EP1114877A1 - Al-Cu-Mg alloy aircraft structural element - Google Patents

Abstract

An aircraft structure component has a rolled, extruded, or forged product, as a starting material, made of an alloy containing (wt.%): copper (4.6-5.3), magnesium (0.10-0.50), manganese (0.15-0.45), silicon (less than 0.10), iron (less than 0.15), zinc (less than 0.20), chromium (less than 0.10), other elements (less than 0.05 each, less than 0.15 total), and aluminum (balance). An aircraft structure component, particularly a lower wing component of an aircraft, consists of a rolled, extruded, or forged product, as a starting material, which is made of an alloy containing (wt.%): copper (4.6-5.3), magnesium (0.10-0.50), manganese (0.15-0.45), silicon (less than 0.10), iron (less than 0.15), zinc (less than 0.20), chromium (less than 0.10), other elements (less than 0.05 each, less than 0.15 total), and aluminum (balance) treated by solution heat treating, quenching, controlled tension to more than 1.5 % permanent deformation, and aging. An Independent claim is also included for a process of manufacturing the aircraft structure component, including casting a plate or billet with the alloy composition, homogenizing the plate or billet, hot transforming the plate by rolling or the billet by extrusion or forging to obtain a product thicker than 10 mm, quenching the hot transformed product; and solution heat treating, controlled tensioning, aging, and machining the product.

Description

Field of the invention

The invention relates to aircraft structural elements, in particular wing skin and stiffeners for large commercial aircraft capacity, produced from rolled, extruded or forged products in AlCuMg alloy at the state treated by dissolving, quenching and tempering, and having, with respect to prior art products used for the same application, an improved compromise between the different properties of use required.

The designation of alloys and metallurgical states used below corresponds to the Aluminum Association nomenclature, taken up by European standards EN 515 and EN 573 part 3.

State of the art

The wings of large commercial aircraft have a top (or upper surface) made of a skin made from thick alloy sheets 7150 in state T651, or in alloy 7055 in state T7751 or 7449 in state T7951, and stiffeners made from profiles of the same alloy, and a lower part (or lower surface) made of a skin made from thick sheets of alloy 2024 to state T351 or 2324 in state T39, and stiffeners made from profiles of same alloy. The two parts are assembled by side members and ribs.

Alloy 2024 according to the designation of the Aluminum Association or standard EN 573-3 has the following chemical composition (% by weight):
If <0.5 Fe <0.5 Cu: 3.8 - 4.9 Mg: 1.2 - 1.8 Mn: 0.3 - 0.9 Cr <0.10 Zn <0.25 Ti <0 , 15

In order to improve the compromise between the different properties required, in particular mechanical strength and toughness, various alternative solutions have been proposed. Boeing has developed the composition 2034 alloy:
If <0.10 Fe <0.12 Cu: 4.2 - 4.8 Mg: 1.3 - 1.9 Mn: 0.8 - 1.3 Cr <0.05 Zn <0.20 Ti <0 , 15 Zr: 0.08 - 0.15

This alloy was the subject of patent EP 0031605 (= US 4,336,075). He presents, by compared to 2024 in state T351, a better specific elastic limit due to increasing the manganese content and adding another anti-recrystallizer (Zr), as well as improved toughness and fatigue resistance.

US patent 5652063 (Alcoa) relates to an airplane structure element produced from an alloy of composition (% by weight):
Cu: 4.85 - 5.3 Mg: 0.51 - 1.0 Mn: 0.4 - 0.8 Ag: 0.2 - 0.8 Si <0.1 Fe <0.1 Zr <0, 25 with Cu / Mg between 5 and 9.

The sheet of this alloy in the T8 state has a yield strength> 77 ksi (531 MPa). The alloy is particularly intended for supersonic aircraft.

US patent 5593516 (Reynolds) relates to an alloy for aeronautical applications containing from 2.5 to 5.5% Cu and 0.1 to 2.3% Mg, in which the contents of Cu and Mg are kept below their limit of solubility in aluminum, and are linked by the equations:
Cu _max = 5.59 - 0.91 Mg and Cu _min = 4.59 - 0.91Mg

The alloy can also contain: Zr <0.20% V <0.20% Mn <0.80% Ti <0.05% Fe <0.15% Si <0.10%

US patents 5,376,192 and US 5,512,112, issued from the same initial application, relate to alloys of this type containing 0.1 to 1% silver. We can notice that the use of silver in this type of alloy leads to an increase in the cost of development and difficulties for the recycling of manufacturing scraps.

