Aluminum alloy for producing semi-finished products or components for motor vehicles, method for producing an aluminium alloy strip from said aluminium alloy, and aluminium alloy strip and uses therefore

Abstract

An aluminium alloy for producing semi-finished products or components for motor vehicles is provided, wherein the alloying components of the aluminium alloy have the following contents in percent by weight: Fe0.80%, Si0.50%, 0.90%Mn1.50%, Mg0.25%, Cu0.125%, Cr0.05%, Ti0.05%, V0.05%, Zr0.05%, the remainder being aluminium, unavoidable impurity elements, individually <0.05%, in total <0.15%, and the combined content of Mg and Cu satisfies the following relation in percent by weight: 0.15%Mg+Cu0.25%, wherein the Mg content of the aluminium alloy is greater than the Cu content of the aluminium alloy. A method for producing an aluminium alloy strip from such an aluminium alloy and an aluminium alloy strip produced by this method are also provided, as well as uses thereof.

Claims

1. An aluminium alloy strip, produced by a method comprising the following method steps: Casting a rolling ingot from an aluminium alloy for producing semi-finished products or components for motor vehicles, wherein the alloying components of the aluminium alloy have the following contents in percent by weight: Fe0.80%, Si0.50%, 0.90%Mn1.50%, Mg0.25% Cu0.125%, Cr0.05%, Ti0.05%, V0.05%, Zr0.05%, the remainder being aluminium, unavoidable impurity elements individually <0.05%, in total <0.15%, and the combined content of Mg and Cu satisfies the following relation in percent by weight 0.15%Mg+Cu0.25% wherein the Mg content of the aluminium alloy is greater than the Cu content of the aluminium alloy; Homogenizing the rolling ingot at 480 C. to 600 C. for at least 0.5 h; Hot rolling the rolling ingot at 280 C. to 500 C. to form an aluminium alloy strip; Cold rolling the aluminium alloy strip to final thickness; and Subjecting the aluminium alloy strip to recrystallizing final annealing; wherein the aluminium alloy strip consists of an alloy having the following contents in percent by weight: Fe0.80%, Si0.50%, 0.90%Mn1.50%, Mg0.25%, Cu0.125%, Cr0.05%, Ti0.05%, V0.05%, Zr0.05%, the remainder being aluminium, unavoidable impurity elements individually <0.05%, in total <0.15%, and the combined content of Mg and Cu satisfies the following relation in percent by weight 0.15%Mg+Cu0.25% wherein the Mg content of the aluminium alloy is greater than the Cu content of the aluminium alloy, and wherein the aluminium strip has an offset yield strength R.sub.p0.2 of at least 45 MPa, a uniform elongation A.sub.g of at least 23%, and an elongation at break A.sub.80 mm of at least 30%.

2. The aluminium alloy strip according to claim 1, wherein the aluminium alloy strip has a thickness in the range from 0.2 mm to 5 mm.

3. A method of fabricating a component in a motor vehicle, comprising the steps of: making a metal sheet from the aluminium alloy strip according to claim 1; and utilizing the metal sheet to fabricate the component in the motor vehicle.

4. The method according to claim 3, wherein component in a motor vehicle is an interior door panel.

5. The aluminium alloy strip according to claim 1, wherein the method further comprises the following method step: Milling the upper and/or lower side of the rolling ingot.

6. The aluminium alloy strip according to claim 1, wherein homogenization is carried out in at least two stages and comprises the following steps: first homogenization at 500 C. to 600 C. for at least 0.5 h; and second homogenization at 450 C. to 550 C. for at least 0.5 h.

7. The aluminium alloy strip according to claim 1, wherein the degree of rolling reduction during cold rolling is between 70% and 90%.

8. The aluminium alloy strip according to claim 7, wherein the degree of rolling reduction during cold rolling is between 80% and 85%.

9. The aluminium alloy strip according to claim 1, wherein the cold rolling is carried out with or without intermediate annealing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows a flowchart for multiple exemplary embodiments of the method according to the invention.

