EP3303646B1 - Metal sheet for a motor vehicle body having high mechanical strength - Google Patents

Description

Field of the invention

The invention relates to the field of Al-Si-Mg alloy sheets, more particularly alloy type AA6xxx according to the designation of the "Aluminum Association", added with hardening elements and intended for the manufacture by stamping of lining parts , of structure or reinforcement of the white box of motor vehicles.

State of the art

In the preamble, all the aluminum alloys referred to below are designated, unless otherwise indicated, in the terms defined by the "Aluminum Association" in the "Registration Record Series" which it publishes regularly. All indications regarding the chemical composition of the alloys are expressed as a percentage by weight based on the total weight of the alloy.
The definitions of the metallurgical states are given in the European standard EN 515.
The static mechanical tensile properties, in other words the ultimate tensile strength Rm, the conventional yield stress at 0.2% elongation Rp0.2, and the elongation at break A% are determined by a tensile test according to standard NF EN ISO 6892-1.

• une formabilité élevée à l'état de livraison, état T4, en particulier pour les opérations d'emboutissage,
• une limite d'élasticité contrôlée à l'état de livraison de la tôle pour maîtriser le retour élastique lors de la mise en forme,
• une résistance mécanique élevée après cataphorèse et cuisson des peintures pour obtenir une bonne résistance mécanique en service tout en minimisant le poids de la pièce,
• une bonne capacité à l'absorption d'énergie en cas de choc pour application à des pièces de structure de caisse,
• un bon comportement dans les divers procédés d'assemblage utilisés en carrosserie automobile tels que le soudage par points, le soudage laser, le collage, le clinchage ou le rivetage,
• une bonne résistance à la corrosion, notamment la corrosion intergranulaire, la corrosion sous contrainte et la corrosion filiforme de la pièce finie,
• une compatibilité avec les exigences du recyclage des déchets de fabrication ou des véhicules recyclés,
• un coût acceptable pour une production en grande série.

Aluminum alloys are increasingly used in the construction of motor vehicles because their use reduces the weight of vehicles and thus reduce fuel consumption and greenhouse gas emissions.
The aluminum alloy sheets are used in particular for the production of many pieces of the "white box" among which we distinguish: the parts body skin (or exterior body panels) such as the front fenders, roof or roof, bonnet, boot or door skin; lining parts such as door, wing, tailgate or hood liners; and finally the structural parts, such as the longitudinal members, the aprons, the load floors and the front, middle and rear feet.
Although many skin and lining parts are already made of aluminum alloy sheets, the conversion of steel to aluminum for reinforcing or structural parts with higher characteristics is more This is particularly difficult because of the poorer formability of aluminum alloys with respect to steels and, secondly, because of the generally lower mechanical properties of steels used for these types of parts.
Indeed, for reinforcement or structure type applications, a set of sometimes antagonistic properties is required such as:

• high formability in the delivery state, state T4, in particular for stamping operations,
• a yield strength controlled in the state of delivery of the sheet to control the springback during shaping,
• a high mechanical strength after cataphoresis and baking of the paints to obtain a good mechanical resistance in service while minimizing the weight of the part,
• a good ability to absorb energy in case of impact for application to body structure parts,
• good behavior in the various assembly methods used in automotive bodywork such as spot welding, laser welding, gluing, clinching or riveting,
• good resistance to corrosion, including intergranular corrosion, stress corrosion and filiform corrosion of the finished part,
• compatibility with the recycling requirements of manufacturing waste or recycled vehicles,
• an acceptable cost for mass production.

However, there are already large-scale motor vehicles with a white box consisting mainly of aluminum alloys. for example the Ford F-150 2014 model is made of AA6111 alloy structure. This alloy was developed by the group "Alcan" in the years 1980-1990. Two references describe this development work:
PE Fortin et al, "An optimized Al alloy for Auto body sheet applications", SAE technical conference, March 1984 describes the following composition: [Fort] Yes Fe Cu mn mg Cr Zn Ti AA6111 0.85 0.20 0.75 0.20 0.72 - - - MJ Bull et al, "Al sheet alloys for structural and skin applications", 25th ISATA symposium, Paper 920669, June 1992 :
The main property remains a strong mechanical strength, even if it is initially intended to resist indentation for skin-type applications: "A yield-strength of 280 MPa is achieved after 2% pre-strain and 30 min at 177 ° C. ".

On the other hand, other alloys of the AA6xxx family with high mechanical properties have been developed for aeronautical or automotive applications.

Thus, the type AA6056 alloy, whose development dates back to the 1980s at "Pechiney" has been the subject of numerous studies and numerous publications, either to optimize the mechanical characteristics, or to improve the resistance to intergranular corrosion. We will retain the automotive application of this type of alloy, which was the subject of a patent application ( WO2004113579A1 ).

Alloys of the type AA6013 have also been the subject of much work.

