ALUMINIUM ALLOY, METHOD FOR PRODUCING AN ALUMINIUM FLAT PRODUCT, ALUMINIUM FLAT PRODUCT AND USE THEREOF

Abstract

An aluminium alloy for superplastic aluminium flat products having the following composition: Si0.4 wt. %, Fe0.4 wt. %, Cu0.1 wt. %, 0.5 wt. %Mn1.0 wt. %, 4.7 wt. %Mg5.5 wt. %, 0.05 wt. %Cr0.25 wt. %, Zn0.25 wt. %, Ti0.20 wt. %, Na2 ppm, unavoidable impurities individually 0.05 wt. %, in total 0.15 wt. %, remainder aluminium. A method for producing an aluminium flat product in which an aluminium melt is provided from the above-mentioned aluminium alloy, in which the aluminium melt is cast to form an ingot, in which the ingot is hot rolled to form a hot strip, in which the hot strip is cold rolled to form a cold strip, and in which the cold strip is levelled. Further disclosed is an aluminium flat product produced by the method and a use thereof.

Claims

1. A method for producing an aluminium flat product, in particular a superplastic aluminium flat product, comprising: providing an aluminium melt from an aluminium alloy having the following composition: TABLE-US-00008 0.03 wt. % Si 0.10 wt. %, Fe 0.4 wt. %, Cu 0.1 wt. %, 0.5 wt. % Mn 1.0 wt. %, 5.2 wt. % Mg 5.5 wt. %, 0.05 wt. % Cr 0.25 wt. %, Zn 0.25 wt. %, Ti 0.20 wt. %, Na 2 ppm, unavoidable impurities individually 0.05 wt. %, in total 0.15 wt. %, remainder aluminium, casting the aluminium melt to form an ingot, hot rolling the ingot to form a hot strip, cold rolling the hot strip to form a cold strip, and levelling the cold strip.

2. The method according to claim 1, wherein the aluminium alloy has an Fe content of 0.05-0.15 wt. % and/or a Cu content of max. 0.05 wt. %.

3. The method according to claim 1, wherein the aluminium alloy has a Mn content of 0.7 wt. % to 1.0 wt. %.

4. The method according to claim 1, wherein the aluminium alloy has a Zn content of max. 0.06 wt. % and/or a Ti content in the range 0.015-0.03 wt. %.

5. The method according to claim 1, wherein the aluminium alloy has a B content of max. 50 ppm and/or a Ca content of max. 15 ppm and/or a Li content of max. 15 ppm.

6. The method according to claim 1, wherein providing comprises melting together a preliminary aluminium melt with additives to achieve the composition of the molten aluminium, wherein at least two of the alloying elements Cr, Mn and Ti, preferably all three alloying elements Cr, Mn and Ti, are charged separately from one another.

7. The method according to claim 1, wherein the degree of rolling during cold rolling is in total in the range of 70% to 80%, wherein the degree of rolling in the last roll pass is preferably less than 33%.

8. The method according to claim 1, further comprising cutting the cold strip into sheets after levelling without intermediate coiling.

9. The method according to claim 1, wherein the levelling of the cold strip is performed by means of levelling rollers with a diameter of more than 60 mm.

10. An aluminium flat product, in particular superplastic aluminium flat product, produced by a method according to claim 1.

11. The aluminium flat product according to claim 10, wherein the aluminium flat product after a heat treatment for 30 minutes at 500 C. has a yield strength R.sub.p0.2 of at least 160 MPa, in particular at least 170 MPa, and a tensile strength R.sub.m of at least 310 MPa, in particular at least 320 MPa.

12. The aluminium flat product according to claim 10, wherein the aluminium flat product after a superplastic forming at a forming temperature of 515 C., a strain rate of 2.510.sup.4s.sup.1 and a total elongation of 100%, has a porosity of less than 1.5%, in particular less than 1%.

13. Use of an aluminium flat product according to claim 10 for producing an aluminium product by superplastic forming of the aluminium flat product, in particular blow moulding.