In addition, we have known for many years alloys of the type "AU6MGT" according to the old designation of alloys in France. The FR patent 1379764, deposited in 1963 by Pechiney, concerns the use of an alloy of this type composition: Cu: 5 - 7 Mg: 0.10 - 0.50 Mn 0.05 - 0.50 Si <0.30 Fe <0.50 Ti: 0.05 - 0.25 for the manufacture of cylinders for compressed gases.

The Aluminum Association registered in 1976 the alloy 2001 of composition:
Cu: 5.2 - 6 Mg: 0.20 - 0.45 Mn: 0.15 - 0.50 Si <0.20 Fe <0.20 Cr <0.10 Zn <0.10 Ni <0.05 Ti <0.20 Zr <0.05

To the knowledge of the inventors, there is no other industrial use of this alloy than compressed gas cylinders produced by reverse spinning.

Problem

In commercial aircraft construction, the current trend is to use growing of very thick products, in the mass of which the structural elements are machined. For example, for some small planes, the skins of wings are machined from relatively thick sheets to allow machining in the mass of the wing stiffeners, while these are usually made from folded sections or sheets, and are then mechanically fixed to the skin. The integral machining in the mass of the skin-stiffener assembly leads to a reduction in manufacturing costs, since the number of parts is reduced and that we avoid assembly. In addition, the use of an unassembled structure allows a reduction in the weight of the whole.

It is therefore desirable that in addition to the properties usually sought for structural elements of aircraft, namely high mechanical strength, good tolerance to damage and good resistance to fatigue and various forms of corrosion, the sheets have homogeneous mechanical characteristics over the entire thickness, i.e. the properties do not vary so significant depending on the thickness, typically between 10 and 120 mm. Else On the other hand, the more machining is used, the more desirable the machining stability, which is obtained by a low level of internal stresses. However, it is known that, for a thick sheet, the mechanical characteristics are all the more homogeneous, and the internal stresses all the more reduced, as the sheet has a low sensitivity to quenching.

Finally, aircraft wings, especially large capacity aircraft, have a curved wing profile, with a curvature both in the longitudinal direction and in the transverse direction. This complex form can be obtained during the operation of returned to an autoclave, by forming on a mold, by depressing the face sheet metal on the mold side with respect to the opposite face, using a partial vacuum. he this operation must be successful, to avoid costly scrap of a part with high added value, especially for large parts. The pledge of success lies in the lowest possible elastic return for a form of mold given, because the elastic return is most often difficult to control.

The object of the present invention is to provide structural elements for aircraft having properties at least equivalent to those of the same elements produced in alloy 2024 in state T351 with regard to mechanical characteristics static, toughness, crack propagation speed and resistance to corrosion, using rolled, extruded or forged products with low level of residual stresses, low sensitivity to hardening and good income training skills.

Subject of the invention

The subject of the invention is a structural element, in particular an aircraft wing underside element, produced from a rolled, spun or forged product, made of an alloy of composition (% by weight):
Cu: 4.6 - 5.3 Mg: 0.10 - 0.50 Mn: 0.15 - 0.45 Si <0.10 Fe <0.15 Zn <0.20 Cr <0.10 other elements < 0.05 each and <0.15 in total, Al remains treated by dissolving, quenching, controlled traction to more than 1.5% permanent deformation and tempering.

• limite d'élasticité R_0,2 (sens TL) > 350 MPa, de préférence > 370 MPa,
• ténacité K_1c (sens L-T) > 42 MPa√m
• résistance à la corrosion intercristalline de type P selon la norme ASTM G110.

This element has at least one of the following properties:

• yield point R _0.2 (TL direction)> 350 MPa, preferably> 370 MPa,
• toughness K _1c (LT direction)> 42 MPa√m
• resistance to type P intercrystalline corrosion according to ASTM G110.

a) la coulée d'une plaque ou d'une billette de la composition mentionnée ci-dessus,

b) l'homogénéisation de cette plaque ou billette,

c) la transformation à chaud de cette plaque par laminage ou de cette billette par filage ou forgeage pour obtenir un produit d'épaisseur supérieure à 10 mm,

d) la trempe du produit transformé à chaud,

e) la mise en solution de ce produit, de préférence à une température inférieure de moins de 10°C à la température de fusion commençante de l'alliage,

f) la traction contrôlée du produit jusqu'à une déformation permanente de plus de 1,5%,

g) le revenu du produit à une température supérieure à 160°C, éventuellement associé à un formage,

h) l'usinage du produit éventuellement formé jusqu'à la forme finale de l'élément de structure.

