EP0504077B1 - Strong, formable, isotropic aluminium alloys for deep drawing - Google Patents

Description

The invention relates to sheets or strips laminated from Al-based alloy intended for stamping and / or drawing and having high strength mechanical characteristics as well as good isotropy (low rate of horns) and good cold formability.

It is known that the alloys usually used for the manufacture of drawn box bodies are alloys 3004 or 3104, according to the designations of the Aluminum Association.

However, current developments push to seek alloys which are both more resistant mechanically, which correlatively makes it possible to reduce the wall thicknesses for a given application, and more isotropic, that is to say at low rate of horns during stamping and / or drawing in order to improve the utilization rate of the alloy, while remaining sufficiently cold formable. However, for the conventional alloys mentioned above, the first and the last 2 properties are relatively contradictory.

Thus in patent US-A-4 318 755 alloys of this type are claimed but their mechanical characteristics in the hardened state remain relatively modest R: 280-300 Mpa, E 0.2: 250-280 Mpa and A%: 2-4% to guarantee acceptable formability in drawing and drawing, while retaining good isotropy.
Patent EP 0121 620 describes an Al Mg Mn alloy intended for the manufacture of bodies, lids and rings for cans of beverage curable by cooking the varnish ("bake hardening" effect), of composition (% by weight): Mg: 0.5 - 2.5 Mn 0.5 - 2 Cu 0.05 0.5 Fe <0.7 If <0.5 Ti <0.05 the microstructure of which, after cold rolling, has an average grain size of less than 25 microns.
The Al-based alloys according to the invention, which exhibit both high mechanical characteristics, good isotropy and good formability belong to two distinct families, one derived from the conventional 3004 alloys which are the subject of the present invention, Another subject of divisional application no. 95105950.0, essentially containing additions of Fe and Mg.

The alloys of the present invention are distinguished from the prior art by a “classic” Mn content associated with a low iron content. In addition, a relatively high Cu content is preferred.

More precisely, the sheets or strips according to the invention have the following weight compositions (%): Mn 0.8 to 1.6 Mg 0.7 to 2.5 eventually Cu 0 to 0.6 Cr 0 to 0.35 Ti from 0 to 0.1 V from 0 to 0.1 Remains Al and unavoidable impurities: Each ≤ 0.05% Total ≤ 0.15% and their Fe and Si contents are each controlled to less than 0.25%. A Cu content greater than or equal to 0.20% or even 0.25% is preferable. Likewise, it is preferable to have an Fe content ≤ 0.20, or better still less than 0.15%.

De même, pour obtenir des taux de déformation à froid, définis par : (épaisseur initiale - épaisseur finale) /épaisseur initiale, supérieurs à 50%, ou même 60 ou 65%, sans recuit(s) intermédiaire(s), tout en conservant une isotropie élevée, il convient de respecter la relation suivante : $$\text{\%Mn-2,25\%Fe≥0,50}$$

Les limitations analytiques se justifient de la façon suivante : Pour Fe ≥ 0,25 et/ou Si ≥ 0,25, il y a apparition dans la structure micrographique de « bandes blanches » après homogénéisation ou réchauffage et laminage à chaud, zones dans lesquelles la teneur en Mn est faible favorisant ainsi l'anisotropie du matériau. Les teneurs en Mn et Mg sont limitées inférieurement pour obtenir une résistance mécanique suffisante ; cependant au-delà de 1,6 % Mn, il y a apparition de particules intermétalliques primaires néfastes vis-à-vis de la formabilité au cours du laminage ou des opérations d'emboutissage et/ou d'étirage, et pour Mg ≥ 2,5 %, il y a apparition de défauts à l'étirage, par exemple le collage sur la filière (aussi appelée bague) et une trop grande anisotropie.In addition, it has been observed that, preferably, the contents of Mn, Fe and Mg must be limited, in order to avoid the rough precipitation of primary compounds, precipitates which are harmful and cause defects during the subsequent rolling and / or stamping-stretching; in this case, the relationship to be observed is as follows: $$\text{\% Mn + 0.9\% Fe + 0.3\% Mg≤1.9}$$

Similarly, to obtain cold deformation rates, defined by: (initial thickness - final thickness) / initial thickness, greater than 50%, or even 60 or 65%, without intermediate annealing (s), while retaining a high isotropy, the following relationship should be observed: $$\text{\% Mn-2.25\% Fe≥0.50}$$

The analytical limitations are justified as follows: For Fe ≥ 0.25 and / or Si ≥ 0.25, there appears in the micrographic structure of "white bands" after homogenization or reheating and hot rolling, zones in which the Mn content is low, thus promoting the anisotropy of the material. The contents of Mn and Mg are limited below to obtain sufficient mechanical strength; however, beyond 1.6% Mn, there is the appearance of primary intermetallic particles which are harmful with regard to formability during rolling or stamping and / or drawing operations, and for Mg ≥ 2 , 5%, there is an appearance of stretching defects, for example bonding on the die (also called ring) and too great anisotropy.