(3) FIG. 2 shows a flowchart for further exemplary embodiments of the method according to the invention.

(4) FIG. 3 shows a diagram with measurement results from exemplary embodiments of the alloy and/or aluminium alloy strip according to the invention.

(5) FIGS. 4a-c show photographic images of three metal sheet samples from three different aluminium alloy strips regarding a test for filiform corrosion.

(6) FIG. 5 shows a component for a motor vehicle according to a further exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows a flowchart for a first exemplary embodiment of the method according to the invention for producing an aluminium alloy strip.

(8) In a first step 2, a rolling ingot is first cast from an aluminium alloy according to the invention. Casting can be carried out for example in a DC continuous casting or strip casting process. After casting, the rolling ingot is homogenized in step 4 at a temperature in the range from 480 C. to 600 C. for at least 0.5 h. In step 6, the rolling ingot is then hot rolled at a temperature in the range from 280 C. to 500 C. to a final thickness between 3 and 12 mm. In step 8, the hot strip that has been hot-rolled from the rolling ingot is then cold rolled to a final thickness of preferably 0.2 mm to 5 mm. Finally, after cold rolling, final annealing of the aluminium alloy strip is performed in step 10, for example in a chamber furnace at a temperature between 300 C. and 400 C. or in a continuous furnace between 450 C. and 550 C.

(9) The upper and/or lower side of the rolling ingot may be milled in an optional step 12 between the casting in step 2 and the homogenization in step 4.

(10) Further, the aluminium alloy strip may undergo intermediate annealing in an optional step 14 during cold rolling in step 8, preferably in a chamber furnace at a temperature between 300 C. and 400 C. Intermediate annealing is particularly useful for improving the material properties of the aluminium alloy strip if the hot strip is relatively thick and if the degree of rolling reduction during cold rolling is thus in total more than 85%, particularly more than 90%.

(11) With a hot strip thickness of 12 mm and a final thickness of 0.4 mm, the total degree of rolling reduction in cold rolling is, for example, 96.7%. In this case, the hot strip may first be rolled for example to a thickness of 2 mm in a first cold rolling pass, then subjected to intermediate annealing and finally rolled to 0.4 mm in a second cold rolling pass. The degree of rolling reduction after the intermediate annealing step is then only 80% and thus lies in a preferred range.

(12) FIG. 2 shows a part of a flowchart for further exemplary embodiments of the method according to the invention. The process flow for these exemplary embodiments is substantially the same as the process flow for the methods described with reference to FIG. 1. In the exemplary embodiments according to FIG. 2, the rolling ingot is however homogenized in step 16 rather than in step 4, wherein step 16 is divided into several substeps. FIG. 2 shows possible sequences of the individual steps in step 16.

(13) Accordingly, after the rolling ingot is cast in step 2 or after the rolling ingot is milled in step 12, in the first substep 18 of step 16, a first homogenization is carried out at a temperature between 550 and 600 C. for at least 0.5 h, preferably for at least for 2 h. In a subsequent step 20, the rolling ingot is cooled to the temperature for the second homogenization in the range from 450 C. to 550 C., before then in subsequent step 22 undergoing the second homogenization at this temperature for at least 0.5 h, preferably at least 2 h.

(14) Alternatively, in a step 24, the rolling ingot may first be cooled to room temperature after the first homogenization in step 18, and then in a subsequent step 26 be reheated to the temperature for the second homogenization. The upper and/or lower side of the rolling ingot may optionally be milled between step 24 and step 26.

(15) In the course of the invention, AA 3xxx type aluminium alloys, particularly basing on AA 3103, were produced with various contents of Mg and Cu. The compositions of these aluminium alloys are summarized in the following Table 1, wherein the contents of the individual alloying components are each indicated in % by weight.

