoods, tailgates, or other structural parts, as well as component parts, are to be proposed, made of an aluminum alloy of type AA 5xxx.

[0011] According to a first teaching of the present invention, the above-mentioned object is achieved by an aluminum alloy strip having a recrystallized structure, wherein the grain size (KG) of the structure in µm satisfies the following dependence on the Mg content (c_Mg) in wt.%: KG ≥ 22 + 2 * c_Mg . and wherein the aluminum alloy of the aluminum alloy strip has the following composition in wt.%: Si ≤ 0.2%, Fe ≤ 0.35%, 0.04% ≤ Cu ≤ 0.08%, 0.2% ≤ Mn ≤ 0.5%, 4.35% ≤ Mg ≤ 4.8%, Cr ≤ 0.1%, Zn ≤ 0.25%, Ti ≤ 0.1%,

[0012] The remainder Al and unavoidable impurities individually not more than 0.05 wt%, in total not more than 0.15 wt%, wherein the aluminum alloy strip is cold rolled and soft annealed and the aluminum alloy strip has a yield strength Rp0.2 of more than 120 MPa and a tensile strength Rm of more than 260 MPa.

[0013] With a Cu content of 0.04 wt% to 0.08 wt%, copper contributes to an increase in strength without significantly reducing corrosion resistance. Furthermore, by limiting the Mg range to 4.35 wt% to 4.8 wt%, very good strength can be achieved with a moderate grain size. Thus, resistance to intergranular corrosion can also be achieved with particularly reliable process reliability, since the required grain sizes of the microstructure can be reliably achieved in the process.

[0014] An aluminum alloy strip with a recrystallized structure can be provided by hot-rolled strip or annealed cold-rolled strip. Extensive studies have shown that there is a relationship between grain size, magnesium content, and resistance to intergranular corrosion. Since the grain size of a material is always distributed, all grain size specifications refer to the average grain size. The average grain size can be determined according to ASTM E1382. With a sufficiently large grain size, i.e. provided the grain size is greater than or equal to the lower limit of the grain size according to the invention in relation to the Mg content of the aluminum alloy strip, resistance to intergranular corrosion can be achieved so that the mass loss in the ASTM G67 test drops to below 15 mg / cm2. Corresponding aluminum strips can therefore be described as resistant to intergranular corrosion.This was demonstrated for the above-mentioned aluminum strips in the undeformed state after a simulated cathodic electrocoating cycle, as well as after a simulated cathodic electrocoating cycle including a subsequent operating load of up to 500 hours at 80°C. The resistance to intergranular corrosion was also demonstrated for the above-mentioned strips when the material was stretched by 15% before the cathodic electrocoating cycle and the operating load to simulate the forming process into a component. As a result, the aluminum alloy strip according to the invention offers high strengths and yield points due to its relatively high Mg content and is simultaneously resistant to intergranular corrosion. It is therefore ideally suited for use in heat-stressed areas in automotive construction.

[0015] According to a next embodiment of the aluminum alloy strip according to the invention, the grain size additionally satisfies the following condition: KG < 253 / 265 − 50 * c_Mg 2 With KG in µm and c_Mg in wt.%, it can be ensured that the yield strength R p0.2 of the aluminum alloy strip is greater than 110 MPa. The tensile strength of the strip is typically above 255 MPa.

[0016] A further advantageous embodiment of the aluminum alloy strip is achieved in that the aluminum alloy of the aluminum alloy strip has the following composition in wt.%: Si ≤ 0.2%, Fe ≤ 0.35%, 0.04% ≤ Cu ≤ 0.08%, 0.2% ≤ Mn ≤ 0.5%, 4.45% ≤ Mg ≤ 4.8%, Cr ≤ 0.1%, Zn ≤ 0.25%, Ti ≤ 0.1%,

[0017] The remainder consists of Al and unavoidable impurities, each containing a maximum of 0.05 wt.%, and a total of 0.15 wt.%. By limiting the Mg content to 4.45 wt.% to 4.8 wt.%, very good strength can also be achieved with a moderate grain size.