For example, at "Alcoa", in the application US2002039664 published in 2002, an alloy comprising 0.6-1.15% Si; 0.6-1% Cu; 0.8-1.2% Mg; 0.55-0.86% Zn; less than 0.1% Mn, 0.2-0.3% Cr and about 0.2% Fe, used in the T6 state, combines good resistance to intergranular corrosion, as well as a Rp _0.2 of 380 MPa. At "Aleris", a request published in 2003, WO03006697 relates to an alloy of the AA6xxx series with 0.2 to 0.45% Cu. The object of the invention is to propose an AA6013 type alloy with a reduced Cu level, targeting 355 MPa of Rm at the T6 state and good resistance to intergranular corrosion. The claimed composition is as follows: 0.8-1.3% Si, 0.2-0.45% Cu, 0.5-1.1% Mn, 0.45-1.0% Mg.

The patent US5888320 discloses a method of manufacturing an aluminum product, comprising: (A) providing an aluminum alloy consisting essentially of about 0.6 to 1.4 by weight. % silicon, no more than about 0.5. % iron, not more than about 0.6 by weight. % copper, about 0.6 to 1.4 by weight. % of magnesium, about 0.4 to 1.4 by weight. % zinc, at least one member selected from the group consisting of about 0.2 to 0.8 by weight. % manganese and 0.05 to 0.3. % chromium, the rest mainly aluminum, secondary elements and impurities; (B) homogenization, (C) heat distortion (D) dissolution and (E) quenching; wherein the product has a ductility loss of at least 5% less than a comparable treated alloy comprising about 0.88 wt% Cu, 0.05 wt%, 0.75 wt% Si, 0 wt. 17 wt.% Fe, 0.42 wt.% Mn, 0.95 wt.% Mg, 0.08 wt.% Ti, and <0.01 wt.% Cr.

The patent application JPH05112840 describes a car body sheet of composition, in% by weight, 0.4 to 1.5% Mg, 0.24 to 1.5% Si, 0.12 to 1.5 % Cu, 0.1 to 1.0% Zn, 0.005 to 0.15% Ti and at most 0.25% Fe, wherein Si and Mg satisfy the Si ratio at most 0.6 Mg (%), and containing at least one of 0.08 to 0.30% Mn, 0.05 to 0.20% Cr, 0.05 to 0.20% Zr, 0 , 04 to 0.10% V and from 0.0002 to 0.05% of B and the remainder of Al with unavoidable impurities.

Note finally that in all the above examples, the achievement of high mechanical characteristics (Rp _0.2 , Rm) is achieved by using alloys containing at least 0.5% copper.

Problem

The object of the present invention is to provide aluminum alloy sheets for lining, reinforcement or automotive body structure having a resistance mechanical operation, after shaping and baking paints, also, or even higher, than the sheets of the prior art, while having a good resistance to corrosion, particularly intergranular or filiform, a formability by stamping at temperature satisfactory ambient and good behavior in various assembly processes such as spot welding, laser welding, gluing, clinching or riveting.

Object of the invention

• Si : 0,85 - 1,20 et de préférence : 0,90 - 1,10
• Fe : < 0,30 et de préférence : 0,15 - 0,25
• Cu : 0,10 - 0,30 et de préférence : 0,10 - 0,20
• Mg : 0,70 - 0,90 et de préférence : 0,70 - 0,80
• Mn : < 0,30 et de préférence : 0,10 - 0,20
• Zn : 0,9 - 1,60, de préférence 1,10 - 1,60 et de manière préférée : 1,20 - 1,50
• V : 0,02 - 0,30, de préférence 0,05 - 0,30 et de manière préférée : 0,10 - 0,20
• Ti : 0,05 - 0,20 et de préférence : 0,08 - 0,15
• autres éléments < 0,05 chacun et < 0,15 au total, reste aluminium,

The subject of the invention is a sheet for a stamped part of a lining, a reinforcement or an automobile bodywork structure, also called a blank body, made of aluminum alloy of the AA6xxx series, having a low Cu content, added with hardening elements. of which in particular Zn, V and Ti, typically of thickness between 1 and 5 mm, and of composition (% by weight):

• If: 0.85 - 1.20 and preferably: 0.90 - 1.10
• Fe: <0.30 and preferably: 0.15 - 0.25
• Cu: 0.10 - 0.30 and preferably: 0.10 - 0.20
• Mg: 0.70-0.90 and preferably 0.70-0.80
• Mn: <0.30 and preferably: 0.10 - 0.20
• Zn: 0.9 - 1.60, preferably 1.10 - 1.60 and preferably: 1.20 - 1.50
• V: 0.02 - 0.30, preferably 0.05 - 0.30 and more preferably: 0.10 - 0.20
• Ti: 0.05 - 0.20 and preferably: 0.08 - 0.15
• other elements <0.05 each and <0.15 in total, remaining aluminum,

• la coulée typiquement semi-continue verticale d'une plaque et son scalpage éventuel,
• l'homogénéisation à une température de 550 à 570°C avec un maintien entre 2 et 12 h, préférentiellement entre 4 et 6 h, suivi d'un refroidissement rapide jusqu'à l'ambiante, typiquement à l'air pulsé ou à l'eau,
• le réchauffage à une température comprise entre 450 et 550°C avec un maintien entre 30 min et 3 h, préférentiellement sensiblement 2 h,
• le laminage à chaud de la plaque en une bande d'épaisseur comprise entre 3 et 10 mm,
• le laminage à froid jusqu'à l'épaisseur finale typiquement comprise entre 1 et 5 mm,
• la mise en solution de la bande laminée à une température au-delà de la température de solvus de l'alliage, tout en évitant la brûlure, soit entre 550 et 570°C pendant 5 s à 5 min, suivi de la trempe à une vitesse de plus de 50°C/s et mieux d'au moins 100°C/s,
• le pré-revenu, ou réversion, par bobinage à une température d'au moins 60°C suivi d'un refroidissement à l'air libre de la bobine obtenue.

