AlMgSi strip for applications having high formability requirements

Abstract

The invention relates to a method for producing a strip made of an AlMgSi alloy in which a rolling ingot is cast of an AlMgSi alloy, the rolling ingot is subjected to homogenization, the rolling ingot which has been brought to rolling temperature is hot-rolled, and then is optionally cold-rolled to the final thickness thereof. The problem of providing a method for producing an aluminum strip made of an AlMgSi alloy and an aluminum strip, which has a higher breaking elongation with constant strength and therefore enables higher degrees of deformation in producing structured metal sheets, is solved in that the hot strip has a temperature of no more than 130 C. directly at the exit of the last rolling pass, preferably a temperature of no more than 100 C., and the hot strip is coiled at that or a lower temperature.

Claims

1. An aluminum strip comprising a AlMgSi alloy, wherein the aluminum ally is one of the alloy type AA6014, AA6016, AA6060, AA6111, or AA6181 produced with a method comprising: casting a rolling ingot; homogenizing the rolling ingot; hot rolling the rolling ingot using multiple hot rolling passes, the rolling ingot having been brought to a hot rolling temperature, and then optionally cold rolling the rolling ingot to a final thickness to produce a hot strip; wherein a cooling operation is performed within the last two hot rolling passes using at least one plate cooler and an emulsion charged hot rolling pass itself, wherein the hot rolling passes are carried out by working rolls of a hot rolling mill, such that immediately after an exit from the last rolling pass from the hot rolling mill the hot strip has an exit temperature not above 130 C. and the hot strip is coiled at this or a lower temperature at the exit of the hot rolling mill to produce a finished rolled aluminum strip; wherein the aluminum strip in a T4 state has a breaking elongation A80 of at least 30% with a yield point of Rp0.2 from 80 to 140 MPa; and wherein the aluminum strip is solution annealed and quenched such that the aluminum strip in the T6 state after artificial aging at 205 C./30 minutes has a yield point of Rp0.2 of more than 185 MPa.

2. The aluminum strip according to claim 1, wherein the hot rolling temperature of the hot strip is at least 230 C. before the penultimate hot rolling pass.

3. The aluminum strip according to claim 1, wherein the final thickness of the finished hot strip is 3 mm to 12 mm.

4. The aluminum strip according to claim 1, wherein the finished rolled aluminum strip is subjected to a heat treatment in which the finished rolled aluminum strip is heated to above 100 C. and is then coiled and aged at a temperature above 55 C.

5. The aluminum strip according to claim 1, wherein the aluminum strip in the T4 state has a uniform elongation Ag of more than 25%.

6. The aluminum strip according to claim 1, wherein the aluminum strip is solution annealed and quenched such that the aluminum strip in the T6 state after artificial aging at 205 C./30 minutes has an yield point difference Rp0.2 between states T6 and T4 of at least 80 MPa.

7. The aluminum strip according to claim 4, wherein the aluminum strip has a uniform elongation Ag of more than 25% with an yield point Rp0.2 of 80 to 140 MPa.

8. The aluminum strip according to claim 7, wherein the aluminum strip has a uniform elongation Ag of more than 25% in at least one of the direction of at rolling, transversely to the direction of rolling, and diagonally to the direction of rolling.

9. The aluminum strip according to claim 1, wherein the aluminum strip has a thickness of 0.5 to 12 mm.

10. The aluminum strip according to claim 1, wherein the aluminum strip has a uniform elongation Ag of more than 25% in at least one of the direction of at rolling, transversely to the direction of rolling, and diagonally to the direction of rolling.

11. The aluminum strip according to claim 1, wherein immediately after the exit from the last rolling pass the hot strip has an exit temperature not above 100 C. and the hot strip is coiled at this or a lower temperature to produce a finished rolled aluminum strip.

12. The aluminum strip according to claim 1, wherein the hot rolling temperature of the hot strip is above 400 C. before quenching during hot rolling, particularly before the penultimate hot rolling pass.

13. The aluminum strip according to claim 1, wherein the final thickness of the finished hot strip is 3.5 mm to 8 mm.

14. The aluminum strip according to claim 1, wherein the finished rolled aluminum strip is subjected to a heat treatment in which the finished rolled aluminum strip is heated to above 100 C. and is then coiled and aged at a temperature above 85 C.