[0018] According to a further embodiment of the aluminum alloy strip according to the invention, the maximum grain size is 50 µm, since the process reliability decreases when producing aluminum strips with grain sizes greater than 50 µm from an AA 5xxx aluminum alloy with a Mg content of at least 4 wt.%. A maximum grain size of 50 µm, however, can be achieved with process stability. The process stability for producing structures with controlled grain size increases with decreasing grain size. Thus, the production of an aluminum alloy strip with a maximum grain size of 45 µm, preferably a maximum of 40 µm, is associated with increasing process stability.

[0019] According to a further embodiment of the aluminum alloy strip according to the invention, it has a thickness of 0.5 mm - 5 mm and is therefore excellently suited for most applications, for example in automotive engineering.

[0020] The aluminum alloy strip according to the invention is cold-rolled and then soft-annealed. Recrystallizing soft annealing typically takes place at temperatures of 300°C - 500°C and allows for the elimination of the hardening introduced during the rolling process and ensures good formability of the aluminum alloy strip. Furthermore, cold-rolled, soft-annealed, and therefore recrystallized strips can produce lower final thicknesses than recrystallized hot strips.

[0021] Finally, the aluminum alloy strip according to the invention exhibits a yield strength R p0.2 of more than 120 MPa and a tensile strength R m of more than 260 MPa. This means that the aluminum alloy strip according to the invention, which is resistant to intergranular corrosion, also exceeds the strength properties required by DIN 485-2 for an aluminum alloy of type AA5182. The elongation values, with a uniform elongation A g of at least 19% and a fracture elongation A 80 mm of at least 22%, also far exceed the values ​​required by DIN 485-2.

[0022] According to a second teaching of the present invention, the above-mentioned object is achieved by a method for producing an aluminum alloy strip comprising the following method steps: Casting a rolling ingot consisting of an aluminum alloy composition according to the invention, homogenizing the rolling ingot at 480 °C to 550 °C for at least 0.5 h, hot rolling the rolling ingot at a temperature of 280 °C to 500 °C, cold rolling the aluminum alloy strip to final thickness with a degree of rolling of less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%, soft annealing the finish-rolled aluminum alloy strip at 300 °C to 500 °C.

[0023] The listed process steps result in a grain size after annealing that satisfies the above-mentioned dependence on the Mg content due to the low degree of rolling during cold rolling of the aluminum alloy strip to final thickness. The degree of rolling to final thickness adjusts the hardening of the strip before annealing, which determines the resulting grain size. As the degree of rolling decreases from less than 40%, to a maximum of 30%, and a maximum of 25%, different grain sizes are achieved, which can be tailored to the alloy composition. In this respect, an aluminum alloy strip can be produced that is resistant to intergranular corrosion.

[0024] According to a further embodiment of the method according to the invention, the following process steps are carried out alternatively after hot rolling: Cold rolling of the hot-rolled aluminum alloy strip with a degree of reduction of at least 30%, preferably at least 50%, intermediate annealing of the aluminum alloy strip at 300 °C to 500 °C, subsequent cold rolling to final thickness with a degree of reduction of less than 40%, preferably at most 30%, particularly preferably at most 25%, soft annealing of the finish-rolled aluminum alloy strip at 300 °C to 500 °C.

[0025] What both of the processes mentioned above have in common is that the degree of rolling before soft annealing, i.e. the degree of rolling of final thickness during cold rolling, is limited to less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%. In the second embodiment of the process according to the invention, an additional cold rolling step takes place after intermediate annealing at 300°C - 500°C. During intermediate annealing, the aluminum alloy strip, which has been strongly hardened by cold rolling, is recrystallized and converted back into a formable state. The subsequent cold rolling step with a degree of rolling of less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%, means that, in conjunction with the Mg content used in the aluminum alloy, the grain size can be adjusted to the required ratio.As a result, a strip can be produced in the annealed state that is both resistant to intergranular corrosion and has the necessary forming and strength properties.