The invention also relates to a method of manufacturing a structural element comprising:

a) pouring a plate or a billet of the composition mentioned above,

b) the homogenization of this plate or billet,

c) the hot transformation of this plate by rolling or of this billet by spinning or forging to obtain a product of thickness greater than 10 mm,

d) the quenching of the hot processed product,

e) dissolving this product, preferably at a temperature of less than 10 ° C below the starting melting temperature of the alloy,

f) the controlled traction of the product up to a permanent deformation of more than 1.5%,

g) the income of the product at a temperature above 160 ° C., possibly associated with forming,

h) the machining of the product possibly formed up to the final shape of the structural element.

In the case where the product is a sheet, the entry temperature for hot rolling is preferably at least 40 ° C lower, and more preferably at least 40 ° C 50 ° C, at the solution temperature.

Description of the invention

The invention is based on the observation that an alloy of the 2001 type, with certain composition changes and a suitable manufacturing range, could present a set of properties making it suitable for use in structures aircraft, and more particularly in the lower surfaces of commercial aircraft wings high capacity, with interesting properties in terms of low sensitivity to quenching, low residual stresses and tempering.

Compared with the 2001 alloy, the copper content range is clearly shifted towards the low, while remaining higher than that of the 2024 or 2034 alloys for lower surface, for compensate, in its influence on the mechanical resistance, the low content of magnesium. It is preferable to choose a copper content higher than 4.8%, or even at 4.9% or even 5%. The magnesium content is of the same order as in the alloy 2001, and preferably between 0.20 and 0.40%. The Cu / Mg ratio is thus almost always greater than 10, contrary to the teaching of the US patent 5652063, which recommends a Cu / Mg ratio between 5 and 9.

The manganese content is controlled within a relatively narrow range. Below 0.15%, we would risk having a grain too large; above 0.45%, we obtain a non-recrystallized structure which is not favorable for the control of constraints residual. A preferred range is between 0.25 and 0.40%. Note that, for the same reason, and contrary to the teaching of US Pat. No. 5,593,516, the alloy does not contain any other anti-recrystallizing element such as vanadium or zirconium.

The iron and silicon contents are maintained respectively below 0.15 and 0.10%, and preferably below 0.09 and 0.08%, to guarantee good tenacity. The alloy can contain up to 0.2% zinc, this addition having an effect favorable on mechanical resistance, without risk for other properties, such as corrosion resistance.

The transformation range includes the casting of a plate or a billet, a reheating or homogenization to a temperature close to the temperature of beginning alloy melting and hot transformation by rolling, spinning or forging. In the case of rolling, this may include a pass, called widening, in the direction perpendicular to that of the other passes, and intended for improve the isotropy of the product. The hot transformation temperature is located, preferably at a level slightly lower than that which the man of profession with reference to the solution temperature. So, with regard to the rolling, the inlet temperature is preferably at least 40 ° C, or even 50 ° C, below the solution temperature, and the outlet temperature of 20 to 30 ° C below the inlet temperature.

The product is then subjected to a solution as complete as possible, to a temperature close, for example less than 10 ° C below, the temperature of beginning of the alloy, while avoiding burns. This temperature is between 520 and 535 ° C. The quality of the solution can be checked by analysis differential enthalpy. The product is then soaked, for example by immersion in cold water, so as to ensure a cooling rate of between 10 and 50 ° C / s. After quenching, the product is pulled up to deformation permanent of at least 1.5%, so as to relax it and improve its flatness.

For the alloy according to the invention, this traction also has the effect of improving, by a work hardening effect, the elastic limit after tempering, so that one can qualify the state obtained from state T851, as if it were a specific pass hardening after quenching. As noted above, income itself can be performed at the same time as the shaping of the lower surface. This income is preferably performed at a temperature above 160 ° C (and above preferably> 170 ° C), of a duration allowing to reach the limit peak of elasticity, as for a state T6. Typically, a time income equivalent to that corresponding to 12 to 24 h at a temperature of 173 ° C is carried out; all time-temperature combination to reach the peak of alloy income is usable.