(16) TABLE-US-00001 TABLE 1 No. Si Fe Cu Mn Mg Cr Zn Ti V Zr Cu + Mg 1 V 0.063 0.54 0.0029 1.07 0.0102 0.0005 0.0051 0.0053 0.0038 0.0005 0.013 2 V 0.23 0.55 0.055 0.93 0.059 0.0096 0.0131 0.0151 0.0099 0.0008 0.114 3 V 0.208 0.546 0.064 1.026 0.071 0.004 0.005 0.018 0.0081 0.0006 0.135 4 E 0.154 0.51 0.152 1.02 0.0019 0.0005 0.0034 0.0602 0.0073 0.0005 0.154 5 E 0.176 0.511 0.092 1.01 0.063 0.003 0.006 0.0169 0.0107 0.0008 0.155 6 E 0.128 0.57 0.031 1.0 0.15 0.006 0.007 0.0166 0.0114 0.0008 0.181 7 E 0.23 0.5 0.18 1.06 0.0109 0.0101 0.0055 0.0093 0.0112 0.0008 0.191 8 E 0.142 0.62 0.0019 1.1 0.19 0.0004 0.0011 0.0066 0.0091 0.0005 0.192 9 E 0.17 0.54 0.19 1.03 0.053 0.0005 0.0032 0.0217 0.0064 0.0005 0.243 10 V 0.42 0.45 0.086 1.01 0.19 0.0331 0.0058 0.028 0.0066 0.0006 0.276 11 V 0.052 0.21 0.28 0.87 0.22 0.0006 0.0028 0.018 0.0061 0.0005 0.5 12 V 0.162 0.59 0.0016 1.1 0.52 0.0002 0.001 0.0055 0.0072 0.0005 0.522 13 V 0.179 0.38 0.116 1.05 0.51 0.003 0.006 0.014 0.0068 0.0006 0.626

(17) In the last column of Table 1, the combined content of copper and magnesium is indicated, which has been found to be particularly important for the desired material properties. Alloys 4-9 are exemplary embodiments of the inventive alloy (E), while alloys 1-3 and 10-13 represent comparative examples (V).

(18) Aluminium alloy strips were then prepared from said aluminium alloys 1-13 using the method described above. Specifically, in each case a rolling ingot having a thickness of 600 mm was cast from each of said alloys 1 to 13 in DC continuous casting, and then homogenized in two stages, first for several hours at about 580 C. and then for several hours at about 500 C. After homogenization, the ingots were hot-rolled at about 500 C. to create aluminium alloy hot-rolled strips with a thickness of 4 to 8 mm. These aluminium alloy hot rolled strips were then each cold rolled to a final thickness of 1.2 mm and finally subjected to recrystallizing final annealing at 350 C. for 1 h.

(19) Then, the mechanical properties of the aluminium alloy strips were tested, in particular their strength and formability.

(20) The results of these tests are summarized in Table 2 below. The last row of Table 2 also shows the corresponding material properties of a type AA 8006 alloy as known from the prior art.

(21) TABLE-US-00002 TABLE 2 R.sub.p0.2 R.sub.m A.sub.g A.sub.80 mm n- r- SZ 32 No. [MPa] [MPa] [%] [%] value value [mm] 1 V 42 101 25.1 41.3 0.214 0.472 16.7 2 V 42 103 24.6 35.7 0.216 0.579 16.3 3 V 43 111 24.5 36.1 0.218 0.484 16.4 4 E 48 111 25.3 35.9 0.214 0.417 16.6 5 E 45 114 24.8 36.4 0.217 0.484 16.5 6 E 46 116 24.5 35.1 0.217 0.662 16.7 7 E 49 115 25.1 34.2 0.218 0.420 16.2 8 E 50 113 24.2 35.0 0.210 0.598 16.4 9 E 53 118 23.8 32.5 0.216 0.344 15.9 10 V 51 119 21.8 29.5 0.207 0.635 15.9 11 V 58 134 21.2 26.9 0.220 0.556 15.4 12 V 57 135 20.8 28.0 0.221 0.652 15.5 13 V 66 152 19.7 21.0 0.225 0.582 14.9 AA V 49 104 27.5 42.0 0.223 0.431 17.3 8006