[0026] According to a further embodiment of the process according to the invention, the soft annealing and / or the intermediate annealing steps take place in a batch furnace, in particular a chamber furnace, or a continuous furnace. Both furnaces produce a sufficiently coarse grain structure, which ensures resistance to intergranular corrosion. Batch furnaces are typically less expensive to operate and purchase than continuous furnaces.

[0027] According to a third teaching of the present invention, the above-described object is achieved by a component for a motor vehicle that consists at least partially of an aluminum alloy strip according to the invention. The component is typically subjected to a coating, preferably a cathodic dip coating. Nevertheless, there are also possible applications for uncoated components made from the aluminum alloy strip according to the invention.

[0028] As already stated above, the aluminum alloy strip exhibits excellent properties in terms of strength, formability, and resistance to intergranular corrosion, so that the thermal stress during painting, a baking process that typically lasts 20 minutes at around 185°C, has only a minor impact on the component's resistance to intergranular corrosion. Forming into a component, which was simulated by stretching by 15% transverse to the original rolling direction, also has only a minor impact on resistance to intergranular corrosion. Even after 15% stretching, the mass loss values ​​according to ASTM G67 are less than 15 mg / cm². Furthermore, operation in temperature-stressed areas, which was simulated by a thermal stress of 200 or 500 hours at 80°C, has only a minor impact on resistance to intergranular corrosion.The mass loss values ​​according to ASTM G67 are less than 15mg / cm 2 even after corresponding temperature stress.

[0029] A component is particularly advantageous if it is designed as a body part or body attachment of a motor vehicle. Typical body parts are the fender or parts of the floor assembly, the roof, etc. Body attachments are generally doors and tailgates, etc., which are not permanently attached to the motor vehicle. Invisible body components or body attachments are preferably produced from the aluminum alloy strip according to the invention. These are, for example, inner door parts or inner parts of tailgates, but also floor panels, etc. A typical heat load for such components of a motor vehicle, for example inner door parts, is caused, for example, by solar radiation during operation of a motor vehicle.Furthermore, the body or body attachments of a motor vehicle are generally exposed to moisture, for example, in the form of splash water or condensation, so resistance to intergranular corrosion is required. The body or body attachments according to the invention, manufactured from an aluminum alloy strip according to the present invention, meet these requirements and also ensure a weight advantage over previously used steel structures.

[0030] The invention will now be explained in more detail using exemplary embodiments in conjunction with the drawing. The drawing shows in Fig. 1 is a schematic flow chart for an embodiment of a manufacturing process, Fig. 2 is a diagram showing the grain size as a function of the magnesium content of the embodiments, and Fig. 3 is a component for a motor vehicle according to a further embodiment.

[0031] Extensive tests were conducted to investigate whether there is a relationship between the grain size of an aluminum alloy strip made from an AA 5xxx aluminum alloy and the Mg content in relation to resistance to intergranular corrosion. Various aluminum alloys were used and different process parameters applied. Table 1 shows the various alloy compositions used to investigate the relationship between grain size, resistance to intergranular corrosion, and yield strength. In addition to the contents of the alloying elements Si, Fe, Cu, Mn, Mg, Cr, Zn, and Ti in wt.%, the aluminum alloys listed in Table 1 contain aluminum as the remainder and impurities individually up to a maximum of 0.05 wt.% and in total up to a maximum of 0.15 wt.%.

[0032] Since the final soft annealing and the final rolling degree in particular influence the grain size, these were varied or measured in the respective tests. For example, the grain size varied from 16 µm to 61 µm, and the final rolling degree from 17% to 57%. The final soft annealing was carried out either in a chamber furnace (KO) or in a continuous strip furnace (BDLO).

[0033] Fig. 1 shows the process flow of examples for the production of aluminum strips. The flow diagram of Fig.1 shows schematically the various process steps of the manufacturing process of the aluminum alloy strip according to the invention.