The metallurgical structure obtained is, unlike that of the alloys 2024 and 2034, highly recrystallized, with a recrystallization rate always exceeding 70%, and the more often 90%, over the entire thickness.

The structural elements according to the invention exhibit a compromise of properties (static mechanical characteristics, toughness, speed of crack propagation, corrosion resistance) which make them suitable for use in construction aeronautics, and in particular the manufacture of lower surfaces of wings. In addition, these elements can be easily made by machining and formed in tempering. Finally, the alloy used is easily weldable by conventional techniques, which can allow reduce the number of riveted connections.

Examples Example 1

Six alloys were prepared, the composition of which is indicated in Table 1. Alloy A is a 2024-T3 alloy of usual composition for the lower surface application of the airfoil. Alloy B is an alloy falling within the composition range described in US Pat. No. 5,652,063, but without the addition of silver. Alloy C is in accordance with the invention. Alloys D and E differ from alloy C only by a higher silicon for D, a higher manganese and copper for E and F, and an addition of zirconium for F. Alloy Yes Fe Cu Mn Mg Ti Zr AT 0.07 0.07 4.11 0.53 1.28 0.008 B 0.06 0.08 4.73 0.30 0.67 0.065 VS 0.05 0.08 5.26 0.30 0.28 0.062 D 0.15 0.08 5.28 0.30 0.31 0.065 E 0.07 0.10 5.64 0.99 0.29 0.012 F 0.06 0.08 5.47 0.67 0.29 0.014 0.11

Cast plates with a 380 x 120 mm section were homogenized, hot rolled to a thickness of 22 mm, dissolved, quenched in cold water, drawn to 2.3% permanent deformation and returned. The parameters for homogenization, hot rolling (inlet temperatures), dissolution and tempering are shown in Table 2. Alloy Homogenized neization Hot Rolling (entry) Dissolution Returned AT 4h 490 ° C 467 ° C 3h at 497 ° C - B 4h 490 ° C 467 ° C 3h at 518 ° C 4 p.m. at 173 ° C VS 4h 490 ° C 467 ° C 6h at 527 ° C 4 p.m. at 173 ° C D 4h 490 ° C 472 ° C 6h at 527 ° C 4 p.m. at 173 ° C E 1h 520 ° C 479 ° C 6h at 527 ° C 4 p.m. at 173 ° C F 1h 520 ° C 474 ° C 6h at 527 ° C 4 p.m. at 173 ° C

The mechanical characteristics were measured on the treated sheets: breaking strength R _m (in MPa), conventional yield strength at 0.2% R _0.2 (in MPa) and elongation at break A (in%) , on tensile specimens of circular section according to standard ASTM B 557, taken at mid-thickness in the directions L and TL (3 specimens per case).

The toughness was also measured by the critical stress intensity factor K _1c (in MPa√m) measured, according to standard ASTM E 399, on CT20 test pieces taken at quarter thickness in the directions LT and TL (2 test pieces per case).

All the results are collated in Table 3. Alloy R _m (L) R _0.2 (L) A (L) R _m (TL) R _0.2 (TL) A (TL) K _1c (LT) K _1c (TL) AT 472 362 21.3 467 321 21.4 41.8 40.5 B 476 439 12.5 475 427 11.2 41.3 34.6 VS 458 396 13.9 463 384 12.6 45.4 42.9 D 460 397 13.6 465 387 12.2 40.5 36.4 E 488 423 10.5 480 403 9.4 36.8 29.3 F 480 418 11.6 481 402 10.1 40.2 33.6

It is found that the alloy C according to the invention leads to an elastic limit significantly higher than that of 2024, and slightly lower than that of alloys B, E and F. The elongation is lower than for 2024, but better than that of alloys B, D, E and F. The toughness is the best of all the alloys tested. So we have a favorable compromise of these various properties. In particular, the results show the unfavorable effect, both on toughness and elongation, of a increased silicon and manganese content, as well as an addition of zirconium.