14. Use according to claim 13, wherein the superplastic forming is performed at a strain rate of at least 10.sup.3s.sup.1, in particular of at least 10.sup.2s.sup.1.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0048] Further features and advantages of the method, of the aluminium flat product and its use emerge from the following description of exemplary embodiments, wherein reference is made to the respective drawing.

[0049] In the drawing

[0050] FIG. 1 shows an exemplary embodiment of the method,

[0051] FIG. 2 shows an exemplary embodiment of the use of the aluminium flat product produced by the method,

[0052] FIG. 3 shows a second exemplary embodiment for the use of the aluminium flat product produced by the method of FIG. 1,

[0053] FIG. 4 shows a micrograph of an aluminium flat product with coarse Cr-containing particles,

[0054] FIGS. 5-7 show micrographs of aluminium flat products before a heat treatment (FIG. 4), after 1 minute (FIG. 5) and after 60 minutes' heat treatment at 500 C. (FIG. 6)

[0055] FIG. 8 shows a diagram with experimental results for the forming temperature-dependent strain rate sensitivity m, and

[0056] FIGS. 9-10 show diagrams with test results for the yield strength R.sub.p0.2 (FIG. 9) and tensile strength R.sub.m (FIG. 10) at room temperature after 30 minutes' heat treatment at various temperatures.

DETAILED DESCRIPTION

[0057] FIG. 1 shows an exemplary embodiment of the method for producing a flat aluminium product in a schematic representation.

[0058] In a first step 2 of the method a preliminary aluminium melt is firstly prepared, in which primary metal 4 and alloying additives 6 are added to an aluminium melting furnace 8 and melted there. The use of aluminium scrap is preferably largely dispensed with for the production of the preliminary aluminium melt 10.

[0059] In the second step 12 the preliminary aluminium melt 10 is homogenised in the melting furnace 8, which is illustrated in FIG. 1 by the schematically illustrated stirrer 14.

[0060] The homogenised preliminary aluminium melt 10 in the aluminium melting furnace 8 has the following composition:

TABLE-US-00002 0.03 wt. % Si 0.10 wt. %, 0.05 wt. % Fe 0.15 wt. %, Cu 0.05 wt. %, 0.7 wt. % Mn 1.0 wt. %, Mg 1 wt. %, Cr 0.05 wt. %, Zn 0.06 wt. %, 0.015 wt. % Ti 0.030%, Na 1.0 ppm,
unavoidable impurities individually up to a maximum of 0.05 wt. %, in total not more than 0.15 wt. %, remainder aluminium. The low Na content can be achieved, for example, by a chlorine treatment of the melt.

[0061] In the third step 16 chromium-containing material 18 is added to the preliminary aluminium melt 10 and the resulting (still preliminary) aluminium melt 22 is in turn homogenised in the fourth step 20 (as illustrated by the stirrer 14). The homogenised aluminium melt 22 has the following composition:

TABLE-US-00003 0.03 wt. % Si 0.10 wt. %, 0.05 wt. % Fe 0.15 wt. %, Cu 0.05 wt. %, 0.7 wt. % Mn 1.0 wt. %, Mg 1 wt. %, 0.12 wt. % Cr 0.18 wt. %, Zn 0.06 wt. %, 0.015 wt. % Ti 0.030%, Na 1.0 ppm,
unavoidable impurities individually up to a maximum of 0.05 wt. %, in total not more than 0.15 wt. %, remainder aluminium.

[0062] A separate charging of Mn or Ti and Cr is achieved with the described steps 2, 12 and 16. In the first step 2 the Mn content and the Ti content are firstly adjusted, while the material 18 to be added for the adjustment of the desired Cr content is added separately only in the third step 16 after a homogenisation of the preliminary melt 10 in step 12. In a similar way Ti can also be charged separately from Mn. In addition to the content of Mn and Ti, in the present example the contents of further alloying elements (in particular Si and Fe) are also adjusted in the first step 2. The charging of these alloying elements (in the present example, in particular Mn, Ti, Si and Fe) can be carried out simultaneously or also separately from one another.