(22) Table 2 shows the following values: offset yield strength R.sub.p0.2 in MPa and the tensile strength R.sub.m in MPa, measured in the tensile test perpendicular to the rolling direction of the sheet according to DIN EN ISO 6892-1:2009, uniform elongation A.sub.g as a percentage and elongation at break A.sub.80mm as a percentage, measured in a tensile test perpendicular to the rolling direction of the sheet with a strip tensile test sample according to DIN EN ISO 6892-1:2009, Appendix B, Form 2, the strain hardening exponent n (n-value) measured in the tensile test perpendicularly to the rolling direction of the sheet according to DIN ISO 10275:2009, the perpendicular anisotropy r (r-value) measured in the tensile test perpendicularly to the rolling direction of the sheet according to DIN ISO 10113:2009, and the cupping SZ 32 achieved during stretch forming in millimetres as a further measure of the ductility of the alloy. Cupping SZ 32 was determined in the Erichsen cupping test according to DIN EN ISO 20482, but with a punch head diameter of 32 mm and die diameter of 35.4 mm tuned to the sheet thickness and with the aid of a Teflon drawing film to reduce friction.

(23) In FIG. 3, the offset yield strengths R.sub.p0.2 (empty squares), elongations at break A.sub.80mm (filled diamonds) and the cupping values SZ 32 (filled triangles) of aluminium alloy strips 1 to 13 are plotted against the combined Cu and Mg content of the respective aluminium alloy. The R.sub.p0.2 values are plotted in MPa according to the scale on the left vertical axis. The A.sub.80mm values are plotted in percent and the SZ 32 values are plotted in mm according to the scale on the right vertical axis. The combined Cu and Mg content is indicated on the abscissa in % by weight.

(24) For better clarity, straight lines of best fit are also added in FIG. 3 for each of the measured values for R.sub.p0.2, A.sub.80mm and SZ 32. Two vertical dashed lines further indicate the upper and lower limits for combined Cu and Mg content according to the present invention.

(25) As the measured values for the aluminium alloy strips from aluminium alloys 4-9 show, adjusting the combined Cu and Mg content in a range from 0.15% by weight to 0.25% by weight has the effect of achieving the desired combination of strength (Rp.sub.0.245 MPa) and formability (A.sub.g23% and A.sub.80mm30%).

(26) With a combined Mg and Cu content of less than 0.15% by weight (No. 1-3) the strength proves to be too low (R.sub.p0.2<45 MPa) and with a combined Mg and Cu content of more than 0.25% by weight (numbers 10-13) the elongation values and therewith also the formability are reduced too much (A.sub.g<23% and/or A.sub.80mm<30%).

(27) The good formability is also particularly evident from the measured cupping value, which preferably has a value SZ 3215.8 mm, preferably 15.9 mm for the alloy according to the invention.

(28) As a result, for the same strength, aluminium alloys 4-9 thus exhibit only slightly worse formability than the comparative alloy AA 8006. However, aluminium alloys 4-9 have an advantage over alloy AA 8006, in that they have significantly better corrosion resistance. Particularly, intercrystalline corrosion generally does not occur in AA 3xxx type aluminium alloys.

(29) Moreover, supplementary laboratory tests for corrosion resistance were performed on the aluminium alloy strips made from aluminium alloys 4-9. These laboratory experiments showed that aluminium alloys 4-9 exhibit much better resistance to filiform corrosion than the alloy type AA 8006. Thus, aluminium alloys like the aluminium alloys 4-9 and aluminium alloy strips produced from said aluminium alloys are particularly suitable for coated components.