[0034] In step 1, a rolling ingot made of an AA 5xxx type aluminum alloy with a Mg content of at least 4 wt.% is cast, for example by DC continuous casting. 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, with typical temperatures of 280 °C to 500 °C being reached. The final thicknesses of the hot strip are, for example, 2 to 12 mm. The final hot strip thickness can be selected such that after hot rolling, only a single cold rolling step 4 takes place, in which the hot strip is reduced in thickness with a degree of reduction of less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%.

[0035] The aluminum alloy strip, cold-rolled to its final thickness, is then subjected to soft annealing. The soft annealing was carried out in a continuous furnace or in a chamber furnace to test the dependence of the corrosion properties on the chamber or continuous furnace. In the exemplary embodiments shown in Table 1, the second method with an intermediate annealing was used. For this purpose, after hot rolling according to process step 3, the hot strip is fed to a cold rolling mill 4a, which has a degree of reduction of more than 30% or more than 50%, so that the aluminum alloy strip preferably recrystallizes throughout during a subsequent intermediate annealing. In the exemplary embodiments, the intermediate annealing was carried out either in a continuous furnace at 400°C to 450°C or in a chamber furnace at 330°C to 380°C.

[0036] The intermediate annealing is in Fig. 1 with process step 4b. In process step 4c according to Fig. 1 The intermediately annealed aluminum alloy strip is finally subjected to cold rolling to final thickness, with the degree of stripping in process step 4c being less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%. The aluminum alloy strip is then returned to the soft state by soft annealing, with the soft annealing being carried out either in a continuous furnace at 400°C to 450°C or in a batch furnace at 330°C to 380°C. In the various tests, different aluminum alloys were used, as well as different degrees of stripping after the intermediate annealing. The values ​​for the degree of stripping after the intermediate annealing are also shown in Table 1. In addition, the grain size of the soft annealed aluminum alloy strip was measured in each case.

[0037] Mechanical properties, in particular the yield strength R p0.2 , tensile strength R m , the uniform elongation A g and the elongation A 80mm, were determined on the correspondingly produced aluminum alloy strips. In addition, the corrosion resistance to intergranular corrosion was measured according to ASTM G67, without additional heat treatment in the as-finished state (starting point 0h). In addition to the mechanical properties of the aluminum alloy strips measured according to EN 10002-1 and ISO 6892, the calculated grain sizes according to the formulas (1) presented below for resistance to intergranular corrosion and formula (2) for achieving the necessary mechanical properties, in particular a sufficiently high yield strength, are shown in Table 2 as columns KG(IK) and KG(Rp). The grain sizes were determined according to ASTM E1382 and are given in µm.

[0038] To simulate use in motor vehicles, the aluminum alloy strips were subjected to various heat treatments prior to the corrosion test. The first heat treatment involved storing the aluminum strips for 20 minutes at 185°C to simulate the e-coating cycle. In a further series of measurements, the aluminum alloy strips were stored at 80°C for an additional 200 hours or 500 hours, respectively, and then subjected to the corrosion test. Since forming of aluminum alloy strips or sheets can further influence corrosion resistance, the aluminum alloy strips were stretched by approximately 15% in a further test, subjected to a heat treatment or storage at elevated temperatures, and then subjected to an intergranular corrosion test according to ASTM G67, in which the mass loss was measured.

[0039] It was shown that there is a close relationship between grain size, Mg content, and resistance to intergranular corrosion. Examples 11 to 19 are all classified as resistant to intergranular corrosion. This also applies to their use in motor vehicles exposed to heat and the presence of moisture or a corrosive medium. In addition, examples 12, 14, 16, and 17 demonstrated the mechanical properties required by DIN EN 485-2 for an aluminum alloy strip of type AA 5182.

[0040] In Fig. 2 The diagram shows the measured grain sizes as a function of the Mg content in wt.%. In addition to the measurement points, the diagram also contains two curves A and B. Line A indicates the grain sizes above which, at a specific Mg content, the aluminum alloy strip can be considered resistant to intergranular corrosion. The corresponding grain size (KG) is determined from the following equation: KG = 22 + 2 * c_Mg , with c_Mg as Mg content in wt%.