[0063] In the fifth step 23 magnesium-containing material 24 is added to the preliminary aluminium melt 22, and the resulting aluminium melt 25 is in turn homogenised in the sixth step 26 (as illustrated by the stirrer 14). The homogenised aluminium melt 25 has the following composition:

TABLE-US-00004 0.03 wt. % Si 0.10 wt. %, 0.05 wt. % Fe 0.15 wt. %, Cu 0.05 wt. %, 0.7 wt. % Min 1.0 wt. %, 5.2 wt. % Mg 5.5 wt. %, 0.12 wt. % Cr 0.18 wt. %, Zn 0.06 wt. %, 0.015 wt. % Ti 0.030%, Na 1.0 ppm,
unavoidable impurities individually up to a maximum of 0.05 wt. %, in total not more than 0.15 wt. %, remainder aluminium.

[0064] In this way, Mg is charged only after Mn/Ti and Cr, preferably as the last alloying element of the aluminium melt, in order to prevent the melting loss of Mg. To this end the temperature of the aluminium melt during the charging of Mg is also preferably less than 740 C., in particular at most 730 C. On the other hand, the temperature of the aluminium melt when charging Cr is preferably more than 740 C., in particular at least 750 C., in order to disperse Cr uniformly in the aluminium melt.

[0065] The aluminium melt 25 is cast in the following step 27 by direct chill casting into an ingot 28. To this end the aluminium melt 25 is poured, for example from a crucible 29, into a cooled and downwardly open frame mould 30 and solidified by spraying with water 31, resulting in the formation of the ingot 28.

[0066] In the following step 32 the ingot 28 undergoes ingot homogenisation and/or ingot preheating in a homogenising furnace 34 and in the following step 36 is hot rolled in a reversing hot rolling stand 38 for example, to form the hot strip 40, preferably at a temperature in the range of 280 C. to 550 C., wherein in particular a hot strip temperature of 280 C. to 350 C. is adjusted. Due to the low Na content of the aluminium alloy of the ingot 28 no edge cracks are formed during hot rolling, despite the high Mg content.

[0067] In the following step 42 the hot strip 40 is cold rolled in multiple passes without intermediate annealing on one or more cold rolling stands 44, so that finally a cold strip 46 with a final thickness in the range of 1 to 3 mm is formed. The overall degree of rolling in the cold rolling is at least 70%, the degree of rolling in the last pass being less than 33%.

[0068] In the following step 48 the cold strip 46 is guided through a levelling system 50 with multiple levelling rollers 52 arranged offset to one another and is thereby levelled. The levelling rollers 52 each have a diameter of 60 mm, so that the formation of surface defects is avoided during levelling. After levelling, the cold-rolled strip 46 is cut directly into sheets 56 by means of a cutting device 54, without an intermediate coiling to a coil. This in turn avoids a unilateral buckling or elongation of the cold strip 46.

[0069] The aluminium sheets 56 produced by the method described in FIG. 1 are particularly well suited for further use in a superplastic forming process.

[0070] FIG. 2 shows an embodiment for a use of an aluminium sheet 56 produced with the method of FIG. 1 for producing a component 66 by means of superplastic forming.

[0071] In a first step 68 the aluminium sheet 56 is heated to a temperature in the range of 450 C. to 520 C. The heating can be carried out for example as exemplified in FIG. 2, in a chamber furnace or a continuous furnace 70. Additionally or alternatively, the heating of the aluminium sheet 56 can also take place directly in a forming tool 78 for forming the aluminium sheet 56. In this case a separate furnace 70 can in particular be dispensed with.