Cu is kept below 0.6% to meet food standards (decree of August 27, 1987), but is preferably held above 0.20% or even 0.25% to obtain the high characteristics mechanical properties required when firing the coverings.
Above 0.35% Cr, there is the appearance of coarse primary intermetallic compounds harmful to formability by damaging effect. The upper limits in Ti and V are justified for this same reason.

A preferred composition contains from 1.2 to 1.6% Mn, from 0.8 to 1.2% Mg, from 0.2 to 0.6% Cu, and up to 0.25% Cr.

The use of the alloys according to the invention is completely analogous to that of the alloys 3004 and 3104, as will appear in detail in the examples.

• coulée, généralement par coulée semi-continue en lingots ou coulée directe en bandes
• homogénéisation ou réchauffage
• laminage à chaud jusqu'à une épaisseur intermédiaire
• laminage à froid avec ou sans recuits intermédiaires ce qui fournit des ébauches adaptées aux opérations d'emboutissage et d'étirage.

The manufacturing range therefore essentially comprises the following operations:

• casting, generally by semi-continuous casting in ingots or direct casting in strips
• homogenization or reheating
• hot rolling to an intermediate thickness
• cold rolling with or without intermediate annealing which provides blanks suitable for stamping and drawing operations.

The invention also relates to the process for obtaining a laminated strip, defined in claim 8.

It should be noted that the products retain good isotropy even if the rates of cold deformation exceed 50%, or even 60 or 65% without intermediate annealing (s).

The essential differences between the conventional 3004 alloys and the alloys according to the invention lie in limited Fe and / or Si contents, which lead to micrographic structures on hot-rolled products (generally sheets or strips of thickness greater than 3 mm) completely different.

The classic 3004 alloy is characterized by the existence of a structure comprising, in addition to the coarse primary intermetallic precipitates situated in the interdendritic zones and the intragranular secondary precipitates, fine "white bands", free of precipitates, in the interdendritic zones. On the contrary, the alloys according to the invention have similar microstructures, but in the total absence of "white bands".

The alloys according to the invention are therefore characterized by a very homogeneous distribution of the primary and secondary precipitates in an Al-based matrix from the ingot stage.

FIGS. 1, 2 and 3 are micrographs at magnification X400 respectively of the conventional alloy 3004 (example 0) and of the alloys of examples 1 (or 3) and 2, according to the invention, in the raw state of rolling to hot.

Figure 4 is a schematic profile of the distribution of precipitates (volume fraction in%) as a function of the distance (in µm) counted from an interdendritic zone on a cross section of a dendrite having about 95 µm wide for a conventional 3004 alloy (thick line) and an alloy according to the invention (thin line).

où Hα = (H + H_180- α + H₁₈₀₊α + H_360- α)/4
H étant la hauteur de l'embouti cylindrique dans une direction faisant un angle α avec la direction de laminage et
$\overline{\text{H}}$

la hauteur moyenne de l'embouti cylindrique définie par

n étant le nombre d'extréma = 2 x nombre de cornes - voir norme NFA 50-301, juin 1976 -The invention will be better understood with the aid of the following examples, compared with an alloy 3004 taken as a reference.
In these examples, the material obtained is characterized by its mechanical tensile characteristics (transverse direction), by the index of horns S 45/90 as defined below, and the values of LDR and LIR spreading defined below.
The rate of horns: $$\text{S α / β =}\frac{\overline{\text{H}}\text{α -}\overline{\text{H}}\text{β}}{\overline{\text{H}}}$$

where Hα = (H + H _180- α + H ₁₈₀₊ α + H _360- α) / 4
H being the height of the cylindrical stamp in a direction making an angle α with the rolling direction and
$\overline{\text{H}}$

the average height of the cylindrical stamp defined by

n being the number of extrema = 2 x number of horns - see standard NFA 50-301, June 1976 -

permettant l'étirage sur poinçon d'un cylindre sans apparition de défauts dans des conditions déterminées de géométrie d'outillage (filière/ poinçon), de lubrification, d'épaisseur initiale, de nombre de passes, (généralement 3), etc...., e_o étant l'épaisseur initiale de la paroi et e_f étant l'épaisseur finale.The LDR (limiting drawing ratio) is the value of the ratio: φ maxi blank / φ punch without appearance of rupture under specific stamping conditions: lubrication, pressure of blank holder, geometry of the punch (rounded), sheet thickness (blank), etc.
The LIR (limiting ironing ratio) in% is the nominal value of the ratio $$\text{LIR = 100 (eo - ef) / eo}$$

allowing the stretching on a punch of a cylinder without the appearance of defects under determined conditions of tool geometry (die / punch), lubrication, initial thickness, number of passes, (generally 3), etc. .., e _o being the initial thickness of the wall and e _f being the final thickness.