[0041] Curve B, on the other hand, shows the limit at which the aluminum alloy strips have a yield strength below 110 MPa, which is too low to be considered alloy AA 5182 according to DIN EN 485-2. Curve B is determined by the following equation: KG = 253 265 − 50 * c _ Mg 2

[0042] All examples to the right of curve B therefore meet the requirement of a yield strength of more than 110 MPa.

[0043] Finally, Fig. 3 A typical component of a motor vehicle, shown schematically in the form of an inner door part. Inner door parts 6 are usually made of steel. However, the aluminum alloy strips produced show that high strengths with resistance to intergranular corrosion can also be achieved, provided the grain size ratio is adjusted in accordance with the invention with respect to the Mg content. The component according to the invention according to Fig. 3 is significantly lighter than a comparable steel component and is nevertheless resistant to intergranular corrosion. Patentansprüche

[0044]

Claims

1. Aluminium alloy strip composed of an AA 5 xxx -type aluminium alloy, which apart from Al and inevitable impurities has an Mg content of at least 4 wt.%, characterised in that the aluminium alloy strip has a recrystallized microstructure, wherein the grain size (GS) of the microstructure satisfies the following dependency on the Mg content (c_Mg) in wt.%: GS > 22 + 2 * c_Mg . and in that the aluminium alloy of the aluminium alloy strip has the following composition in wt.%: Si ≤ 0.2%,Fe ≤ 0.35%,0.04% ≤Cu ≤ 0.08%,0.2% ≤Mn ≤ 0.5%.4.35% ≤Mg ≤ 4.8%,Cr ≤ 0.1%,Zn ≤ 0.25%,Ti ≤ 0.1%, the remainder being Al and inevitable impurities, amounting to a maximum of 0.05 wt.% individually and a maximum of 0.15 wt.% in total, whereas the aluminium alloy strip is cold rolled and soft annealed and the aluminium alloy strip has a yield point Rp0.2 of greater than 120 MPa and a tensile strength Rm of greater than 260 MPa.

2. Aluminium alloy strip according to Claim 1, characterised in that the grain size (GS) of the microstructure of the aluminium alloy strip also satisfies the following dependency on the Mg content (c_Mg) in wt.%: GS < 253 265 − 50 * c _ Mg 2 3. Aluminium alloy strip according to any one of Claims 1 or 2, characterised in that the aluminium alloy of the aluminium alloy strip has 4.45% ≤ Mg ≤ 4.8%.

4. Aluminium alloy strip according to any one of Claims 1 to 3, characterised in that the grain size is a maximum of 50 µm, preferably a maximum of 40 µm.

5. Aluminium alloy strip according to any one of Claims 1 to 4, characterised in that the aluminium alloy strip has a thickness of 0.5 mm to 5 mm.

6. Method for producing an aluminium alloy strip according to any one of Claims 1 to 5 comprising the following process steps: - casting a rolling ingot; - homogenisation of the rolling ingot at 480°C to 550°C for at least 0.5 hours; - hot rolling of the rolling ingot at a temperature of 280°C to 500°C - cold rolling of the aluminium alloy strip to the final thickness with a degree of rolling of less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%; - soft-annealing of the finished-rolled aluminium alloy strip at 300°C to 500°C..

7. Method according to Claim 6, wherein after the hot rolling alternatively the following process steps are carried out: - cold rolling of the hot-rolled aluminium alloy strip with a degree of rolling of at least 30%, preferably at least 50%; - intermediate annealing of the aluminium alloy strip at between 300°C and 500°C; - subsequent cold rolling to the final thickness with a degree of rolling of less than 40%, preferably a maximum of 30%, particularly preferably a maximum of 25%; - soft annealing of the finish-rolled aluminium alloy strip at between 300°C and 500°C.

8. Method according to Claim 6 or 7, characterised in that the intermediate annealing and / or the soft annealing is / are carried out in a batch furnace or a continuous furnace.

9. Component for a motor vehicle at least partially composed of an aluminium alloy strip according to any one of Claims 1 to 5.

10. Component according to Claim 9, characterised in that the component is a body part or a body accessory of a motor vehicle.

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