[0072] Owing to the high dislocation density introduced into the material during the cold rolling step 42 of FIG. 1, when heating the aluminium sheet 56, for example in the furnace 70 or in the tool 78, a spontaneous recrystallisation of the aluminium sheet 56 occurs with formation of a very fine microstructure, which has an advantageous effect on the subsequent superplastic forming. In contrast to a chamber furnace, in particular the heating in the tool or in the continuous furnace favoured the superplastic forming, since the transfer and residence times during which the material is exposed to high (forming) temperatures are minimised, and grain growth before the actual forming is thereby further minimised.

[0073] In a second step 72 the aluminium sheet 56 is arranged between a first die half 74 and a second die half 76 of the forming tool 78 for the superplastic forming, unless this has already happened beforehand for heating the aluminium sheet 56 in the forming tool 78. The first die half 74 has in FIG. 2 by way of example a concavity 80 and the second die half 76 has a bulge 82 corresponding thereto. However, the two die halves 74, 76 may instead also have more complex contours for producing a more complex shaped component.

[0074] In the next step 84 the two die halves 74, 76 are brought together, wherein the aluminium sheet 56 is superplastically formed. In particular, the degree of forming of the aluminium sheet 56 is locally in part 100% or more. On account of the good properties of the aluminium sheet 56 for the superplastic forming, in particular the fine and uniform microstructure, the aluminium sheet 56 does not constrict or crack despite the high degree of forming. After the two die halves 74, 76 have been separated a damage-free finished component 66 can thus be removed from the forming die 78 in the last step 86. In addition, the component 66 produced in this way also has a high surface quality without noticeable surface defects.

[0075] The properties of the aluminium sheet 56 enable the superplastic forming to be carried out very quickly. In particular, the bringing together of the two die halves 74, 76 can be effected within a few minutes, preferably in at most 5 minutes. The production time of the component 66 can thus be shortened and the cycle rate of the forming operations can be increased.

[0076] FIG. 3 shows a further exemplary embodiment for the use of an aluminium sheet 56 produced according to the method of FIG. 1 by means of superplastic forming.

[0077] In the first step 90 of the method an aluminium sheet 56, for example as illustrated by way of example in FIG. 3, is heated in a chamber furnace, a continuous furnace or a furnace of other type of construction to a temperature in the range of 450 C. and 520 C., so that a fine grain distribution is formed. Additionally or alternatively, the heating can also take place directly in a forming tool 98.

[0078] In contrast to heating in the chamber furnace, heating in the tool or in the continuous furnace favours the superplastic forming since the transfer and residence times during which the material is exposed to high (forming) temperatures is minimised, thereby further minimising grain growth before the actual forming.

[0079] The aluminium sheet 56 is then positioned in step 92 between a first tool half 94 and a second tool half 96 of the forming tool 98 for the blow moulding, unless the aluminium sheet 56 has not already been arranged there for heating in the forming tool 98. The first tool half 94 has by way of example a concavity 100 corresponding to the target shape of the component to be produced. The illustrated shape of the first tool half 94 is merely by way of example and can be significantly more complex in practice. In the second tool half 96 a channel 102 is provided for blowing in a gas.

[0080] In the next step 104 the first and second tool halves 94, 96 are brought together and a gas 106 is blown at a pressure of for example 2 bar through the channel 102 in the region of the concavity 100 against the aluminium sheet 56, so that the aluminium sheet 56 is superplastically formed until it abuts the contour of the concavity 100. The degree of forming of the aluminium sheet 56 is locally in part 100% or more.

[0081] Because of the good properties of the aluminium sheet 56 for superplastic forming, in particular the fine and uniform microstructure, there is no constriction or tearing of the aluminium sheet 56 despite the high degree of forming. After the two mould halves 94, 96 have been separated, a damage-free finished component 110 can thus be taken out from the forming tool 98 in the last step 108. Furthermore, the component 110 produced in this way also has a high surface quality without conspicuous surface defects.

[0082] The properties of the aluminium sheet 56 enable the superplastic forming to be carried out very quickly. In particular, the gas 106 can be introduced at such a pressure through the channel 102 that the aluminium sheet 56 adopts the contour of the concavity 100 within a few minutes, preferably in at most 5 minutes. The production time of the component 110 can thus be shortened and the cycle rate of the forming operations can be increased.