It also relates to a method of manufacturing said sheets as above, comprising the following steps:

• the typical vertical semi-continuous casting of a plate and its possible scalping,
• homogenization at a temperature of 550 to 570 ° C with a hold between 2 and 12 h, preferably between 4 and 6 h, followed by rapid cooling to ambient, typically to the pulsed air or 'water,
• reheating at a temperature between 450 and 550 ° C with a maintenance between 30 min and 3 h, preferably substantially 2 h,
• the hot rolling of the plate in a strip of thickness between 3 and 10 mm,
• cold rolling until the final thickness typically between 1 and 5 mm,
• dissolving the rolled strip at a temperature above the solvus temperature of the alloy, while avoiding the burn, ie between 550 and 570 ° C. for 5 seconds to 5 minutes, followed by quenching at a temperature of speed of more than 50 ° C / s and better still of at least 100 ° C / s,
• the pre-income, or reversion, by winding at a temperature of at least 60 ° C followed by cooling in the open air of the coil obtained.

According to another variant, the homogenization and reheating steps above are replaced by a single reheating step at a temperature between 550 and 570 ° C with a maintenance between 2 and 12 h, preferably between 4 and 6 h, followed by hot rolling as above.

According to an advantageous embodiment, the sheet obtained by the above process has, after optional maturation at room temperature of between 72 h and 6 months, a controlled tensile pre-deformation of 2% to simulate the shaping, and treatment of typically paints for 20 min at 185 ° C, a yield strength Rp _0.2 of at least 300 MPa.

Equally advantageously, the sheet obtained by the aforementioned method has, in the metallurgical state T6 according to the European standard EN 515, typically after a heat treatment complementary to 205 ° C for 2 hours or equivalent, a yield strength Rp _{0 , 2} of at least 350 MPa.

Also as advantageously, the sheet obtained by the aforementioned method has a good resistance to corrosion, especially intergranular and filiform.

Finally, such a sheet in a thickness of 2 mm, obtained by the aforementioned method, after optional maturation at ambient temperature of between 72 h and 6 months, a controlled tensile pre-deformation of 10%, and baking treatment of the paints, typically for 20 min at 185 ° C., has a "three-point bending angle" α _10% , measured according to the NF EN ISO 7438 standard and the VDA 238-100 procedure, of at least 60 °.

Description of figures

• The figure 1 represents the device for "three-point folding test" consisting of two rollers R, a punch B of radius r to proceed to the folding of the sheet T of thickness t.
• The figure 2 represents the T-plate after the "three-point folding" test with the internal angle β and the external angle, the measured result of the test: α, also called α _10% .
• The figure 3 specifies the dimensions in mm of the tools used to determine the value of the parameter known to those skilled in the art under the name of LDH (Limit Dome Height) characteristic of the drawability of the material.

Description of the invention

The invention is based on the finding made by the applicant that a narrow composition range within the composition of an alloy of the AA6xxx family registered with the "Aluminum Association", combined with a combined addition of Zn, V and Ti, allowed to obtain all the desired properties, namely high mechanical strength in service, after shaping and baking paints, related in particular to the addition of zinc but combined surprisingly and unexpectedly, because a priori of the simultaneous presence of V and Ti, to a resistance to corrosion, intergranular and filiform, very satisfactory and formability in stamping at satisfactory ambient temperature.