In addition, accelerated intercrystalline corrosion tests were carried out on samples of the 6 alloys, in the state T351 for the alloy 2024 (A) and T851 for the others, at the surface and at the core, according to ASTM G110. The type of corrosion observed is noted: P for pitting, I for intercrystalline corrosion and P + I for both. The maximum depth (P max in µm), the depth of intercrystalline corrosion (P CI in µm) and the percentage of intercrystalline corrosion on the sample are measured. The results are shown in Table 4: All. Surf. Surf. Surf. Surf. Heart Heart Heart Heart Type P max P CI % THIS Type P max P CI % THIS AT I + P 160 70 10 I + P 260 260 60 B P + I 130 30 10 P + I 160 50 10 VS P 150 - - P 120 - - D P 150 - - P 120 - - E P 200 - - P 140 - - F P 220 - - P 170 - -

It is observed that the alloy according to the invention has the second best resistance to inter-crystalline corrosion on the surface, and the best at heart. The difference between core and surface results is low, which is a favorable property when the structural element is manufactured by machining.

Finally, for the alloys A and C, the propagation speeds of fatigue cracks da / dn in the direction TL were compared, in mm / cycle, for values of ΔK between 15 and 30 MPa√m, according to the standard. ASTM E647. The results (2 tests per alloy) are shown in Table 5. Alloy 10 MPa√m 15 MPa√m 20 MPa√m 25 MPa√m 30 MPa√m AT 6.2 10 ^-5 3.8 10 ^-4 8.3 10 ^-4 1.8 10 ^-3 3.8 10 ^-3 AT 6.3 10 ^-5 3.8 10 ^-4 8.7 10 ^-4 1.9 10 ^-3 3.6 10 ^-3 VS 1.2 10 ^-4 4.0 10 ^-4 8.6 10 ^-4 1.5 10 ^-3 2.6 10 ^-3 VS 1.2 10 ^-4 4.2 10 ^-4 9.5 10 ^-4 1.8 10 ^-3 3.1 10 ^-3

We observe that the results are roughly comparable for the two alloys.

Example 2

The residual stress level was measured on 40 mm thick sheets of alloy 2024, 2034 and according to the invention, all three treated in the same state T351. The compositions (% by weight) are given in Table 6: Alloy Yes Fe Cu Mn Mg Ti Zr 2024 0.12 0.20 4.06 0.54 1.36 0.02 2034 0.05 0.07 4.30 0.98 1.34 0.02 0.10 Invent. 0.05 0.07 5.12 0.35 0.29 0.02

The method for measuring residual stresses is the bar method described in the applicant's patent EP 0731185. The arrows f _L and f _TL were measured in the directions L and TL (in microns) and the quotient fe / l ^{2 was} calculated in both cases, the thickness e and the length l of the bar being expressed in mm. The results are given in Table 7: alloy e (mm) 1 (mm) f _L (µm) f _L e / l ² f _TL (µm) f _TL e / l ² 2024 40 180 210 0.26 120 015 2034 40 180 147 0.18 129 0.16 invention 40 180 46 0.06 4 0.005 invention 80 385 84 0.05 136 0.07

It is noted that, unlike the 2024 or 2034 alloy test pieces, those according to the invention have a deflection such that the product fe is less than 0.10 l ² , which is, as can be seen in the patent EP 0731185 mentioned above, the indication of a low rate of internal stresses.

We measured, by image analysis on micrographs of the 4 samples above, the recrystallization rate (in%) at the surface, at quarter thickness and at the core.

The results are shown in Table 8: Alloy e (mm) Area Recryster rate (quarter-thick.) Recryster rate (to heart) 2024 40 80 60 30 2034 40 12 0 0 Inv. 40 100 100 100 Inv. 80 100 100 100

It is found that the alloy according to the invention has, in the treated state, a structure completely recrystallized throughout the thickness of the product.

Example 3

Was measured on samples according to the invention, of thickness 15, 40 and 80 mm, treated in the T851 state, with an entry temperature for hot rolling of 475 ° C., a solution treatment of 2 h at 528 ° C, and a 24 h income at 173 ° C, static mechanical characteristics (yield strength R _0.2 and breaking strength R _m in MPa and elongation A in%)) at quarter thickness and mid-thickness, in the L and TL directions. All the results are reproduced in Table 9. They show the small change in properties as a function of thickness, resulting from low sensitivity to quenching. e (mm) Pre-lev. R _{0.2 (L)} R _{m (L)} A _(L) R _{0.2 (TL)} R _{m (TL)} A _(TL) 15 ½ th. 400 451 13.6 392 458 12.1 40 ½ th. 387 439 13.7 376 448 11.2 80 ½ th. 388 436 11.4 376 443 9.8 80 ¼ th. 410 466 11.9 467 400 9.7

These sheets are particularly suitable for the manufacture of wing lower elements of aircraft by a manufacturing range comprising machining and one or more shaping operations.