[0083] In experiments, the formation of coarse particles in the aluminium melt was investigated depending on the charging of the dispersoid formers Cr, Mn and Ti.

TABLE-US-00005 TABLE 1 (all data in wt. %) Si Fe Cu Mn Mg Cr Zn Ti Na B Ca Li Al 0.060 0.126 0.001 0.576 4.282 0.185 0.004 0.017 <0.0001 0.001 0.0001 <0.0001 remainder

[0084] For this purpose an aluminium melt A of the composition mentioned in Table 1 was first produced by melting primary aluminium in an aluminium melting furnace and simultaneously adding to it additives to achieve the desired Mn, Mg and Cr contents. Furthermore, an aluminium melt B of the same composition was produced, wherein Mn and Cr were charged separately, i.e., the Cr-containing additives to achieve the desired Cr content were added only after adjusting the desired Mn content and then homogenising the aluminium melt by stirring. The Cr content in the preliminary molten aluminium during the adjustment of the desired Mn content and during the subsequent homogenisation of the melt was thereby less than 0.05 wt. %, and was only later adjusted to the target value.

[0085] From each of the two aluminium melts A and B produced in different ways, ingots were cast and strips were produced by hot and cold rolling. The strips exhibited coarse particles both on the surface and in the interior, the composition of which was analysed by WDX analysis (wavelength dispersive X-ray spectroscopy). The following Table 2 shows the results of the WDX analysis on six different coarse particles (Nos. 1-6) of a strip from the aluminium melt A, of which the particles Nos. 1-4 were on the surface and the particles 5 and 6 were in the interior of the strip:

TABLE-US-00006 TABLE 2 Particle No. Mg Al Ti Cr Mn Fe 1 607 58061 354 5232 2909 223 2 5890 57001 3339 4806 3086 280 3 7729 51707 185 4339 1356 4 7194 54343 403 4607 1167 5 445 58683 313 5020 3342 300 6 499 57399 332 5084 3089 240

[0086] The numbers given in Table 2 are in each case pulse numbers of the WDX analysis for the respective elements. The numbers are approximately proportional to the content of the elements in the respective particle.

[0087] In addition, a microsection was prepared from a piece of the strip produced from the aluminium melt A. FIG. 4 shows an image of this polished and barked microsection. The micrograph clearly shows a coarse Cr-containing phase. The phase has a size of 46 m210 m in the microsection.

[0088] The aforedescribed WDX analyses show that the strips from the aluminium melt A had significant fractions of high-melting and sparingly soluble Cr-containing phases, in some cases also with certain proportions of Ti and Mg. Such phases (cf. FIG. 4) re-dissolveonce formedonly with difficulty and form coarse, brittle particles in the strip, which adversely affect the superplastic properties of the strip or sheet produced therefrom.

[0089] The strips from the aluminium melt B exhibited practically no coarse particles or phases, i.e. only very fine but practically no coarse Al(Mn,Fe,Cr)Si phases have formed owing to the separate charging of Mn and Cr in the melt.

[0090] The investigated alloy with the composition of Table 1 has a lower Mg content than is envisaged according to the present teaching. For alloys with a Mg content of 5 wt.-% and otherwise identical composition as in Table 1 similar results are found however, with the formation of coarse Cr-containing phases with co-charging of the alloying elements Mn and Cr and only slight or in some cases without the formation of coarse Cr-containing phases with separate charging of Mn and Cr. The separate charging of Ti has also proved to be beneficial in order to prevent the formation of coarse phases.

[0091] In further experiments, an aluminium melt C was produced having the composition shown in Table 3 below, wherein (as in the previously described aluminium melt B) Mn and Cr were charged separately from one another, with intermediate homogenisation of the melt.