15. A chassis element, structural element, a panel of an automobile, aircraft, or railcar, a component, an exterior or interior panel of a car, or a bodywork structural element made of an aluminum strip according to claim 1.

16. The aluminum strip according to claim 1, wherein the hot rolling temperature of the hot strip is at least 230 C. before quenching during hot rolling.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1a-1d illustrate schematically an exemplary of embodiment of a method according to the teachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) In the drawing, the only FIG. 1 shows a schematic flowchart of an exemplary embodiment of the method according to the invention for producing a strip made from an MgSi aluminium alloy in steps a) producing and homogenizing the rolling ingot, b) hot rolling, c) cold rolling and d) solution annealing with quenching.

(3) First a rolling ingot is cast from an aluminium alloy having the following alloy components a percent by weight: 0.35%Mg0.6%, 0.3%Si0.6%, 0.1%Fe0.3% Cu0.1%, Mn0.1%, Cr0.05%, Zn0.1%, Ti0.1% and
the remainder being Al and unavoidable impurities, constituting not more than 0.15% in total and not more than 0.05% individually.

(4) The rolling ingot made in this way is homogenized in a furnace 2 at a homogenizing temperature of about 550 C. for 8 h so that the alloying components are distributed completely homogeneously throughout the rolling ingot FIG. 1a).

(5) FIG. 1b) shows how rolling ingot in the present embodiment of the method according to the invention is hot rolled by reversing through a hot rolling mill 3, wherein the rolling ingot reaches a temperature from 230 to 550 C. during the hot rolling. In this embodiment, hot strip 4 preferably has a temperature of at least 400 C. after it leaves hot roller 3 and before the penultimate hot rolling pass. The quenching of warm strip 4 preferably takes place at this hot strip temperature of at least 400 C. using a plate cooler 5 and the working rollers of hot rolling mill 3. Plate cooler 5, which is shown only diagrammatically, sprays hot strip 4 with cooling rolling emulsion and ensure that hot strip 4 cools down quickly. The working rollers of roller mill 3 are loaded with emulsion and cool hot strip 4 further. After the last rolling pass, at the exit from plate cooler 5 in the present example, hot strip 4 has a temperature of just 95 C. and will then be coiled on recoiler 6.

(6) Since hot strip 4 has a temperature not above 130 C. or not above 100 C. immediately at the exit from the last hot rolling pass or is optionally cooled to a temperature not above 130 C. or not above 100 C. in the last two hot rolling passes by the use of plate cooler 5 and the working rollers of hot rolling mill 3, the crystal microstructure of hot strip 4 is frozen, as it were, since no additional energy in the form of heat is available for subsequent precipitating steps. The hot strip, with a thickness of 3 to 12 mm, preferably 3.5 to 8 mm, is coiled on recoiler 6. As was explained previously, the coiling temperature in the present embodiment is below

(7) 95 C.

(8) In the method according to the invention, now no or very few coarse Mg.sub.2Si precipitates are able to form in the coiled hot strip 4. Hot strip 4 has a crystalline state that lends itself very well to further processing and may be decoiled by decoiler 7, fed to a cold rolling mill 9, for example, and then coiled again on coiler 8, FIG. 1c).

(9) The resulting, cold rolled strip 11 is coiled. It is then transported to solution annealing and quenching 10, FIG. 1d). For this purpose, it is decoiled again from coil 12, solution annealed in a furnace 10, quenched and returned to a coil 13. Then, after natural aging at room temperature, aluminium strip may then in state T4 be shipped with maximum formability. Alternatively (not shown), the aluminium strip 11 may be separated into individual sheets, which will then be available in state T4 after natural aging.

(10) With larger aluminium strip thicknesses, for example for chassis applications or components such as backing plates, alternatively piecewise annealing may be carried out and the sheets quenched directly afterwards.

(11) In state T6, the aluminium strip, or the aluminium panel, is heated to 100 C. to 220 C. g in an artificial aging process in order to obtain maximum values for the yield point. For example, artificial aging may be performed at 205 C./30 min.