(30) In particular, the test for filiform corrosion as described in the following was conducted on sheet samples of each of the various aluminium alloy strips. The test comprises the following steps in the given order: 1. Etching of the rolled and soft annealed sheet samples for 30 s in an acid etching medium with material removal of 0.5 g/m.sup.3. (This material removal corresponds roughly to a typical material removal during pretreatment of semi-finished products and components for motor vehicles, for example in an OEM pretreatment process, so that the filiform results of the test described here correlate well with the results in the actual component.) 2. Coating the etched sheet sample with a transparent acrylic resin paint. 3. Baking the applied paint for 5 min. at 160 C. 4. Using a scriber needle to make a scratch in the sheet sample transversely to the direction of rolling. 5. Droplet seeding of an aqueous, 18% hydrochloric acid solution in the scratch. 6. Ageing of the sheet sample in a climatic exposure test cabinet, a) initially for 24 h at 40 C. and 80% relative humidity, and b) then for 72 h at 23 C. and 65% relative humidity. 7. Visual evaluation of the sheet sample, namely evaluation of the infiltration depth (spread of corrosion under the paint) originating from the scratch.

(31) The test described in the preceding was conducted in particular on sheet samples of exemplary embodiments 5 and 6 listed in tables 1 and 2, and on a sheet sample produced in corresponding manner from the comparison alloy AA8006. FIGS. 4a-c are photographic images of the sheet sample surfaces at the end of the test. FIG. 4a shows the sheet sample from the comparison alloy AA8006, FIG. 4b shows the sheet sample according to exemplary embodiment 5 and FIG. 4c shows the sheet sample according to exemplary embodiment 6.

(32) The scratch made in each sheet sample is visible in each of FIGS. 4a-c (dark line extending from top to bottom). Filiform corrosion emanates from the scratch substantially transversely to the direction of extension of the scratch and appears in the figures as pale, thread-like structures. To facilitate size comparison, each figure shows a ruler with centimetre scale placed on the sheet sample.

(33) The sheet sample from the comparison alloy AA8006 exhibits substantial filiform corrosion. The scratch in FIG. 4a is almost completely surrounded by the white, thread-like structures of filiform corrosion. The infiltration depth, that is to say the length of the thread-like structures originating from the scratch, is up to 6 mm.

(34) In contrast, the level of the filiform corrosion on the sheet sample produced from alloy 5 is considerably lower. The density of the thread-like filiform corrosion structures is much smaller on the scratch shown in FIG. 4b than on the scratch shown in FIG. 4a, indicating that the sheet sample in FIG. 4b is much more resistant to filiform corrosion than the sheet sample in FIG. 4a. Nevertheless, some thread-like filiform corrosion structures still appear on this sheet sample as well, and in some regions the infiltration depth is quite extensive, up to about 6 mm.

(35) The best results with regard to filiform corrosion were obtained with the exemplary embodiments in which the Mg content of the alloying composition is greater than the Cu content. Accordingly, the sheet sample for exemplary embodiment 6 with an Mg content of 0.15% by weight and a Cu content of 0.031% by weight exhibits only minimal filiform corrosion. Only very few short, thread-like filiform corrosion structures less than 3 mm in length sporadically surround the scratch in FIG. 4c. The sheet sample of exemplary embodiment 6 thus exhibits very good resistance to filiform corrosion.

(36) Finally, the measured values in table 2 show that the exemplary embodiments of the aluminium alloy according to the invention can also return good values for tensile strength R.sub.m as well as for the n value and the r value, which in particular are in the same range as conventional AA 3xxx alloys, or even better.

(37) FIG. 5 is a schematic representation of a typical component of a motor vehicle in the form of an interior door panel. Such interior door panels 40 are usually made of steel. However, for the same stiffness, steel components are heavy and prone to corrosion.

(38) It has been found that the aluminium alloys described in the preceding, such as for example aluminium alloys 4-9, can be used to produce aluminium alloy strips that have very good formability, are of medium strength and highly resistant to corrosion, particularly intercrystalline as well as filiform corrosion.

(39) The material properties of these aluminium alloy strips and the sheets prepared from them are thus particularly advantageous for producing motor vehicle components, such as interior door panel 40. The good resistance to filiform corrosion is especially advantageous when the aluminium alloys are used for coated, particularly painted, parts such as interior door panel 40.

(40) In particular, the components produced from these aluminium alloys have better resistance to corrosion than corresponding components made of steel or an AA 8006 type alloy. At the same time, they are considerably lighter than steel components.