TABLE-US-00007 TABLE 3 (all data in wt. %) Si Fe Cu Mn Mg Cr Zn Ti Na B Ca Li Al 0.057 0.136 0.009 0.805 5.282 0.136 0.013 0.025 <0.0001 0.001 0.0004 <0.0001 remainder

[0092] The aluminium melt C was cast into an ingot by direct chill casting. The ingot was preheated, and by subsequent hot and cold rolling without intermediate annealing a cold strip having a thickness of 1.5 mm was produced, with an overall thickness reduction in the cold rolling of 75%. The cold strip was then levelled by levelling rollers with a diameter of more than 60 mm in each case and cut into sheets.

[0093] Some of these sheets were then subjected to a heat treatment at 450 C. for various durations in order to investigate the formation of the fine grain distribution important for superplastic forming. FIG. 5 shows an image of a polished and barked microsection of one of the sheets in the hard-rolled H19 state, i.e. before the heat treatment. The grains elongated by rolling are clearly visible.

[0094] Micrographs were taken of the heat-treated sheets and the average grain diameters in each case were determined according to ASTM E112. FIG. 6 shows an image of a polished and barked microsection of a sheet heat-treated at 450 C. for 1 minute. The fine-grain microstructure with grain sizes between 5 and 15 m and an average grain diameter of 7 m can readily be seen. This shows that the fine-grain microstructure important for superplastic forming is achieved practically instantaneously on heating up to the temperature for superplastic forming (typically 450 C.-520 C.). FIG. 7 shows an image of a polished and barked microsection of a sheet that was heat treated at 450 C. for 60 minutes. The microstructure is just as fine-grained as in FIG. 6, with a mean grain diameter of also 7 m. This shows the stability of the fine microstructure over time at the superplastic forming temperature. This stability is achieved in the investigated metal sheets in particular by the contents of Mn and Cr and their fine distribution in the aluminium matrix, in particular by the separate charging of Mn, Ti and Cr, which permanently prevent the growth of the aluminium grains.

[0095] Furthermore, the metallographic investigations show that the sheets have no coarse particles that would lead to pore formation during superplastic forming. This is achieved in particular by the low contents of Fe and Si as well as by the separate charging of Cr. The micrographs in FIGS. 6 and 7 show that the sheets exhibit a fine-grain microstructure at the forming temperature, which has a very stable mean grain diameter even at high forming temperatures.

[0096] On the sheets as described above produced from the aluminium melt C, superplastic forming tests according to Lederich were carried out by means of the incremental strain rate test (Lederich et al. Superplastic Formability Testing Journal of Metals Vol. 34 Issue 8, pp. 16-20, 1982) under in each case successive use of the strain rates 510.sup.4s.sup.1, 110.sup.4s.sup.1, 510.sup.4s.sup.1, 110.sup.3s.sup.1 510.sup.3s.sup.1, 110.sup.2s.sup.1, 510.sup.4s.sup.1 and 110.sup.1s.sup.1 and an ISO 20032:2007 compliant testing machine and sample geometry for determining the strain rate sensitivity m at four different forming temperatures (450 C., 475 C., 500 C. and 525 C.). The aforementioned strain rate sequence was thus run through for a first sheet sample at a forming temperature of 450 C., for a second sheet sample at a forming temperature of 475 C. etc. In this connection the strain rate 510.sup.4s.sup.1 in the above-mentioned strain rate sequence was used a total of three times in each case to detect any hardening or softening due to the high-temperature forming.

[0097] In order to determine the strain rate sensitivity m dependent on the strain rate, in each case the values for the yield stress 6 measured for a sheet thickness at the different strain rates were plotted double-logarithmically against the associated strain rates and the function F({dot over ()})=ln()/ln({dot over ()}), which is dependent on the strain rate, was determined by fitting a second degree Polynomial to the measured values. The derivative of the function F({dot over ()}), i.e.