(12) The aluminium strips produced in accordance with the embodiment presented have, for example, a thickness of 0.5 to 4.5 mm after natural aging. Strip thicknesses from 0.5 to 2 mm are typically used for bodywork applications and strip thicknesses from 2.0 mm to 4.5 mm are used for chassis parts in car manufacturing. In both application areas, the improved elongation values represent a decisive advantage in parts manufacturing, since most operations with the sheets involve extensive forming but at the same time high strengths in the application state (T6) of the end product are imperative.

(13) Table 1 shows the alloy compositions of aluminium alloys from which aluminium strips have been produced by conventional or inventive methods. Besides the contents of alloy components shown, the remaining composition of the aluminium strips is made up of aluminium and impurities, which are present in individual quantities not exceeding 0.05% by weight and altogether in a quantity not exceeding 0.15% by weight.

(14) TABLE-US-00001 TABLE 1 Strips Si %/wt Fe %/wt Cu %/wt Mn %/wt Mg %/wt Cr %/wt Zn %/wt Ti %/wt 409 1.29 0.17 0.001 0.057 0.29 <0.0005 <0.001 0.02 410 1.30 0.17 0.001 0.056 0.29 <0.0005 <0.001 0.0172 491-1 1.39 0.18 0.002 0.062 0.30 0.0006 0.01 0.0158 491-11 1.40 0.18 0.002 0.063 0.31 0.0006 0.0104 0.0147

(15) Strips (specimens) 409 and 410 were produced according to a method according to the invention in which in the last two hot rolling passes the hot strip was cooled from about 400 C. to 95 C. using a plate cooler and the hot rollers themselves and coiled. The measured values for this strips are marked Inv. in Table 2. They were then cold rolled to a final thickness of 1.04 mm.

(16) The strips (specimens) 491-1 and 491-11 were produced using a conventional hot rolling and cold rolling method and are identified with the label Conv..

(17) The results of the mechanical properties presented in Table 2 clearly show the difference in achievable elongation values A.sub.80.

(18) TABLE-US-00002 TABLE 2 T6 T4 205 C./30 min. Thickness Rp0.2 R.sub.m A.sub.g A.sub.80 Rp0.2 R.sub.m A.sub.80 Rp0.2 Strips (mm) (MPa) (MPa) (%) (%) (MPa) (MPa) (%) (MPa) 409 Inv. 1.04 100 220 26.3 31.3 187 251 16.2 87 410 Inv. 1.04 98 217 25.6 30.3 195 256 15.5 97 491-1 Conv. 1.04 92 202 23.1 27.8 180 235 14.7 88 491-11 Conv. 1.04 88 196 23.0 27.4 179 232 14.3 91

(19) In order to achieve the T4 state, the strips underwent solution annealing with subsequent quenching followed by natural aging at room temperature. The T6 state was achieved with artificial aging at 205 C. for 30 minutes.

(20) It was found that the advantageous microstructure that was created in strips 409 and 410 via the method according to the invention, not only offered a higher yield point Rp0.2 and increased strength Rm but also enabled increased elongation A.sub.80. This microstructure results in a particularly advantageous combination of high breaking elongation A.sub.80 of at least 30% or at least 30% with very high values for the yield point Rp0.2 from 80 to 140 MPa. In the state T6, the yield point may rise to more than 185 MPa, in which case the elongation A.sub.80 still remains above 15%. The hardenability with a Rp0.2 of 87 or 97 MPa shows that the embodiments according to the invention exhibit a very good increase in the yield point of the artificially aged state T6 under artificial aging at 205 C./30 min. despite the increased elongation values of more than 15%.

(21) A comparison of the uniform elongations A.sub.g of the strips according to the invention and of the conventional strips also shows that the uniform elongation A.sub.g, with values of more than 25%, the inventive strips 409 and 410 significantly outperform the conventional strips, for which values of 23% were measured. Table 2 shows the value for uniform elongation transversely to the direction of rolling. Values greater than 25% for uniform elongation A.sub.g also diagonally and in the direction of rolling were also recorded on strips, not listed in the Table 2, which were measured with the method according to the invention. These results underscore the exceptional formability of the strips according to invention.

(22) Breaking elongation values A.sub.g and Ag.sub.80, the yield point values Rp0.2 and the tensile strength values Rm in the following tablew were measured according to DIN EN.