• Si : Les caractéristiques mécaniques des alliages d'aluminium augmentent régulièrement avec la teneur en silicium. Le silicium est, avec le magnésium, le second élément d'alliage des systèmes aluminium-magnésium-silicium (famille AA6xxx) pour former les composés intermétalliques Mg₂Si ou Mg₅Si₆ qui contribuent au durcissement structural de ces alliages. La présence de silicium, à une teneur comprise entre 0,85 % et 1,20 %, combinée à la présence de magnésium, à une teneur comprise entre 0,70% et 0,90% permet d'obtenir le ratio Si/Mg requis pour atteindre les propriétés mécaniques recherchées tout en garantissant une bonne résistance à la corrosion et une mise en forme en emboutissage à température ambiante satisfaisante.
  La fourchette de teneur la plus avantageuse est de 0,90 à 1,10 %.
• Mg : Le niveau de caractéristiques mécaniques des alliages de la famille des AA6xxx est proportionnel à la teneur en magnésium. Combiné au silicium pour former les composés intermétalliques Mg₂Si ou Mg₅Si₆, le magnésium contribue à l'accroissement des propriétés mécaniques. Une teneur minimum de 0,70 % est nécessaire pour obtenir le niveau de caractéristiques mécaniques requis et former suffisamment de précipités durcissants. En outre, la température de solvus, correspondant à la température de mise en solution, de ces alliages est très dépendante de la teneur en magnésium. Au-delà de 0,90 %, la température de solvus devient trop élevée posant ainsi des problèmes de mise en solution industrielle.
  La fourchette de teneur la plus avantageuse est de 0,70 à 0,80 %.
• Fe : Il est toujours présent comme impureté dans « l'aluminium primaire », puisqu'il provient, comme le silicium, du minerai, la bauxite, dont l'alumine est extraite. Une teneur minimum de 0,05 %, et mieux 0,15 %, diminue sensiblement la solubilité du manganèse en solution solide, ce qui permet d'obtenir une sensibilité à la vitesse de déformation positive, retarde la rupture lors de la déformation après striction, et donc améliore la ductilité et la formabilité. Le fer est également nécessaire à la formation d'une forte densité de particules intermétalliques garantissant une bonne « écrouissabilité » au cours de la mise en forme. Dans ces teneurs le fer permet également de contrôler la taille des grains. Au-delà d'une teneur de 0,30 %, trop de particules intermétalliques sont créées avec un effet néfaste sur la ductilité et la résistance à la corrosion.
  La fourchette de teneur la plus avantageuse est de 0,15 à 0,25 %.
• Mn : sa teneur est limitée à 0,30 %. Une addition de manganèse au-delà de 0,05 % peut accroître les caractéristiques mécaniques par effet de solution solide, mais au-delà de 0,3 %, elle ferait très fortement décroitre la sensibilité à la vitesse de déformation et donc la ductilité.
  Une fourchette avantageuse est de 0,10 à 0,20 %.
• Cu : Dans les alliages de la famille des AA6000, le cuivre est un élément durcissant efficace en participant à la précipitation durcissante. A une teneur minimum de 0,10 %, sa présence permet d'obtenir des caractéristiques mécaniques plus élevées. Au-delà de 0,30 % le cuivre a une influence négative sur la résistance à la corrosion.
  La fourchette de teneur la plus avantageuse est de 0,10 à 0,20 %.
• Zn : l'effet de l'ajout de Zn dans les AA6xxx sur les propriétés mécaniques et sur la résistance à la corrosion n'est pas complètement compris. Une teneur minimum de 0,9 % est nécessaire pour obtenir le niveau de caractéristiques mécaniques requis, par durcissement par solution solide. De préférence la teneur minimum en Zn est 1,10 %. Par ailleurs, l'ajout de Zn dans les alliages d'aluminium de la famille des AA6xxx modifie la température du solidus. Plus on ajoute de Zn, plus la température de solidus diminue, réduisant ainsi la différence entre température de solvus et de solidus et rendant difficile l'industrialisation d'un tel alliage. Au-delà de 1,60 %, cette différence devient trop critique. La fourchette de teneur la plus avantageuse est de 1,20 à 1,50 %.
• V et Ti : une teneur minimum de 0,02 % de vanadium et de 0,05 % de titane est nécessaire pour obtenir un durcissement par solution solide conduisant au niveau de caractéristiques mécaniques requis et, combiné à l'ajout de Zn, chacun de ces éléments a de plus un effet favorable sur la ductilité en service et la résistance à la corrosion. De préférence la teneur minimum en vanadium est 0,05 %. Par contre, une teneur maximum de 0,20 % pour Ti et 0,30 % pour V est requise pour ne pas former des phases primaires lors de la coulée verticale, qui ont un effet néfaste sur l'ensemble des propriétés revendiquées. La fourchette de teneur la plus avantageuse est de 0,10 à 0,20 % pour V et 0,08 à 0,15 pour Ti.

The concentration ranges imposed on the constituent elements of this type of alloy are explained for this reason for the following reasons:

• Si: The mechanical properties of aluminum alloys increase steadily with the silicon content. Silicon is, along with magnesium, the second alloying element of aluminum-magnesium-silicon systems (family AA6xxx) to form the intermetallic compounds Mg ₂ Si or Mg ₅ Si ₆ which contribute to the structural hardening of these alloys. The presence of silicon, at a content of between 0.85% and 1.20%, combined with the presence of magnesium at a content of between 0.70% and 0.90% makes it possible to obtain the Si / Mg ratio. required to achieve the desired mechanical properties while ensuring good corrosion resistance and stamping forming at satisfactory ambient temperature.
  The range of the most advantageous content is from 0.90 to 1.10%.
• Mg: The level of mechanical characteristics of the alloys of the AA6xxx family is proportional to the magnesium content. Combined with silicon to form the Mg ₂ Si or Mg ₅ Si ₆ intermetallic compounds, magnesium contributes to the increase in mechanical properties. A minimum content of 0.70% is necessary to obtain the required level of mechanical characteristics and to form sufficient hardening precipitates. In addition, the solvus temperature, corresponding to the dissolution temperature, of these alloys is very dependent on the magnesium content. Beyond 0.90%, the solvus temperature becomes too high thus posing problems of industrial solution.
  The range of the most advantageous content is 0.70 to 0.80%.
• Fe: It is always present as impurity in "primary aluminum", since it comes, like silicon, ore, bauxite, whose alumina is extracted. A minimum content of 0.05%, and better still 0.15%, appreciably decreases the solubility of manganese in solid solution, which makes it possible to obtain a sensitivity to the rate of positive deformation, delays the rupture during the deformation after necking, and thus improves ductility and formability. Iron is also necessary for the formation of a high density of intermetallic particles guaranteeing good "hardenability" during shaping. In these grades iron also makes it possible to control the size of the grains. Above a content of 0.30%, too many intermetallic particles are created with a detrimental effect on ductility and corrosion resistance.
  The most preferred range is 0.15 to 0.25%.
• Mn: its content is limited to 0.30%. An addition of manganese above 0.05% can increase the mechanical characteristics by the effect of solid solution, but beyond 0.3%, it would greatly decrease the sensitivity to the rate of deformation and thus the ductility.
  An advantageous range is from 0.10 to 0.20%.
• Cu: In alloys of the AA6000 family, copper is an effective hardener by participating in hardening precipitation. At a minimum content of 0.10%, its presence makes it possible to obtain higher mechanical characteristics. Above 0.30% copper has a negative influence on the corrosion resistance.
  The most favorable range of content is 0.10 to 0.20%.
• Zn: the effect of Zn addition in AA6xxx on mechanical properties and corrosion resistance is not fully understood. A minimum content of 0.9% is necessary to obtain the required level of mechanical characteristics, by hardening by solid solution. Preferably the minimum content of Zn is 1.10%. In addition, the addition of Zn in aluminum alloys of the AA6xxx family modifies the temperature of the solidus. The more Zn is added, the lower the solidus temperature, thus reducing the difference between solvus and solidus temperature and making the industrialization of such an alloy difficult. Beyond 1.60%, this difference becomes too critical. The best value range is 1.20 to 1.50%.
• V and Ti: a minimum content of 0.02% vanadium and 0.05% titanium is necessary to obtain a solid solution hardening leading to the required mechanical characteristics and, combined with the addition of Zn, each of these elements also have a favorable effect on the ductility in service and the resistance to corrosion. Preferably the minimum vanadium content is 0.05%. On the other hand, a maximum content of 0.20% for Ti and 0.30% for V is required in order not to form primary phases during vertical casting, which have a detrimental effect on all the properties claimed. The most advantageous range of content is 0.10 to 0.20% for V and 0.08 to 0.15 for Ti.

• soit son homogénéisation à une vitesse d'au moins 30°C/h jusqu'à une température de 550 à 570°C avec un maintien entre 2 et 12 h, préférentiellement entre 4 et 6 h, suivi d'un refroidissement rapide à l'air pulsé ou à l'eau jusqu'à l'ambiante, puis du réchauffage à une température comprise entre 450 et 550°C avec un maintien entre 30 min et 3 h, préférentiellement sensiblement 2 h,
• soit directement le réchauffage à une température de 550 à 570°C avec un maintien entre 2 et 12 h, préférentiellement entre 4 et 6 h.

Intervient ensuite le laminage à chaud de la plaque en une bande d'épaisseur comprise entre 3 et 10 mm, le laminage à froid jusqu'à l'épaisseur finale typiquement comprise entre 1 et 5 mm, la mise en solution de la bande laminée à une température au-delà de la température de solvus de l'alliage, tout en évitant la brûlure, soit entre 550 et 570°C pendant 5 s à 5 min et de préférence 30 s à 5 mn, la trempe à une vitesse de plus de 50°C/s et mieux d'au moins 100°C/s, et enfin le pré-revenu, ou réversion, par bobinage à une température d'au moins 60°C suivi d'un refroidissement à l'air libre de la bobine obtenue.The method of manufacturing the sheets according to the invention typically comprises the casting of a plate, possibly the scalping of this plate, followed by:

• it is homogenized at a rate of at least 30 ° C./h up to a temperature of 550 to 570 ° C. with a hold of between 2 and 12 hours, preferably between 4 and 6 hours, followed by rapid cooling at 100.degree. pulsed air or water until ambient, then reheating at a temperature between 450 and 550 ° C with a maintenance between 30 min and 3 h, preferably substantially 2 hours,
• or directly reheating at a temperature of 550 to 570 ° C with a maintenance between 2 and 12 h, preferably between 4 and 6 h.

Then comes the hot rolling of the plate in a strip of thickness between 3 and 10 mm, the cold rolling to the final thickness typically between 1 and 5 mm, the dissolution of the strip laminated to a temperature above the solvus temperature of the alloy, while avoiding the burn, ie between 550 and 570 ° C for 5 s to 5 min and preferably 30 s to 5 min, quenching at a speed of more 50 ° C / s and better still at least 100 ° C / s, and finally the pre-income, or reversion, by winding at a temperature of at least 60 ° C followed by cooling in the open air of the coil obtained.

Thus, the sheets according to the invention have a satisfactory ability to draw at room temperature. Just as advantageously, they have, in use, after shaping, assembly and baking paints, high mechanical properties, good resistance to corrosion, in particular intergranular corrosion and filiform corrosion.

