CA3221029A1 - Strip made of 6xxx alloy and manufacturing process - Google Patents

Abstract

The invention relates to the field of 6xxx series aluminium alloy strips and to a process for manufacturing same. These strips are particularly useful for producing bodywork parts for motor vehicles on account of the compromise they offer between corrosion resistance and formability.

Description

Description Title: Strip made of 6xxx alloy and manufacturing process Field of the invention The invention relates to the field of aluminum alloy strips intended for the manufacture by stamping of car body-in-white parts of automobile vehicles.
State of the art Aluminum alloys are increasingly used in automobile construction to reduce vehicle weight, thereby cutting fuel consumption and greenhouse gas emissions.
Aluminum alloy strips are used especially for the manufacture of numerous "body-in-white"
parts, including car body skin parts (or car body outer panels) such as front fenders, roofs or body roofs, hood skins, trunk skins or door skins.
While many parts are already made from aluminum alloy strips, the transposition from steel to aluminum is still tricky, due to the poorer formability of aluminum alloys compared with steels.
Indeed, this type of application requires a set of properties, sometimes antagonistic, such as:
- high formability in the strip delivery temper, T4 temper, in particular for stamping operations, - a controlled yield strength in the strip delivery temper, T4 temper, to control spring back effect during forming, - good behavior in the various assembly processes used in automobile car body, such as spot welding, laser welding, bonding, clinching or riveting, - sufficient mechanical strength after cataphoresis and paint bake to achieve good in-service mechanical strength while minimizing weight of the part, - good resistance to corrosion, especially filiform corrosion of the finished part, - compatibility with recycling requirements for manufacturing waste or recycled vehicles, - acceptable cost for mass production.
Application W02013/037919 discloses a method for manufacturing an AlMgSi alloy strip, consisting in casting a rolling slab from an AlMgSi alloy, subjecting said rolling slab to homogenization, bringing the rolling slab to rolling temperature for hot rolling and then, optionally, cold rolling to final thickness. The objective of providing an improved method for manufacturing an aluminum strip made of an AlMgSi alloy, enabling the process of manufacturing AlMgSi aluminum strips with very good deformation properties to be made more

2 reliable, is achieved by making a hot strip whose temperature is greater than 130 C, preferably between 135 C and a maximum of 250 C, preferably not exceeding 230 C, when it comes out of the last hot rolling pass and then rolling up said hot strip at this temperature.
Application JP11172390 discloses an alloy having a composition consisting of, by weight, one or more types selected from 0.2-2.0 % Mg, 0.3-3.0 % Si, <+0.8 % Cu, 0, 01-10.4 %
Mn, 0.01-0.4 %
Cr, 0.01-0.4 % Zr, 0.01-0.4 % V, 0.03-0.5 % Fe, 0.005-0.2 % Ti and 0.01-3.0 %
Zn and balance Al with unavoidable impurities. The alloy is rolled, and the resulting alloy sheet is subjected to dissolution heat with treatment at >=480 C for <+5 min. Subsequently, first-step cooling is applied to the foil down to 50-150 C at an average cooling rate of 150 C/min.
Immediately after the end of cooling, cooling of the second step is performed down to 35 C
according to the inequality -1<log(R)<(0.0178T-1.289), where R is the average cooling rate ( C/h) at cooling of the second step and T is the finishing temperature in C of the first cooling step.
Application JP10060567 discloses an aluminum alloy with a composition of, by weight, 0.35 to 1.6 % Mg and 0.35 to 1.6 % Si (where Si/Mg > = 0.65), further containing at least one or more of <+0, 8 % Cu, <+0.1 % Ti, <+0.3 % Fe, <+0.3 % Cr, <+0.8 % Mn and <+0.15 % Zr, and the remainder Al with unavoidable impurities (respectively of <+0.05 %). Then, the size of Si precipitates at grain boundaries is regulated to <+1.01im, the distance between precipitates is regulated to >=51.tm, and its electrical conductivity is regulated to 40-45 %.
The problem The aim of the invention is to achieve an excellent compromise between all the properties required, and in particular between formability and corrosion resistance. The strip's formability is assessed in the T4 temper after natural ageing, natural ageing corresponding to the transport and storage duration between the strip's quenching and its stamping as a part.
Corrosion is assessed on the finished part, therefore after stamping of the strip, painting and paint bake.
Paint bake is also known to those skilled in the art as "bake hardening", as it simultaneously allows the hardening, by ageing, of the stamped strip to obtain the properties required for use of the part on an automobile vehicle.
Object of the invention One object of the invention is an aluminum alloy strip with a composition of, in % by weight:
Si: 1,2 ¨ 1,5, Fe: 0,25, Cu: 0,05, Mn: 0,15,

3 Mg: 0,20 ¨ 0,45, Cr: 0,002 ¨ 0,09, Ni: 0,15, Zn: 0,15, Ti: 0,15, Zr: 0,15, unavoidable elements and impurities in a maximum of 0.05 % each, together 0.15 % maximum, balance aluminum, Another object of the invention is a method for manufacturing an aluminum alloy strip according to the invention comprising the steps of:
a. Casting a slab, preferably by vertical semi-continuous casting, of alloy according to the invention, b. Homogenizing the slab preferably at a temperature between 500 C and 600 C, more preferably between 540 and 580 C and preferably between 1 hour and 12 hours, optionally followed by a second stage between 420 C and 550 C with a maximum duration of