(23) The measured values were verified in state T4 by means of measurements taken on other strips. The aluminium alloy of strips A and B had the following composition: 0.25%Mg0.6%, 1.0%Si1.5%, Fe0.5%, Cu0.2%, Mn0.2%, Cr0.1%, Zn0.1%, Ti0.1%
the remainder being Al and unavoidable impurities, constituting not more than 0.15% in total and not more than 0.05% individually.

(24) Strips A and B underwent quenching of the hot strip to 95 C. by application of the method according to the invention during the last two reduction phases and were coiled and then cold rolled to final thicknesses of 1.0 mm and 3.0 mm respectively. In order to achieve state T4, strips A and B were solution annealed and then naturally aged following quenching.

(25) The following measured values were determined for the two strips:

(26) TABLE-US-00003 TABLE 3 T4 Thickness Rp0.2 R.sub.m A.sub.80 Strips (mm) (MPa) (MPa) (%) A 1.0 107 221 31.1 B 3.0 108 212 32.0

(27) The further increase in elongation values A.sub.80 shows how ideally suited these aluminium strips are for producing components in which very high degrees of deformation in state T4 during manufacturing must be combined with maximum tensile strengths Rm and yield points Rp0.2 in state T6.

(28) In addition, an examination was made of other aluminium strips that had undergone additional heat treatment, which was carried out on the aluminium strip preferably immediately after the product was produced, for example directly after the solution annealing and quenching. For this, the aluminium strips were briefly heated to above 100 C. and then coiled at a temperature above 85 C., in the present case 88 C., and aged naturally.

(29) Table 4 shows the composition of strip 342, which underwent the additional heat treatment after solution annealing and quenching.

(30) TABLE-US-00004 TABLE 4 Strip Si %/wt Fe %/wt Cu %/wt Mn %/wt Mg %/wt Cr %/wt Zn %/wt Ti %/wt 342 1.3 0.17 0.00 0.06 0.3 0.0005 0.001 0.02

(31) The heat treatment, called a pre-bake step, did lead to a worsening of the breaking elongation properties, since the breaking elongation A.sub.80 was now below 30%. Surprisingly, the uniform elongation of aluminium strip P342 remained at over 25%, unchanged from the variant that did not undergo heat treatment, as is shown in Table 5. Uniform elongation is a very important factor in forming aluminium strip into a part, because improved uniform elongation enables higher degrees of deformation and thus either greater process reliability in manufacturing or fewer forming steps.

(32) Table 5 shows various measured values. On the one hand, three measurements were taken at the start of the strip P342-BA and at the end of the strip P342-BE. The State column indicates that the strips were in state T4, that is to say they were solution annealed and quenched, and had undergone natural aging for 8 days at room temperature. The strips from the strip start and strip end were cut out and measured in the longitudinal direction (L), that is to say in the direction of rolling, transversely to the direction of rolling (Q), and diagonally to the direction of rolling (D). It was found that while there was a fall in breaking elongation values A.sub.80 mm in some cases to below 30%, the uniform elongation A.sub.g still remained above 25% when measured in all directions and surprisingly was constant compared to the breaking elongation of the strip that had not under gone heat treatment.

(33) TABLE-US-00005 TABLE 5 Strip/ a.sub.o R.sub.p0.2 R.sub.m A.sub.80 mm Position State Pos (mm) (MPa) (MPa) A.sub.g % % P342- T4 (8 d RT) L 1,009 97 209 25.3 28.9 BA P342- T4 (8 d RT) Q 1.006 90 206 25.5 28.5 BA P342- T4 (8 d RT) D 1.005 92 207 25.6 29.1 BA P342- T4 (8 d RT) L 1.002 95 208 25.9 30.1 BE P342- T4 (8 d RT) Q 1.000 89 204 25.3 28.3 BE P342- T4 (8 d RT) D 1.000 90 205 25.7 29.8 BE

(34) In a subsequent artificial aging step, the state T6 was reached after 20 minutes at 185 C. Typical values for the tensile yield point measured in state T6 were higher than 140 MPa after artificial aging and higher than 165 MPa after artificial aging following by further stretching of 2%. The aluminium strip prepared according to the invention that also underwent heat treatment, therefore combines to important properties. In the T4 state it is very readily deformable because of its high uniform elongation, and at the same time it reaches the desired strength after artificial aging at 185 C. for 20 min.