Examples Preamble

Table 1 summarizes the nominal chemical compositions (% by weight) of the alloys used in the tests.

The foundry plates of these different alloys were obtained by vertical semi-continuous casting.

After scalping, these different plates have undergone a heat treatment homogenization and / or reheating whose temperatures are given in Table 2. The plates of cases 1, 6, 7, 8 and 10 have undergone a homogenization treatment to 570 ° C consisting of a rise in temperature at a speed of 30 ° C / h up to 570 ° C, a maintenance of the order of 5 hours at 570 ° C and a controlled cooling with forced air up to ambient temperature. This homogenization step is followed by a heating step consisting of a rise in temperature at a speed of 70 ° C./h up to 480 ° C. with a holding time of the order of 40 minutes, directly followed by hot rolling. The plates of case 2 underwent a homogenization treatment at 562 ° C. consisting of a temperature rise at a rate of 30 ° C./h up to 562 ° C., a maintenance of about 5 hours at 562 ° C. C then controlled cooling to room temperature. The homogenization step is followed by a heating step consisting of a rise in temperature at a speed of 60 ° C./h up to 530 ° C. with a maximum temperature retention of 2 hours, followed by rolling. hot. The plates of cases 3 and 5 were reheated consisting of a rise at respectively 565 ° C and 550 ° C with minimum maintenance of 2 hours at these temperatures, directly followed by hot rolling. The plates of cases 4 and 9, made of AA6016 and AA5182 type alloys, have undergone standard homogenizations for these types of alloys.

The next hot rolling step takes place on a reversible rolling mill followed according to the case of a hot tandem rolling mill with 4 stands up to a thickness of between 3 and 10 mm. The hot rolling output thicknesses of the tested cases are given in Table 2.

This hot rolling step is followed by a cold rolling step which makes it possible to obtain sheets having thicknesses of between 1.7 and 2.5 mm. The cold rolling output thicknesses of the tested cases are given in Table 2.

The rolling steps are followed by a solution heat treatment step and quenching. The dissolution is done at a temperature above the solvus temperature of the alloy, while avoiding burning. The dissolved sheet is then quenched at a minimum speed of 50 ° C / s. For all cases, except for cases 4 and 9, this step is carried out in a passing furnace by raising the temperature of the metal to 570 ° C in less than about one minute directly followed by quenching. For case 4, alloy AA6016 type, the cold rolling was also followed by a heat treatment at the end of the range and consists of a solution and quenching carried out in a furnace to pass by raising the temperature of the metal until at 540 ° C in about 30 seconds and quenching at a minimum speed of 50 ° C / sec. For case 9, of AA5182 type alloy, the recrystallization annealing took place in a pass-through furnace and consisted in bringing the metal to a temperature of 365 ° C. in approximately 30 seconds and then cooling it.

The quenching is followed by a pre-tempered heat treatment, intended to improve the curing performance during the baking of the paints. For all the cases tested, except case 9, this step is performed by winding at a temperature of at least 60 ° C followed by cooling in the open air. The winding temperatures are described in Table 2. <b> Table 1 </ b> Composition Yes Fe Cu mn mg Zn Ti V Invention 1 0.92 0.19 0.16 0.18 0.72 1.47 0.08 0.15 Invention 2 0.94 0.20 0.17 0.17 0.72 1.52 0.11 0.15 Invention 3 0.95 0.20 0.16 0.18 0.74 1.20 0.10 0.14 Alloy 4 1.05 0.25 0.09 0.17 0.37 0.02 0.02 0.00 Alloy 5 1.08 0.25 0.18 0.18 0.57 0.01 0.02 0.00 Alloy 6 0.81 0.15 0.16 0.17 0.79 0.01 0.02 0.00 Alloy 7 0.63 0.19 0.16 0.17 0.97 1.46 0.09 0.15 Alloy 8 0.93 0.20 0.16 0.18 0.78 0.05 0.03 0.01 Alloy 9 <0.20 <0.35 0.07 0.33 4.65 0.01 0.02 0.00 Alloy 10 0.79 0.29 0.80 0,003 0.71 0.49 0.05 0.01 homogenization Reheating Thickness output LAC Thickness output LAF Pre-Revenue Invention 1 570 ° C 480 ° C 10 mm 2.0 mm 85 ° C Invention 2 562 ° C 530 ° C 10 mm 2.5 mm 65 ° C Invention 3 X 565 ° C 10 mm 2.0 mm 80 ° C Alloy 4 - - 6.0 mm 2.0 mm 70 ° C Alloy 5 X 550 ° C 3.0 mm 1.7 mm 60 ° C Alloy 6 570 ° C 480 ° C 10 mm 2.0 mm 85 ° C Alloy 7 570 ° C 480 ° C 10 mm 2.0 mm 85 ° C Alloy 8 570 ° C 480 ° C 10 mm 2.0 mm 85 ° C Alloy 9 - - 4.3 mm 2.5 mm - Alloy 10 570 ° C 480 ° C 8 mm 2,0mm 85 ° C

Traction tests

Tensile tests at ambient temperature were carried out according to standard NF EN ISO 6892-1 with non-proportional specimens, of widely used geometry for the sheets, and corresponding to the type of specimen 2 of Table B.1 of the appendix. B of said standard. These specimens have in particular a width of 20 mm and a calibrated length of 120 mm.