The following examples (1 to 3) illustrate the invention with respect to the alloy 3004 taken as a reference (example 0).

The alloys, the chemical composition of which is given in Table I, were cast in trays of 1100 x 300 x 2650 mm3, homogenized or reheated, scalped, hot rolled up to 3mm thick and cold up to 0.3 mm thick, with or without intermediate annealing under the conditions detailed in Table 2 (H lx state).
A simulation of the firing of the varnishes was carried out by maintaining for 10 minutes at 204 ° C. (state H 28).

The results obtained are reported in Table 3.

• que l'exemple 1 présente des caractéristiques mécaniques élevées et une faible anisotropie avec une formabilité comparable à celle du 3004.
• que l'exemple 2 présente des caractéristiques mécaniques très élevées associées à une bonne formabilité, l'isotropie étant notablement plus forte que celle du 3004
• que l'exemple 3 présente une isotropie particulièrement élevée, les caractéristiques de résistance mécanique et de formabilité étant équivalentes à celle de 3004

It can be seen :

• that Example 1 has high mechanical characteristics and low anisotropy with a formability comparable to that of 3004.
• that Example 2 has very high mechanical characteristics associated with good formability, the isotropy being notably higher than that of 3004
• that Example 3 has a particularly high isotropy, the mechanical strength and formability characteristics being equivalent to that of 3004

The invention finds a main application in the manufacture of stretched cans, particularly of drink cans, lighter and / or more resistant with an increased saving of material, with a manufacturing range entirely similar to that of conventional alloys (3004 -3104), with a simplification of the manufacturing range by avoiding intermediate annealing.

Claims (10)

1. Al-based alloy sheet or strip intended for drawing and/or ironing, characterised in that the alloy contains (in weight %):

   Mn from 0,8 to 1,6 - Mg from 0,7 to 2,5 - optionally Cr from 0 to 0,35 -

   from 0 to 0,1 - V from 0 to 0,1 - other elements: each ≤ 0,05, total ≤ 0,15, remainder Al,

   that Fe and Si contents are each controlled to be less than 0,25,

   and that in the homogenised state or in the hot-rolled state, its structure consists of an Al-based matrix containing evenly distributed primary and secondary precipitates, in the absence of «white bands».
2. Sheet or strip according to claim 1, characterised in that its composition is such as: %Mn - 2,25%Fe ≥ 0,50,
3. Sheet or strip according to claims 1 or 2, characterised in that the Fe content is controlled to be less than 0,20%.
4. Sheet or strip according to claim 3, characterised in that the Fe content is controlled to be less than 0,15%.
5. Sheet or strip according to claims 1 to 4, charactetised in that the Cu content is greater than 0,25%.
6. Sheet or strip according to claims 1 to 5, characterised in that its composition is such as: %Mn + 0,9%Fe + 0,3%Mg ≤ 1,9.
7. Sheet or strip according to claims 1 to 5, characterised in that it contains from 1,2 to 1,6% Mn, from 0,8 to 1,2% Mg and up to 0,25% of Cr.
8. Process for obtaining a rolled strip of Al alloy intended for drawing and/or ironing, comprising:

   - the casting of an alloy containing (in weight %): Mn from 0,8 to 1,6 - Mg from 0,7 to 2,5 - optionally Cu from 0 to 0,6 - Cr from 0 to 0,35 - Ti from 0 to 0,1 - V from 0 to 0,1 - other elements = each ≤ 0,05 - total ≤ 0,15, remainder Al, wherein Fe and Si contents are controlled to be less than 0,25%, and %Mn - 2,25%Fe ≥ 0,50,

   - homogenisation or reheating,

   - hot rolling,

   - cold rolling without intermediate annealing, the degree of cold deformation being greater than 50%.
9. Process according to claim 8 wherein the cast alloy contains from 1,2 to 1,6% of Mn, from 0,8 to 1,2% of Mg and up to 0,25% of Cr.
10. Process according to claims 8 or 9, characterised in that the degree of cold deformation is higher than 65%.

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