4 hours, c. Cooling the slab to the hot rolling start temperature between 350 C and 550 C at a cooling rate preferably greater than 150 C/h or cooling the slab to room temperature and then reheating the slab to the temperature of said hot rolling start temperature, d. Hot rolling the slab into a strip at the hot rolling end temperature of between 250 C and 450 C, e. Cold rolling the strip, optionally in two parts separated by intermediate annealing, preferably in a coil, f. Solution heat treating the strip, preferably between 500 C and 600 C
for preferably 10 s to 60 s, followed by quenching, g. Pre-ageing the strip to a temperature by coiling the strip between 50 and 100 C, followed by cooling to room temperature, h. Naturally ageing the strip at room temperature between 72 hours and 6 months.
Another object of the invention is a car body part a. Supplying an aluminum alloy strip according to the invention, b. Stamping, c. Painting, d. Paint baking between 15 and 60 minutes at a temperature of between 120 and 200 C, characterized in that the average length of filiform corrosion filaments, after a corrosion test according to EN 3665, in the sanded zone is less than 2 mm, preferably less than 1 mm, or that the average length of filiform corrosion filaments in the unsanded zone is less than 1 mm, preferably less than 0.8 mm.
Brief description of the figures [Fig. 1]: This figure is a photo of a sample of the strip after a streaking test.
[Fig. 2]: This figure specifies the dimensions in mm of the tools used to determine the value of the parameter known to those skilled in the art as LDH (Limit Dome Height), characteristic of the material's stampability.
[Fig. 3]: This figure shows the variation in elongation during the natural ageing duration with the data from Table 5.
[Fig. 4]: This figure shows the variation in strain-hardening coefficient during the natural ageing duration with data from Table 6.
[Fig. 5]: This figure shows the variation in yield strength and tensile strength during the natural ageing duration with data from Table 5.
[Fig. 6]: This figure shows the variation in the bending angle during the natural ageing duration with data from Table 9.
Description of the invention All aluminum alloys referred to hereafter are designated, unless otherwise stated, according to the rules and designations defined by the Aluminum Association in its regularly published Registration Record Series. Unless otherwise stated, compositions are expressed in % by weight. The expression 1.4 Cu means that the copper content expressed in % by weight is multiplied by 1.4.
The metallurgical tempers referred to are designated according to European standard EN-515.
Static tensile mechanical characteristics, in other words ultimate tensile strength Rm, conventional yield strength at 0.2 % elongation Rp0.2, striction elongation Ag % and elongation at rupture A %, are determined by a tensile test according to EN ISO 6892-1, the sampling and test direction being defined by EN 485-1.
The strain-hardening coefficient n is evaluated according to EN ISO 10275.
Modulus of elasticity is measured according to ASTM 1876.

5 The Lankford anisotropy coefficient is measured according to EN ISO 10113.
Filiform corrosion is characterized with EN 3665. Samples for this characterization are prepared as follows: 2 % pre-stretching in the rolling transverse direction, sanding typical of a surface defect repair, followed by usual surface treatments in the automobile industry and paint bake with the typical treatment of 180 C for 20 minutes. Sanding is carried out with P150 grit paper for 10 seconds. Some samples are not sanded. Before the corrosion test, painted samples are scratched to a width of 1 mm to expose the metal of the aluminum strip sample through the paint layer.
Bending angles, called alpha norms, are determined by 3-point bending test according to NF EN
ISO 7438 and procedures VDA 238-100 and VDA 239-200 version 2017.
Unless otherwise stated, the definitions of EN 12258 apply. A thin strip, or for simplicity a strip, is a rolled product of rectangular transverse cross-section with a uniform thickness of between 0.20 mm and 6 mm.
The LDH parameter is widely used to evaluate the stampability of strips. It has been the subject of numerous publications, especially that of R. Thompson, "The LDH test to evaluate sheet metal formability - Final Report of the LDH Committee of the North American Deep Drawing Research Group", SAE conference, Detroit, 1993, SAE Paper n 930815. This is a stamping test on a blank clamped at the periphery by a ring. The blank holder pressure is adjusted to prevent slip in the ring. The blank, of dimensions 120 mm x 160 mm, is loaded in a mode close to plane deformation. The punch used is hemispherical. Fig. 2 shows the dimensions of the tools used for this test. The hemispherical punch has a radius of 50.8 mm. The ring between the die and the blank holder has a diameter of 132.6 mm, the axis of which is common with the axis of the punch and the axis of a bore in the blank holder and die. The bore of the blank holder and die has a diameter of 101.6 mm. A chamfer with a radius of 6.3 mm is arranged on the die, around the bore and opposite the blank holder. Lubrication between the punch and the strip is provided by graphite grease, such as Shell HDM2 grease. The punch's drop rate is 50 mm/min. The so-called LDH value is the value of the punch displacement at rupture, that is, the limit depth of stamping.
Rupture is detected by a 20 daN reduction in stamping force. The so-called LDH
value is in fact the average of three tests, giving a 95 % confidence interval on the 0.2 mm measurement.
The invention is based on the fact that, by virtue of an adapted composition tolerant to the presence of copper, it is possible to obtain strips combining excellent stampability after solution heat treatment, quenching and natural ageing at room temperature, and very good corrosion resistance after paint bake treatment. In particular, resistance to filiform corrosion is an

6 important property for use on car body parts. These parts are exposed to accidental or even malicious scratches or impacts. When the scratch or impact is sufficiently deep in the paint, the metal is exposed to the external environment, and filiform corrosion can occur. Filiform corrosion is a mode of corrosion that starts from the scratch or impact and spreads to the metal surface beneath the paint. A small scratch or impact can therefore result in a large, highly visible damaged surface.
In a preferred embodiment, the strip according to the invention has excellent resistance to filiform corrosion after deformation, painting and paint bake. Deformation is 2 % perpendicular to the rolling direction. Part of the surface of the samples is sanded, as this corresponds to repairs for a surface defect during production of the car body parts. These sanded surfaces are generally more susceptible to filiform corrosion. Painting includes all the operations know per se of surface preparation, cataphoresis and painting. Paint bake, also known as bake hardening, can be simulated by treatment at 180 C for 20 minutes. The average length of the filiform corrosion filaments in the sanded zone is less than 2 mm, preferably 1 mm. In the unsanded zone, the average length of filiform corrosion filaments is less than 1 mm, preferably less than 0.8 mm.
In a preferred embodiment, the aluminum alloy strip of thickness between 0.7 and 1.0 mm according to the invention in T4 temper has a minimum LDH of at least 26.0 mm.
In another embodiment, the strip of thickness between 1.1 and 1.5 mm according to the invention in T4 temper has a minimum LDH of at least 26.5 mm. This property is important when stamping complex geometries.
In a preferred embodiment, the strip according to the invention in T4 temper is characterized by a strain-hardening coefficient at relatively high deformations between 14 and 16 % greater than 0.26.
In one embodiment, the average anisotropy rm = (r0 + 2r45 + r90)/4 of the strip in T4 temper during natural ageing is between 0.54 and 0.66 and the planar anisotropy Ar =
(r0 - 2r45 + r90)/2 is less than 0.25. This property is important for stable stamping behavior.
Measurements are carried out for deformations between 8 and 12 % and according to ISO EN 10113.
In one embodiment, the strip according to the invention in T4 temper has a bending angle TT of at least 1000, preferably at least 120 , or a bending angle TL of at least 120 , preferably at least 145 .
The concentration ranges imposed on the constituent elements of this type of alloy are described below:

7 Si: Together with magnesium, silicon is the first alloy element in aluminum-magnesium-silicon systems (AA6xxx family) to form the intermetallic compounds Mg2Si or Mg5Si6, which contribute to the structural hardening of these alloys during paint bake. Si content is between 1.2 and 1.5 %. A higher content degrades bendability and mechanical strength after paint bake, as the Si cannot be properly solution heat treated. When the Si content is close to the above-mentioned maximum, it is necessary to increase the solution heat treatment duration, which degrades productivity, to ensure proper solution heat treatment of Si. A compromise between formability and productivity is a Si content of 1.25 % to 1.45 %, preferably 1.25 % to 1.40 %, more preferably 1.30 % to 1.35 %. In one embodiment, the minimum Si content is 1.25 % and the maximum is 1.50% or 1.45% or 1.40% or 1.35% or 1.30%. In another embodiment, the minimum Si content is 1.30 % and the maximum is 1.50 % or 1.45 % or 1.40% or 1.35 %. In another embodiment, the minimum Si content is 1.35 % and the maximum is 1.50 % or 1.45 % or 1.40 %. In another embodiment, the minimum Si content is 1.40% and the maximum is 1.50% or 1.45%.
In another embodiment, the minimum Si content is 1.45 % and the maximum is 1.50 %.
Preferably, the Si content is in excess of the Mg content to achieve the required formability.
The excess of Si content over Mg content is the difference between Si content minus Mg content.
Preferably, the excess of Si content over Mg content is at least 0.95 % by weight, preferably at least 1.00 %
by weight.
Fe: Iron is generally considered an undesirable impurity. The presence of iron-containing intermetallic compounds is generally associated with a reduction in local formability. However, very pure alloys are expensive. A compromise is an Fe content of less than or equal to 0.25 %, preferably less than or equal to 0.20 % and preferably greater than or equal to 0.05 %, more preferably greater than or equal to 0.10 %. In one embodiment, the Fe content is a minimum of 0.05 % and a maximum of 0.25 % or 0.20 % or 0.15 % or 0.10 %. In another embodiment, the Fe content is a minimum of 0.10 % and a maximum of 0.25 % or 0.20 % or 0.15 %. In another embodiment, the Fe content is a minimum of 0.15 % and a maximum of 0.25 % or 0.20 %. In another embodiment, the Fe content is a minimum of 0.20 % and a maximum of 0.25 %.
Mn: Manganese has a similar effect to iron through its contribution to common intermetallic precipitates. The maximum Mn content is 0.15 %. In one embodiment, the Mn content is a minimum of 0.05 % and a maximum of 0.15 % or 0.10 %. In another embodiment, the Mn content is a minimum of 0.10 % and a maximum of 0.15 %.
Mg: Generally speaking, the level of mechanical characteristics in alloys of the AA6xxx family increases with magnesium content. Combined with silicon to form the intermetallic compounds Mg2Si or Mg5Si6, magnesium helps increase mechanical properties such as mechanical strength

8 after paint bake. Mg content is between 0.20 and 0.45 %. Too high a Mg content reduces the solubility of Si during solution heat treatment, degrading strip formability.
A compromise between Si solubility and increased mechanical strength after paint bake is a content preferably less than or equal to 0.39 %, more preferably 0.35 % more preferably 0.34 %, more preferably 0.33 %. In one embodiment, the Mg content is a minimum of 0.25 % and a maximum of 0.45 %
or 0.40 % or 0.35 % or 0.30 %. In one embodiment, the Mg content is a minimum of 0.30 % and a maximum of 0.45 % or 0.40 % or 0.35 %. In one embodiment, the Mg content is a minimum of 0.35 % and a maximum of 0.45 % or 0.40 %. In one embodiment, the Mg content is a minimum of 0.40 % and a maximum of 0.45 %.
The balance between Mg and Si is also important because, surprisingly, it allows the presence of Cu in the alloy, as described below.
Cu: In alloys of the AA6000 family, copper is an element involved in hardening precipitation, but is known to degrade corrosion resistance. The copper content is a maximum of 0.05 %. Allowing the presence of copper in the alloy is economically interesting, as it enables the recycling of aluminum scrap and waste containing it. The presence of copper can come from scrap and waste as such, but can also be the result of accidental introduction. For example, during the dismantling of an out-of-service vehicle, it is sufficient to inadvertently leave a copper wire with the aluminum parts to pollute a slab obtained with recycled aluminum alloy. In one embodiment, the Cu content is a minimum of 0.01 % and a maximum of 0.05 % or 0.04 % or 0.03 % or 0.02 %. In one embodiment, the Cu content is a minimum of 0.02 % and a maximum of 0.05 % or 0.04 % or 0.03 %. In one embodiment, the Cu content is a minimum of 0.03 % and a maximum of 0.05 % or 0.04 %. In one embodiment, the Cu content is a minimum of 0.04 %
and a maximum of 0.05 %.
Ti: This element can promote solid solution hardening leading to the required level of mechanical characteristics, and also has a favorable effect on service ductility and corrosion resistance. On the other hand, a maximum Ti content of 0.15 % is required to avoid the conditions of primary phase formation during vertical casting, which have a detrimental effect on all the properties claimed. In one embodiment, the Ti content is a minimum of 0.01 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 %
or 0.02 %. In another embodiment, the Ti content is a minimum of 0.02 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 %. In another embodiment, the Ti content is a minimum of 0.03 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 %. In another embodiment, the Ti content is a minimum of 0.03 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 %. In another embodiment, the Ti content is a