The results of these tensile tests in terms of the 0.2% yield strength, Rp _0.2 , and measured on the sheets as manufactured according to the conditions described in the preceding paragraph, ie after quenching, pre-tempering, maturation at room temperature for a minimum of 72 hours, then strain hardening of 2% in controlled tension to simulate shaping, and hold for 20 min. at 185 ° C to simulate the baking of the paints, are given in Table 3 below. <b> Table 3 </ b> Rp _0.2 [MPa] Alloy 4 217 Alloy 5 264 Alloy 6 282 Alloy 7 288 Alloy 8 291 Invention 1 309 Invention 2 316 Invention 3 307

It is clearly noted that the yield strengths of alloy sheets 1, 2 and 3, according to the invention, are greater than 300 MPa, as claimed, which is not the case for other alloys.

The results of these tensile tests, again in terms of the 0.2% yield strength, Rp _0.2 , but measured on the sheets as manufactured under the conditions described in the previous paragraph, in the T6 state, after quenching, pre-tempering, ripening at room temperature for a minimum of 72 hours, and returning to the T6 state at the peak of hardening, ie 2 hours at 205 ° C., are given in Table 4 below. after. <b> Table 4 </ b> Rp _0.2 [MPa] Alloy 3 249 Alloy 4 310 Alloy 5 336 Alloy 6 347 Alloy 7 343 Alloy 9 344 Invention 1 355 Invention 2 357 Invention 3 354

It is clearly noted that the yield strengths of alloy sheets 1, 2 and 3, according to the invention, are greater than 350 MPa, as claimed, which is not the case for other alloys.

Evaluation of ductility in service

The ductility in service can be estimated by a "three-point bend test" according to the NF EN ISO 7438 standard and the VDA 238-100 procedure.

The folding device is as presented in figure 1 .

It is first carried out on a sheet in the T4 state, either after quenching, pre-tempering and maturation at ambient temperature for 72 hours, a controlled tensile pre-deformation of 10% in the direction perpendicular to the rolling direction. then a hold for 20 min. at 185 ° C to simulate the baking of the paints, and the "three-point bending" is carried out properly using a punch B of radius r = 0.4 mm, the sheet being supported by two rollers R, the bending axis being perpendicular to the pre-traction direction. The rollers have a diameter of 30 mm and the distance between the axes of the rollers is equal to 30 + 2t mm, where t is the initial thickness of the sheet tested T.

At the beginning of the test the punch is brought into contact with the sheet with a pre-force of 30 Newtons. Once the contact is established, the displacement of the punch is indexed to zero. The test then consists in moving the punch so as to perform the "three-point folding" of the sheet.

The test stops when a micro-cracking of the sheet leads to a force drop on the punch of at least 30 Newtons, or when the punch has moved 14.2 mm, which corresponds to the stroke maximum allowed.

At the end of the test, the sheet sample is thus folded as illustrated in figure 2 . The ductility in service is then evaluated by measuring the angle of folding α, here called α _10% , in degrees. The higher the angle α _10% , the better the crimping or folding ability of the sheet.

The results of these folding tests on the sheets as manufactured according to the conditions described in the "Preamble" paragraph are given in Table 5 below. <b> Table 5 </ b> α _10% (°) Alloy 4 63 Alloy 7 52 Invention 1 61

It is clearly noted that the angle α _10% of the sheet according to the invention is greater than 60 °.

Measurement of LDH (Limit Dome Height)

These measurements of LDH (Limit Dome Height) were carried out in order to characterize the stamping performance at the T4 state of the various sheets of this example.

The LDH parameter is widely used for the evaluation of the drawability of sheets with a thickness of 0.5 to 3.0 mm. It has been the subject of numerous publications, in particular 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 No. 930815 .

This is a trial of stamping a blank blocked at the periphery by a ring. The blanking pressure is controlled to prevent slippage in the rod. The blank, dimensions 120 x 160 mm, is biased in a mode close to the plane strain. The punch used is hemispherical.

The figure 3 specifies the dimensions of the tools used to perform this test.

The lubrication between the punch and the plate is ensured by graphited grease (Shell HDM2 grease). The speed of descent of the punch is 50 mm / min. The value called LDH is the value of the displacement of the punch at break, the limit depth of the stamping. It actually corresponds to the average of three tests, giving a 95% confidence interval on the 0.2 mm measurement.

Table 6 below shows the values of the LDH parameter obtained on test pieces of 120 × 160 mm cut from the above-mentioned sheets with a thickness of 2.5 mm and for which the dimension of 160 mm was positioned parallel to the rolling direction. <b> Table 6 </ b> LDH (mm) Alloy 8 37.1 Invention 2 36.5

These results highlight the fact that the sheet according to the invention has an LDH value similar to the LDH value obtained for an alloy sheet of the AA5182 (alloy 8) type, reference alloy when it comes to panels. bodywork for severe stamping.