Claims (24)

Structural element, in particular an aircraft wing underside element, produced from a rolled, extruded or forged product, made of an alloy of composition (% by weight):
Cu: 4.6 - 5.3 Mg: 0.10 - 0.50 Mn: 0.15 - 0.45 Si <0.10 Fe <0.15 Zn <0.20 Cr <0.10 other elements < 0.05 each and <0.15 in total, Al remainder, treated by dissolution, quenching, controlled traction to more than 1.5% permanent deformation and tempering. Structural element according to claim 1, characterized in that Si <0.08%. Structural element according to one of claims 1 or 2, characterized in that Fe <0.09%. Structural element according to one of claims 1 to 3, characterized in that Cu > 4.8%, and preferably> 4.9%. Structural element according to one of claims 1 to 4, characterized in that Cu > 5%. Structural element according to one of claims 1 to 5, characterized in that Mg is between 0.20 and 0.40%. Structural element according to one of Claims 1 to 6, characterized in that Mn is between 0.25 and 0.40%. Structural element according to one of claims 1 to 7, characterized in that it has an elastic limit R _0.2 (TL direction)> 350 MPa, and preferably> 370 MPa. Structural element according to one of claims 1 to 8, characterized in that it has a toughness K _1c (direction LT)> 42 MPa√m. Structural element according to one of claims 1 to 9, characterized in that it has a P-type intercrystalline corrosion resistance according to ASTM standard G110. Structural element according to one of claims 1 to 10, characterized in that dissolution takes place at a temperature less than 10 ° C below the starting melting temperature of the alloy. Structural element according to one of claims 1 to 11, characterized in that tempering is carried out at a temperature> 160 ° C (preferably> 170 ° C). Structural element according to one of claims 1 to 12, characterized in that the income is carried out at the same time as a forming operation. Structural element according to one of claims 1 to 13, characterized in that it has a recrystallization rate greater than 70% throughout the thickness, and preferably 90%. Structural element according to one of claims 1 to 14, characterized in that it is part of an aircraft wing underside. Structural element according to one of claims 1 to 14, characterized in that it is obtained by machining. Aircraft wing underside element according to claim 16, characterized in that the skin and the stiffeners are obtained by machining the same starting material. Structural element according to one of claims 16 or 17, characterized in that after machining, it has an arrow f in the directions L and TL such that: fe <0.10 l 2 f being expressed in µm, e being the thickness of the element and l the length of the bar-shaped test piece in mm. Method of manufacturing a structural element comprising: a) pouring a plate or billet of the composition:
Cu: 4.6 - 5.3 Mg: 0.10 - 0.50 Mn: 0.15 - 0.45 Si <0.10 Fe <0.15 Zn <0.20 Cr <0.10 other elements < 0.05 each and <0.15 in total, aluminum remains, b) the homogenization of this plate or billet, c) the hot transformation of this plate by rolling or of this billet by spinning or forging to obtain a product of thickness greater than 10 mm, d) the quenching of the hot processed product, e) dissolving this product, preferably at a temperature of less than 10 ° C below the starting melting temperature of the alloy, f) the controlled traction of the product up to a permanent deformation of more than 1.5%, g) the income of the product at a temperature above 160 ° C., possibly associated with forming, h) the machining of the product possibly formed up to the final shape of the structural element. Method according to claim 19, characterized in that the plate or the billet casting has a Cu content> 4.8%, and preferably> 4.9%. Method according to claim 19 or 20, characterized in that the plate or the billet billet has an Mg content of between 0.20 and 0.40%. Method according to one of claims 19 to 21, characterized in that the plate or the cast billet has an Mn content of between 0.25 and 0.40%. Method according to one of claims 19 to 22, characterized in that the income is carried out at a temperature> 170 ° C. Method according to one of claims 19 to 23, characterized in that the product is a sheet obtained by hot rolling with a lower inlet temperature of at least minus 40 ° C (and preferably at least 50 ° C) at the solution temperature.