9 minimum of 0.04% and a maximum of 0.15 % or 012 % or 0.10% or 0.08 % or 0.06 %. In another embodiment, the Ti content is a minimum of 0.06 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 %. In another embodiment, the Ti content is a minimum of 0.08 % and a maximum of 0.15 % or 0.12 % or 0.10 %. In another embodiment, the Ti content is a minimum of 0.10 % and a maximum of 0.15 % or 0.12 %. In another embodiment, the Ti content is a minimum of 0.12 %
and a maximum of 0.15 %.
Cr: The Cr content is a minimum of 0.002 % and a maximum of 0.09 % as it is used as a hardening element. It can be added to refine the grains and stabilize the structure. In one embodiment, the Cr content is a minimum of 0.002 % and a maximum of 0.09 % or 0.08 % or 0.06 % or 0.04 %
or 0.03 % or 0.02 % or 0.01 %. In another embodiment, the Cr content is a minimum of 0.01 %
and a maximum of 0.09 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 %. In another embodiment, the Cr content is a minimum of 0.02 % and a maximum of 0.09 % or 0.08 % or 0.06 % or 0.04 % or 0.03 %. In another embodiment, the Cr content is a minimum of 0.03 % and a maximum of 0.09 % or 0.08 % or 0.06 % or 0.04 %. In another embodiment, the Cr content is a minimum of 0.04% and a maximum of 0.15 % or 0.12 % or 0.10% or 0.08 % or 0.06 %. In another embodiment, the Cr content is a minimum of 0.06 % and a maximum of 0.09 % or 0.08 %. In another embodiment the Cr content is a minimum of 0.08 % and a maximum of 0.09 %.
Ni: The Ni content is a maximum of 0.15 %. The alloy is tolerant to the presence of nickel, which can be introduced by recycling. In one embodiment, the Ni content is a minimum of 0.002 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 % or 0.01 % or 0.005 %. In another embodiment, the Ni content is at least 0.005 %
and at most 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 % or 0.01 %. In another embodiment, the Ni content is a minimum of 0.01 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 %. In another embodiment, the Ni content is a minimum of 0.02 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 %. In another embodiment, the Ni content is a minimum of 0.03 % and a maximum of 0.15% or 0.12% or 0.10% or 0.08% or 0.06% or 0.04%. In another embodiment, the Ni content is a minimum of 0.04 % and a maximum of 0.1 5 % or 0.12 % or 0.10 % or 0.08 %
or 0.06 %. In another embodiment, the Ni content is a minimum of 0.06 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 %. In another embodiment, the Ni content is a minimum of 0.08 % and a maximum of 0.15 % or 0.12 % or 0.10 %. In another embodiment, the Ni content is a minimum of 0.10 % and a maximum of 0.15 % or 0.12 %. In another embodiment, the Ni content is a minimum of 0.12 % and a maximum of 0.15 %.

10 Zn: The content is a maximum of 0.15 % in order not to degrade corrosion resistance. As Zn is an alloying element in aluminum alloys, it is interesting to accept it with a view to recycling aluminum scrap and waste, in particular from out-of-service vehicles. Indeed, Zn is used in some alloys of some components such as heat exchangers. In one embodiment, the Zn content is a minimum of 0.001 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 % or 0.01 % or 0.005 % or 0.002 %. In another embodiment, the Zn content is a minimum of 0.002 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 %
or 0.06 % or 0.04 % or 0.03 % or 0.02 % or 0.01 % or 0.005 %. In another embodiment, the Zn content is a minimum of 0.005 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04 % or 0.03 % or 0.02 % or 0.01 %. In another embodiment, the Zn content is a minimum of 0.01 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 % or 0.04% or 0.03 % or 0.02 %. In another embodiment, the Zn content is a minimum of 0.02 % and a maximum of 0.15 % or 0.12% or 0.10% or 0.08% or 0.06% or 0.04% or 0.03%. In another embodiment, the Zn content is a minimum of 0.03 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 %
or 0.06 % or 0.04 %. In another embodiment, the Zn content is a minimum of 0.04 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 % or 0.06 %. In another embodiment, the Zn content is a minimum of 0.06 % and a maximum of 0.15 % or 0.12 % or 0.10 % or 0.08 %. In another embodiment, the Zn content is a minimum of 0.08 % and a maximum of 0.15 % or 0.12 % or 0.10 %.
In another embodiment, the Zn content is a minimum of 0.10 % and a maximum of 0.15 % or 0.12 %. In another embodiment, the Zn content is a minimum of 0.12 % and a maximum of 0.15 %.
Zr: The Zr content is a maximum of 0.15 %. Its content must be limited in view of the effect on grain size. As Zr is an alloying element in some aluminum alloys, it is interesting to accept it for the purpose of recycling aluminum scrap and waste. In one embodiment, the minimum Zr content is 0.0005 % and the maximum is 0.15 % or 0.10 % or 0.05 % or 0.02 % or 0.01 % or 0.005 % or 0.001 %. In another embodiment, the minimum Zr content is 0.001 % and the maximum is 0.15% or 0.10% or 0.05 % or 0.02 % or 0.01 % or 0.005 %. In another embodiment, the minimum Zr content is 0.005 % and the maximum is 0.15 % or 0.10 % or 0.05 % or 0.02 %
or 0.01 %. In another embodiment, the minimum Zr content is 0.01 % and the maximum is 0.15 %
or 0.10 %
or 0.05 % or 0.02 %. In another embodiment, the minimum Zr content is 0.02 %
and the maximum is 0.15 % or 0.10 % or 0.05 %. In another embodiment, the minimum Zr content is 0.05 % and the maximum is 0.15 % or 0.10 %. In another embodiment, the minimum Zr content is 0.10 % and the maximum is 0.15 %.
The other elements are typically impurities whose content is kept below 0.05 %, the total being less than 0.15 %; the remainder is aluminum.