Evaluation of corrosion resistance

The intergranular corrosion test according to ISO 11846 consists of immersing the test pieces for 24 h in a solution of sodium chloride (30 g / l) and hydrochloric acid (10 ml / l) at a temperature of 30 ° C ( obtained by means of holding in a drying oven), after stripping with hot soda (5% by mass) and with nitric acid (70% by mass) at room temperature.

The samples have a dimension of 40 mm (rolling direction) x 30 mm x thickness.

The type and depth of corrosion caused is determined by a micrographic sectional examination of the metal. The maximum depth of corrosion is measured.

The results are summarized in Table 7 below. <b> Table 7 </ b> Maximum depth of attack in μm Alloy 9 250 Invention 1 140

The maximum depth of attack appears significantly lower for the alloy according to the invention, reflecting a better resistance to intergranular corrosion.

Claims (14)

1. Sheet for a stamped part for an automobile body lining, reinforcement or structure, also referred to as body-in-white, made from aluminium alloy in the AA6xxx series, with the composition (% by weight) of:

   Si: 0.85-1.20 Fe: < 0.30 Cu: 0.10-0.30 Mg: 0.70-0.90 Mn: < 0.30 Zn: 0.9-1.60 V: 0.02-0.30 Ti: 0.05-0.20

   other elements < 0.05 each and < 0.15 in total, the remainder aluminium.
2. Sheet according to claim 1, characterised in that the Si content is between 0.90% and 1.10%.
3. Sheet according to one of claims 1 or 2, characterised in that the Cu content is between 0.10% and 0.20%.
4. Sheet according to one of claims 1 to 3, characterised in that the Mg content is between 0.70% and 0.80%.
5. Sheet according to one of claims 1 to 4, characterised in that the Zn content is between 1.10% and 1.60% and preferably between 1.20% and 1.50%.
6. Sheet according to one of claims 1 to 5, characterised in that the V content is between 0.05% and 0.30% and preferably between 0.10% and 0.20%.
7. Sheet according to one of claims 1 to 6, characterised in that the Ti content is between 0.08% and 0.15%.
8. Sheet according to one of claims 1 to 7, characterised in that the Mn content is between 0.10% and 0.20%.
9. Sheet according to one of claims 1 to 8, characterised in that the Fe content is between 0.15% and 0.25%.
10. Method for manufacturing a sheet according to one of claims 1 to 9, comprising the following steps:

    - the typically semi-continuous vertical casting of a rolling ingot and optional scalping thereof,

    - homogenisation of this rolling ingot at a temperature of 550° to 570°C with holding between 2 and 12 hours, preferentially between 4 and 6 hours, followed by rapid cooling,

    - reheating to a temperature of between 450°C and 550°C with holding between 30 minutes and 3 hours, preferably substantially 2 hours,

    - hot rolling of the rolling ingot into a strip with a thickness of between 3 and 10 mm,

    - cold rolling to the final thickness,

    - solution heat treatment of the rolled strip at a temperature beyond the solvus temperature of the alloy, while avoiding incipient melting, that is to say between 550° and 570°C for 5 seconds to 5 minutes, followed by quenching at a rate of more than 50°C/s and preferentially more than 100°C/s,

    - pre-ageing, or reversion, by coiling at a temperature of at least 60°C followed by cooling of the coil obtained in the open air.
11. Method for manufacturing a sheet according to one of claims 1 to 9, comprising the following steps:

    - the typically semi-continuous vertical casting of a rolling ingot and optional scalping thereof,

    - heating of this rolling ingot to a temperature of between 550° and 570°C with holding between 2 and 12 hours, preferentially between 4 and 6 hours,

    - hot rolling of the rolling ingot into a strip with a thickness of between 3 and 10 mm,

    - cold rolling to the final thickness,

    - solution heat treatment of the rolled strip at a temperature beyond the solvus temperature of the alloy, while avoiding incipient melting, that is to say between 550° and 570°C for 5 seconds to 5 minutes, followed by quenching at a rate of more than 50°C/s and preferentially more than 100°C/s,

    - pre-ageing, or reversion, by coiling at a temperature of at least 60°C followed by cooling of the coil obtained in the open air.
12. Sheet obtained by the method according to one of claims 10 or 11, characterised in that, after optional natural ageing at ambient temperature of between 72 hours and 6 months, 2% controlled tensile pre-stressing, and paint baking treatment, typically 20 minutes at 185°C, it has a yield strength Rp_0.2 of at least 300 MPa.
13. Sheet obtained by the method according to one of claims 10 or 11, characterised in that, at temper T6 according to the European standard EN 515, it has a yield strength Rp_0.2 of at least 350 MPa.
14. Sheet with a thickness of 2 mm, obtained by the method according to one of claims 10 or 11, characterised in that, after optional natural ageing at ambient temperature of between 72 hours and 6 months, 10% controlled tensile pre-stressing, and paint baking treatment, typically 20 minutes at 185°C, it has a "three-point bending angle" α_10% measured in accordance with NF EN ISO 7438, and the VDA 238-100 procedure, of at least 60°.

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