[00002]$\frac{\mathrm{dP}\left(\stackrel{.}{\mathrm{.Math.}}\right)}{d.Math.\stackrel{.}{\mathrm{.Math.}}}=\frac{d.Math..Math.\ln \left(\right)}{d.Math..Math.\ln \left(\stackrel{.}{\mathrm{.Math.}}\right)},$

or the derivative of the polynomial fitted for it, respectively, then corresponds to the strain rate sensitivity m() as a function of the strain rate {dot over ()}.

[0098] The results of the forming experiments are shown in the diagram in FIG. 8, in which the forming temperature T of the superplastic forming in C. is plotted on the X-axis and the (dimensionless) strain rate sensitivity m is plotted on the Y-axis. Here in FIG. 8 the values of the previously described determined function m() for the strain rates 110.sup.4s.sup.1 (+ symbols), 110.sup.3s.sup.1 (x-symbols), 110.sup.2s.sup.1 (o-symbols) and 110.sup.1s.sup.1 (squared symbols) are plotted for each forming experiment. The lines plotted in FIG. 8 connect the m-values of the four forming experiments, which were each determined respectively for the same strain rate.

[0099] As FIG. 8 shows, at the forming temperatures in the range 450 C.-520 C. typical for superplastic forming, strain rate sensitivities m>0.3 were achieved not only at the typical forming rates 10.sup.4s.sup.1 to 10.sup.3s.sup.1 but also at higher forming rates, in particular forming rates of 10.sup.2s.sup.1 or higher. Accordingly, the sheets are suitable not only for superplastic forming at conventional strain rates, but also for high-speed superplastic forming with very high strain rates, as a result of which the forming times can be significantly reduced and thus higher production rates can be achieved.

[0100] To investigate the porosity and the mechanical properties after superplastic forming, sheets produced as described above from the aluminium melt C were superplastically formed at a forming temperature of 515 C. with an ISO 20032:2007 compliant testing device in a uniaxial tensile test, in which the sample geometry was based on the abovementioned Standard (ISO 20032:2007 S-Type sample shape). The strain rate was 2.510.sup.4s.sup.1 and the total strain at the end of the forming was 100%.

[0101] On some of these sheets that were superplastically formed at a forming temperature of 515 C., the porosity was determined by means of the metallographic microsection and cutting test in accordance with the VDG-Merkblatt (VDG leaflet) P201. The tested sheets showed a very low porosity in the range of 0.3% to 0.7%.

[0102] Furthermore, tensile tests were carried out on some of the sheets in order to determine the yield strength R.sub.p0.2 and the tensile strength R.sub.m according to DIN EN ISO 6892-1:2017, wherein the test was carried out transversely to the rolling direction. The tensile tests were carried out in each case after heating the sheets in order to achieve the desired microstructure for the superplastic forming. The sheets were not superplastically formed before the tensile tests.

[0103] The results of the tensile tests are shown in the diagrams in FIGS. 9 and 10, in which the superplastic forming temperature T in C. is plotted on the X-axis, and the yield strength R.sub.p0.2 and the tensile strength R.sub.m, in each case in MPa, are plotted on the Y-axis. As the test results show, the sheets had a yield strength R.sub.p0.2 of more than 160 MPa over the entire forming temperature range investigated, and even a yield strength R.sub.p0.2 of more than 170 MPa at a forming temperature of 500 C. The tensile strength of the sheets was significantly above 310 MPa, even above 320 MPa, over the entire forming temperature range investigated. The good mechanical properties after superplastic forming are due in particular to the advantageous Mn content of at least 0.7 wt. %, the advantageous Mg content of at least 5.2 wt. %, and the separate charging of Cr and Mn.

[0104] Due to the Mn content of at least 0.7 wt. % and the separate charging of Cr and Mn, in particular the formation of coarse particles and thereby a pore formation adversely affecting the mechanical properties is also reduced or even prevented in the superplastic forming. In superplastic forming there is therefore virtually no further softening over and above the softening induced by the heating, so that the measured values for R.sub.p0.2 and R.sub.m shown in FIGS. 9 and 10 are achieved by the sheets after the heating and also the superplastically formed sheets.

[0105] All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0106] The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0107] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.