11 The method for manufacturing strips according to the invention typically includes casting a slab preferably by vertical semi-continuous casting, also known as Direct Chill casting or DC casting, preferably scalping this slab to remove the foundry cortical layer, followed by homogenizing the same.
The slab is cast with an alloy according to the composition previously described. Preferred dimensions of the slabs according to the invention are 200 mm to 600 mm thick, 1000 to 3000 mm wide and 2000 to 8000 mm long. The slabs are then cut to length and scalped.
The slab is then homogenized. Too low a homogenization temperature and too short a duration will impose an increase of the solution heat treatment duration. Too long a duration degrades productivity. Too high a temperature may cause incipient melting, degrading both the mechanical strength after paint bake and the formability of the strip.
Homogenization of the slab is performed at a temperature between 500 C and 600 C. The homogenization duration is advantageously a minimum of 1 hour. An advantageous compromise is homogenization between 540 C and 580 C for a duration of 1 to 4 hours. In one embodiment, the homogenization temperature is between 520 C and 600 C or 580 C or 560 C or 540 C. In another embodiment, the homogenization temperature is between 540 C and 600 C
or 580 C
or 560 C. In another embodiment, the homogenization temperature is between 560 C and 600 C or 580 C. In another embodiment, the homogenization temperature is between 560 C
and 600 C.
The homogenization duration is preferably a minimum of 1 hour. In one embodiment, the maximum homogenization duration is 12 hours or 10 hours or 8 hours or 6 hours or 4 hours or 2 hours. In another embodiment, the maximum homogenization duration is a minimum of 2 hours and a maximum of 12 hours or 10 hours or 8 hours or 6 hours, or 4 hours.
In another embodiment, the maximum homogenization duration is a minimum of 4 hours and a maximum of 12 hours or 10 hours or 8 hours or 6 hours. In another embodiment, the maximum homogenization duration is a minimum of 6 hours and a maximum of 12 hours or 10 hours or 8 hours. In another embodiment, the maximum homogenization duration is a minimum of 8 hours and a maximum of 12 hours or 10 hours. In another embodiment, the maximum homogenization duration is a minimum of 10 hours and a maximum of 12 hours.
Homogenization can optionally include a second stage between 420 C and 550 C
with a maximum duration of 4 hours. This second stage enables the slab temperature to be decreased towards its hot rolling temperature when there are production contingencies that slow down production. In one embodiment, this second stage has a maximum temperature of 550 C and

12 440 C or 460 C or 480 C or 500 C or 520 C or 540 C. In another embodiment, this second stage has a maximum temperature of 540 C and 440 C or 460 C or 480 C or 500 C or 520 C. In another embodiment, this second stage has a maximum temperature of 520 C and 440 C or 460 C or 480 C or 500 C. In another embodiment, this second stage has a maximum temperature of 500 C and 440 C or 460 C or 480 C. In another embodiment, this second stage has a maximum temperature of 480 C and 440 C or 460 C. In another embodiment, this second stage has a maximum temperature of 460 C and 440 C. The purpose of this second stage is to avoid double pass through the following cooling machine such as that described in application W02016012691.
Then, either the slab is cooled to room temperature and then reheated to a hot rolling start temperature below the homogenization temperature, or the slab is cooled directly from the homogenization temperature to the hot rolling start temperature, which improves productivity as hot rolling can start immediately. Direct cooling to the hot rolling start temperature is preferably carried out at a direct cooling rate of at least 150 C per hour.
Advantageously, the direct cooling rate is a maximum of 500 C/h. Direct cooling can typically be performed by a machine such as that described in application W02016012691. Preferably, this direct cooling is carried out in two steps, one of sprinkling and the other of uniformization.
Optionally, this direct cooling can be performed in two passes through the machine as described in W02016012691.
The slab is then transferred to the hot rolling mill at the hot rolling start temperature. The hot rolling start temperature is between 350 C and 550 C. Preferably, the hot rolling start temperature is between 500 C and 400 C. Limiting the too high hot rolling start temperature causes risks of cracks on the slab during hot rolling, which can lead to rejection of the slab. A too low hot rolling start temperature can make the hot rolling end temperature insufficient, by making the slab too difficult to roll. In one embodiment, the hot rolling start temperature is a minimum of 350 C and a maximum of 500 C or 480 C or 460 C or 440 C or 420 C or 400 C or 380 C. In another embodiment, the hot rolling start temperature is a minimum of 380 C and a maximum of 550 C or 500 C or 480 C or 460 C or 440 C or 420 C or 400 C. In another embodiment, the hot rolling start temperature is a minimum of 400 C and a maximum of 550 C
or 500 C or 480 C or 460 C or 440 C or 420 C. In another embodiment, the hot rolling start temperature is a minimum of 420 C and a maximum of 550 C or 500 C or 480 C or 460 C or 440 C. In another embodiment, the hot rolling start temperature is a minimum of 440 C and a maximum of 550 C or 500 C or 480 C or 460 C. In another embodiment, the hot rolling start temperature is a minimum of 460 C and a maximum of 550 C or 500 C or 480 C. In another embodiment, the hot rolling start temperature is a minimum of 480 C and a maximum of 550 C

13 or 500 C. In another embodiment, the hot rolling start temperature is a minimum of 500 C and a maximum of 550 C.
At the end of hot rolling, the slab has been rolled into a strip with the final hot rolling thickness of between 3 and 10 mm. The hot rolling end temperature is between 250 C and 450 C. Cooling between the start and end of hot rolling results from the usual thermal exchange between the slab and the strip with air at room temperature of the plant, with hot rolling mill equipment such as, but not limited to, rolls or conveyor rollers, and with the usual lubricating or cooling fluids.
In one embodiment, the hot rolling end temperature is a minimum of 270 C and a maximum of 450 C or 400 C or 380 C or 360 C or 340 C or 320 C or 300 C. In another embodiment, the hot rolling end temperature is a minimum of 300 C and a maximum of 450 C or 400 C
or 380 C or 360 C or 340 C or 320 C. In another embodiment, the hot rolling end temperature is a minimum of 320 C and a maximum of 450 C or 400 C or 380 C or 360 C or 340 C. In another embodiment, the hot rolling end temperature is a minimum of 340 C and a maximum of 450 C
or 400 C or 380 C or 360 C. In another embodiment, the hot rolling end temperature is a minimum of 360 C
and a maximum of 450 C or 400 C or 380 C. In another embodiment, the hot rolling end temperature is a minimum of 380 C and a maximum of 450 C or 400 C. In another embodiment, the hot rolling end temperature is a minimum of 400 C and a maximum of 450 C.
A first embodiment is the combination of a hot rolling start temperature of 400 to 450 C, preferably 400 to 430 C, a rolling end temperature of 350 to 450 C, preferably 350 to 420 C, cooling during hot rolling of less than 100 C, preferably 70 C, and the absence of intermediate annealing during cold rolling. This combination makes it possible to obtain recrystallized tempers at the end of hot rolling, which recrystallize during solution heat treatment to obtain good surface quality after painting.
A second embodiment is the combination of a hot rolling start temperature of 450 to 500 C, preferably 460 C to 500 C, a hot rolling end temperature of 250 to 350 C, preferably 260 to 320 C, cooling during hot rolling greater than 100 C, preferably greater than 125 C, more preferably greater than 150 C, and intermediate annealing during cold rolling.
This combination makes it possible to obtain fibrous tempers at the end of hot rolling, which recrystallize during solution heat treatment to obtain good surface quality after painting.
The first embodiment is preferred over the second because the intermediate annealing operation is absent, which is more economical.
The strip is then cold-rolled to a final thickness of between 0.8 and 2 mm.
Optionally, cold rolling is performed in two parts, separated by an intermediate annealing operation between 300 C

14 and 500 C, preferably between 300 C and 400 C, more preferably between 340 C
and 380 C.
This intermediate annealing is preferably carried out on the strip rolled up as a coil instead of in a continuous furnace, as the furnace for coil annealing is simpler to build.
The strip is then solution heat treated in a continuous furnace and quenched.
The solution heat treatment temperature is between 500 C and 600 C. In one embodiment, the solution heat treatment temperature is a minimum of 520 C and a maximum of 580 C or 570 C or 560 C or 550 C or 540 C. In another embodiment, the solution heat treatment temperature is a minimum of 540 C and a maximum of 580 C or 570 C or 560 C or 550 C. In another embodiment, the solution heat treatment temperature is a minimum of 550 C and a maximum of 580 C or 570 C
or 560 C. In another embodiment, the solution heat treatment temperature is a minimum of 560 C and a maximum of 580 C or 570 C. In another embodiment, the solution heat treatment temperature is a minimum of 570 C and a maximum of 600 C. The solution heat treatment duration is between 10s and 60s. A solution heat treatment duration of less than 10s does not allow sufficient solution heat treatment of the strip, and the strip's formability and mechanical strength properties after paint bake are not achieved. Too long a solution heat treatment duration degrades productivity and therefore production costs. Quenching is preferably performed with air. Air quenching is advantageous for surface quality of the strip, which is an important characteristic when used for car body skin parts. Quenching with water causes high cooling rates which deform the strip. The deformations resulting from water quenching impose the use of a leveler, which can damage the surface quality. The quench rate up to the temperature of 100 C is at least 15 C/s, preferably more than 20 C/s, more preferably more than 30 C/s. Considering the preferred air quenching, the preferred maximum quench rate is 95 C/s.
The strip is then pre-aged. Pre-ageing is achieved by coiling the strip at a pre-ageing temperature of between 50 C and 100 C, followed by cooling to room temperature. This pre-ageing is used to stabilize the strip's mechanical properties and formability during natural ageing. In one preferred embodiment, the strip is reheated to the pre-ageing temperature and then directly coiled at said temperature. This reheating is advantageous for controlling the coiling temperature. Indeed, on the one hand, the temperature after rapid cooling, such as quenching, is difficult to control, reheating makes it possible to finely control the temperature of the strip.
Moreover, the solution heat treatment and quenching machine is usually separated from the machine performing reheating by an accumulator in which the strip continues to cool, depending on the length of the accumulated strip. On the other hand, a surface treatment step, known to those skilled in the art and useful for the strip's use by the automaker, often takes

15 place after quenching and before pre-ageing. Reheating then enables a pre-ageing temperature to be selected independently of the final surface treatment temperature. In one embodiment, the pre-ageing temperature is a minimum of 60 C and a maximum of 100 C or 95 C
or 90 C or 85 C or 80 C or 75 C or 70 C or 65 C. In another embodiment, the pre-ageing temperature is a minimum of 65 C and at most 100 C or 95 C or 90 C or 85 C or 80 C or 75 C or 70 C. In another embodiment, the pre-ageing temperature is a minimum of 70 C and a maximum of 100 C or 95 C or 90 C or 85 C or 80 C or 75 C. In another embodiment, the pre-ageing temperature is a minimum of 75 C and a maximum of 100 C or 95 C or 90 C or 85 C or 80 C. In another embodiment, the pre-ageing temperature is a minimum of 80 C and a maximum of 100 C or 95 C or 90 C or 85 C. In another embodiment, the pre-ageing temperature is a minimum of 85 C
and a maximum of 100 C or 95 C or 90 C. In another embodiment, the pre-ageing temperature is a minimum of 90 C and a maximum of 100 C or 95 C. In another embodiment, the pre-ageing temperature is a minimum of 95 C and a maximum of 100 C. Pre-ageing takes place during natural cooling of the coil in the room temperature of the workshop for a duration of between 8 and 24 hours.
Room temperature is a temperature compatible with human activity. Room temperature is typically between 0 and 45 C. Cooling the coil at a pre-ageing temperature to a temperature of 45 C is advantageous because it does not require the use of a cooling medium such as an air conditioner during the warmer seasons.
As a result, the strip is in the 14 temper and naturally ages at room temperature between 72 hours and 6 months. This duration corresponds to the usual storage duration prior to the manufacture of car body parts.
The strip is then used to manufacture a car body part. The car body part manufacturing method therefore includes the following successive steps of - Supplying the strip according to the invention, - Stamping the strip, - Painting, which includes all the surface treatment, cataphoresis and painting operations known to those skilled in the art, - Paint baking, known to those skilled in the art as "bake hardening", between 15 and 30 minutes at a temperature of between 170 and 200 C.

16 The car body part has excellent resistance to filiform corrosion, at the end of a corrosion test according to EN 3665. The average length of filiform corrosion filaments in the sanded zone is less than 2 mm, preferably less than 1. In the unsanded zone, the average length of filiform corrosion filaments is less than 1 mm, preferably less than 0.8 mm. Sanding is representative of repairs to a surface defect that have occurred during part production. These sanding repairs are carried out by workers in production plants and are well known to those skilled in the art.
Examples Slabs of Aluminum alloy according to the mass percentage composition in Table 1 have been cast by vertical semi-continuous casting. Alloys C, D, E and F have a composition according to the invention. Slab dimensions were 1820 * 520 *3500.
[Table 1]
Si Fe Cu Mn Mg Cr Ni Zn Ti Zr A 1.27 0.19 0.089 0.09 0.29 0.012 0.004 0.006 0.03 0.0009 B 1.30 0.15 0.164 0.07 0.28 0.015 0.003 0.010 0.02 0.0009 C 1.34 0.14 0.002 0.08 0.33 0.010 0.007 0.004 0.03 0.0006 D 1.34 0.14 0.002 0.08 0.33 0.010 0.007 0.004 0.03 0.0006 E 1.33 0.15 0.029 0.06 0.32 0.011 0.03 F 1.33 0.15 0.029 0.06 0.32 0.011 0.03 G 0.94 0.22 0.092 0.15 0.42 0.037 0.004 0.011 0.02 0.0013 The slabs have then been cut to length, scalped and then homogenized for 2 hours at 560 C. The homogenizing furnace has then been set to 540 C. After 2 hours, the slabs have been removed from the homogenizing furnace at 540 C and cooled to the hot rolling start temperature according to Table 2. Cooling has been performed by a machine such as that described in application W02016012691. Slabs A, B and C have required a double pass and the others have passed through said machine only once. The cooling rate was approximately 350 C/h. Cooling has been carried out in two steps, a sprinkling step followed by a uniformizing step. The slabs have then been hot-rolled into a strip. Temperatures at the start of rolling for slabs A, B and C
are between 400 and 450 C, while temperatures at the start of rolling for the other slabs are between 450 and 500 C. At the end of hot rolling, slabs A, B and C have a temperature between 350 and 400 C, while the other slabs have a hot rolling end temperature below 300 C. The hot

17 rolling end thickness of the strip is given in Table 2. The strip is then cold-rolled to the intermediate cold rolling thickness. Some strips, according to Table 2, are thermally treated at 350 C in coils for 1 hour. The strip is then rolled to the final thickness in Table 2.
[Table 2]
Hot rolling Hot rolling Hot rolling start end end intermediate Final cold rolling temperature temperature thickness annealing thickness (mm) ( C) ( C) (mm) A 413 367 5.5 no 1.0 B 406 375 5.5 no 1.5 C 413 374 5.5 no 1.2 D 477 298 5.0 yes 1.0 E 482 299 7.3 yes 1.5 F 482 290 5.0 yes 1.0 G 7.4 yes 1.0 The strips have then been solution heat treated and air-quenched in a continuous furnace. The solution heat treatment duration is given in Table 3. The strips have then been pre-aged. Pre-ageing has been carried out by coiling the strip at pre-ageing temperature, with the resulting coil cooling naturally to room temperature within 12 hours. As the strip has been produced under industrial conditions, the room temperature has varied between 15 and 26 C. Pre-ageing temperatures are given in Table 3. The coils have then naturally aged at room temperature and samples have been taken for different characterizations.
[Table 3]
Solution heat Pre-ageing coiling treatment duration (s) temperature ( C) G

18 The stamping performance of strips in T4 temper is tested using the LDH (Limit Dome Height) test.
Specimens were 120 x 160 mm in dimensions, with the 160 mm dimension positioned either in the long direction, which is the rolling direction, or in the transverse direction, which is the direction perpendicular to the rolling direction, or in the 45 direction between the previous two directions. The results are set out in Table 4.
Coils according to the invention E and F show better stampability than coil G, and the latter does not degrade with the natural ageing duration.
[Table 4]
LDH (mm) Natural ageing Long . Transverse (day) 45 direction direction direction E 101 28.0 29.5 28.7 F 71 27.0 28.3 28.0 G 71 25.9 26.5 Formability can also be observed in the following analyses.
Tables 5 and 6 show the results of mechanical characterizations after different natural ageing durations. These results demonstrate the stability of mechanical properties during natural ageing, a characteristic essential for forming, and in particular stamping, irrespective of strip storage duration. Static tensile mechanical characteristics are determined by a tensile test according to NF EN ISO 6892-1.
To guarantee formability, these mechanical properties vary little during natural ageing. In particular, the strain-hardening coefficient at high elongation between 14 and 16 % varies by less than 0.04 during natural ageing. The anisotropy of the strain-hardening coefficient is also low.
Fig. 3 shows the low time sensitivity of elongations during natural ageing in Table 5.
Fig. 4 shows the low time sensitivity of the strain-hardening coefficient at high elongation between 14 and 16 % in Table 6.
Fig. 5 shows the low sensitivity to natural ageing of tensile strength and yield strength in Table 5.

19 [Table 5]
Natural ageing Rp0.2 ST Rm ST Ag ST ( A80 ST ( (days) (Mpa) (Mpa) %) %) E 4 101 216 24.6 28.6 F 6 98 217 25.0 28.5 D 6 99 217 24.2 27.6 F 15 101 221 24.1 28.1 D 15 103 222 23.7 26.9 D 29 107 225 23.6 27.7 F 29 105 225 24.1 28.1 E 29 108 224 24.1 28.4 E 59 113 229 23.7 28.4 D 62 112 231 23.8 27.4 F 62 111 231 24.2 28.4 D 70 111 230 23.9 28.8 F 70 109 228 24.3 29.1 E 89 116 233 23.9 28.8 D 90 115 233 23.5 27.1 F 90 113 232 24.0 28.0 E 100 115 231 23.7 29.3 D 149 117 236 23.7 28.6 F 149 116 235 23.7 28.5 E 179 121 236 23.4 29.0 [Table 6]
n15(14- n15(14-16 Natural 16%) n15(14- %) ageing Long 16 %) 45 Transverse (days) direction direction E 4 0.27 0.28 0.28 F 6 0.27 0.28 0.28 D 6 0.28 0.28 0.28 D 62 0.26 0.27 0.27 F 62 0.27 0.27 0.27 D 90 0.26 0.27 0.27 F 90 0.26 0.27 0.27

20 Anisotropy has also been evaluated for the strip in 14 temper using the Lankford coefficient between 8 and 12 % in the rolling direction, transverse to rolling and 45 %
between said directions. Average anisotropy and planar anisotropy could therefore be calculated during natural ageing. The results are shown in Table 8, and these characteristics remain stable throughout natural ageing.
[Table 8]
r8-12 Natural ageing 0 45 90 rm Ar (days) D 6 0.75 0.50 0.62 0.59 0.19 D 15 0.78 0.51 0.66 0.61 0.21 D 29 0.76 0.51 0.64 0.61 0.19 D 62 0.74 0.48 0.62 0.58 0.20 D 70 0.74 0.48 0.64 0.59 0.21 D 90 0.74 0.49 0.62 0.59 0.19 D 149 0.76 0.51 0.68 0.61 0.21 E 4 0.77 0.51 0.69 0.62 0.22 E 100 0.75 0.48 0.63 0.58 0.21 E 179 0.78 0.50 0.65 0.61 0.22 F 6 0.75 0.52 0.65 0.61 0.18 F 15 0.76 0.53 0.65 0.62 0.17 F 29 0.75 0.53 0.66 0.62 0.18 F 62 0.73 0.51 0.63 0.60 0.17 F 70 0.74 0.51 0.65 0.60 0.18 F 90 0.74 0.52 0.65 0.61 0.17 F 149 0.58 0.51 0.68 0.57 0.11 Samples have also been taken in order to carry out filiform corrosion tests.
Corrosion has been quantified in Table 7 by measuring the average length of filiform corrosion filaments and the length of filiform corrosion. These results clearly show the influence of copper on filiform corrosion.

21 [Table 7]
sanding strip maximum average length length of of filaments filaments no A 3.6 1.4 no F 2.5 0.5 yes A 5.8 2.9 yes F 3.0 0.6 yes C 1.9 0.5 Streaking is measured as follows. A sample measuring about 270 mm (crosswise to rolling direction) by 50 mm (in rolling direction) is cut from the strip. A 15 %
tensile pre-deformation is then applied, perpendicular to the rolling direction, that is, along the length of the sample. The sample is then subjected to the action of a P800 abrasive paper to reveal streaking. Streaking has been measured on sample D, the result of which is shown in Fig. 1. The strip therefore has a satisfactory surface quality for the surface quality after painting of a part made with this strip.
Strips D and F in T4 temper have also been characterized by bending. Strips D, E and F have a /0 bending angle TT of at least 120 and a bending angle TL of at least 145 in the long rolling direction. Fig. 6 shows the low sensitivity to natural ageing of bending angles with the data in Table 9.
[Table 9]
Natural Bending angle TT ( ) - Bending angle TL ( ) -ageing (day) norm t=1.00 mm norm t=1.00 mm

Claims (14)

Claims

1. An aluminum alloy strip with a composition of, in % by weight:
Si: 1,2 ¨ 1,5, Fe: 0,25, Cu: 0,05, Mn: 0,15, Mg: 0,20 ¨ 0,45, Cr: 0,002 ¨ 0,09 Ni: 0,15 Zn: 0,15 Ti: 0,15 Zr: 0,15, unavoidable elements and impurities in a maximum of 0.05 % each, together 0.15 % maximum, remainder aluminum,

2. The aluminum alloy strip according to claim 1 characterized in that Si:
1.25- 1.45, preferably 1.25 - 1.40.

3. The aluminum alloy strip according to claim 1 or 2 characterized in that Fe: 0.10 - 0.20, or 0.10 - 0.15 or 0.15-0.20.

4. The aluminum alloy strip according to any of claims 1 to 3 characterized in that Mn: 0.05 -0.10.

5. The aluminum alloy strip according to any of claims 1 to 4 characterized in that Mg: is less than or equal to 0.39 %, more preferably 0.35 %, more preferably 0.34 %, more preferably 0.33 %.

6. The aluminum alloy strip according to any of claims 1 to 5 characterized in that Cu:
maximum 0.03, or Cu 0.01 - 0.04 or Cu 0.02 - 0.04.

7. The aluminum alloy strip according to any of claims 1 to 6 characterized in that Ti 0.10, or 0.06, or 0.02 - 0.08.

8. The aluminum alloy strip according to any of claims 1 to 7 characterized in that Cr 0.005 -0.03 or Cr 0.01 - 0.05.

9. The aluminum alloy strip according to any of claims 1 to 8 characterized in that Ni 0.002 -0.01 or Ni 0.005 - 0.02.

10. The aluminum alloy strip according to any of claims 1 to 9 characterized in that the excess of Si content over Mg content is at least 0.95 % by weight, preferably at least 1.00 % by weight.

11. The aluminum alloy strip according to any of claims 1 to 10 characterized in that the strip of thickness between 0.7 and 1.0 mm in T4 temper has a minimum LDH of at least 26.0 mm, or in that the strip of thickness between 1.1 and 1.5 mm according to the invention in T4 temper has a minimum LDH of at least 26.5 mm.

12. The aluminum alloy strip according to any of claims 1 to 11, characterized in that the average length of the filiform corrosion filaments, characterized according to EN 3665, in the unsanded zone is less than 2 mm, preferably less than 1 mm, after the strip has been pre-deformed by 2 % in the rolling transverse direction, then painted, then paint bake at 180 C for 20 minutes, or that the average length of the filiform corrosion filaments, characterized according to 3665, in the unsanded zone is less than 1 mm, preferably less than 0.8 mm, after the strip has been pre-deformed by 2 % in the rolling transverse direction, then painted, then paint bake at 180 C for 20 minutes.

13. A method for manufacturing an aluminum alloy strip according to any of claims 1 to 12 comprising the steps of:
a. Casting a slab, preferably by semi-continuous vertical casting, b. Homogenizing the slab preferably at a temperature between 500 C and 600 C, more preferably between 540 and 580 C and preferably between 1 hour and 12 hours, optionally comprising a second stage between 420 C and 550 C with a maximum duration of 4 hours, c. Cooling the slab to the hot rolling start temperature between 350 C and 550 C at a cooling rate preferably greater than 150 C/h or cooling the slab to room temperature and then reheating the slab to said hot rolling start temperature, d. Hot rolling the slab into a strip, with the hot rolling end temperature between 250 C and 450 C, e. Cold rolling the strip, optionally in two parts separated by intermediate annealing, said annealing being preferably carried out on the strip rolled up into a coil, f. Solution heat treating the strip, preferably between 500 C and 600 C for 10 to 60 s, followed by quenching, g. Pre-ageing the strip by coiling the slab to a temperature of between 50 and 100 C, followed by cooling to room temperature, h. Naturally ageing the strip at room temperature between 72 hours and 6 months.

14. A car body part obtained by a. Supplying an aluminum alloy strip according to any of claims 1 to 12, b. Stamping c. Painting d. Paint baking between 15 and 60 minutes at a temperature of between 120 and 200 C, characterized in that the average length of filiform corrosion filaments, after a corrosion test according to EN 3665, in the sanded zone is less than 2 mm, preferably less than 1 mm, or that the average length of filiform corrosion filaments in the unsanded zone is less than 1 mm, preferably less than 0.8 mm.