EP0679199B1 - Aluminium-silicon-magnesium alloy having improved ductility and deep-drawing properties, and method for producing same - Google Patents

Description

The invention relates to aluminum alloys of the Al-Si-Mg type with ductility. and improved stampability, used in the form of sheets or strips, as well as a process for obtaining them. The sheets or bands are particularly intended for stamping and in particular for automobile body.

For a given mechanical strength, ductility and drawability are the essential characteristics of the sheets or bands intended for be cold formed, before surface coatings such as painting and "baking" of these.

The conventional alloys used in this field, such as alloys 6009, 6016, 6111 (described in US Patent 4,614,552), according to the designation of the Aluminum Association still exhibit mechanical characteristics of use and formability insufficient.

de 0,9 à 1,35 % Si

de 0,1 à 0,5 % Mg

Mn ≤ 0,3 %

de 0,5 à 0,8 % Cu

Fe ≤ 0,35 %

Patent EP 0 375 572 of the applicant describes an alloy containing (by weight%):

from 0.9 to 1.35% Si

0.1 to 0.5% Mg

Mn ≤ 0.3%

from 0.5 to 0.8% Cu

Fe ≤ 0.35%

de 0,15 à 0,65 Mn

de 0,3 à 0,6 Mg

de 0,7 à 1,2 Si

de 0,1 à 0,5 Cu

jusqu'à 0,4 Fe

jusqu'à 0,05 (chacun) et 0,15 (au total) d'autres éléments

reste Al

The alloy according to the invention contains (by weight%):

from 0.15 to 0.65 Mn

0.3 to 0.6 Mg

from 0.7 to 1.2 Si

0.1 to 0.5 Cu

up to 0.4 Fe

up to 0.05 (each) and 0.15 (total) other items

stay Al

de 0,25 à 0,45 Mn

de 0,3 à 0,5 Mg

de 0,85 à 1,10 Si

de 0,1 à 0,3 Cu

jusqu'à 0,3 Fe

reste: Al+ impuretés inévitables.

The preferred composition is as follows:

from 0.25 to 0.45 Mn

from 0.3 to 0.5 Mg

from 0.85 to 1.10 Si

0.1 to 0.3 Cu

up to 0.3 Fe

rest: Al + unavoidable impurities.

It is known that the presence of Mn is favorable to mechanical strength and to deformability; this action is sensitive beyond 0.1%; however, above 0.8% Mn, coarse compounds (Al, Mn, Fe) are formed which impair formability. The Applicant has also found that high contents of Mn lead to homogenization of the microscopic deformation, which is favorable to a good distribution of the deformations.
For values of Mg less than 0.25%, the elastic limit after curing of the coatings is too low; for values greater than 0.8%, the formability becomes insufficient and the maturation too rapid.
If the Si content is less than 0.5%, the mechanical characteristics are too low; if Si> 1.3%, coarse primary compounds appear and impair formability.
If the Cu content is greater than 0.9%, the corrosion resistance (intercrystalline) is insufficient.
If the Fe content is greater than 0.5%, this results in coarse precipitation which is detrimental to formability.

• coulée d'un alliage de composition donnée sous forme de lingots ou de bandes
• homogénéisation éventuelle
• réchauffage et laminage à chaud
• laminage à froid
• mise en solution
• mise en forme à froid à l'état T4
• revêtement superficiel éventuel et sa "cuisson", par exemple une peinture (laquelle contribue au durcissement de l'alliage) -voir par exemple US 4614552, US 4784921, US 4840852, WO 87/02712.

The manufacturing process usually used includes the following operations:

• casting of an alloy of given composition in the form of ingots or strips
• possible homogenization
• reheating and hot rolling
• cold rolling
• dissolution
• cold forming in T4 state
• possible surface coating and its "baking", for example a paint (which contributes to the hardening of the alloy) - see for example US 4614552, US 4784921, US 4840852, WO 87/02712.

The Applicant has found that this range could be simplified and / or improved, on the one hand by reducing the homogenization step to a reheating before hot rolling, or on the other hand, by introducing a rapid quenching and a pre-tempering step after quenching and before maturation.

Thus, the method according to the invention, comprising the casting operations, reheating, hot rolling and possibly cold rolling, dissolution and quenching, maturing and possibly surface coating and "cooking" thereof, is characterized in that the temperature of the reheating before hot rolling and the inlet temperature to the rolling mill hot is between 460 and 520 ° C.

A temperature rise at a speed between 10 ° C / h and 150 ° C / h and a holding temperature limited between 460 ° C and 520 ° C lead to effect at a maximum of the volume density of the precipitates, at Mn: Al (Fe, Mn) Si; their maximum size is less than 0.2 µm and their size median is less than 0.07 µm.

After the temperature has risen, the temperature holding time is between 30 min and 24 h.
The temperature at the end of hot rolling is preferably less than 400 ° C. and even 350 ° C.

The fine manganese precipitates remain until the final stage, and the plaintiff hypothesizes that the presence of these is at the origin improved cold forming characteristics.

The dissolution is preferably carried out between 520 and 570 ° C. and in particular, between 550 and 570 ° C, for 5 min to 1 h. Average speed quenching temperature is preferably greater than 100 ° C / sec.

For short holding times, a pass-through oven can be used.

Typically the alloy matures at room temperature and reaches a hardness stationary in about 15 days, state in which it is able to undergo formatting.

After forming and possibly a surface coating, the alloy may undergo tempering hardening during the baking treatment of the coating (around 180 ° C. for 30 min). It has however been noted that in the case of an alloy homogenized in a conventional manner, the practice of a pre-annealing between 70 and 150 ° C for 0.5 to 5 h after quenching leads to a significant increase in the coefficient d 'hardening n (after maturation) and a significant increase in the mechanical strength characteristics (after curing of the coatings).
The coefficient of work hardening is equal to n = d (Lnσ) / dε, σ being the stress of Von Misès and ε the equivalent strain of Von Misès for strains in tension ranging between 5 and 20% (ε = Ln (1 / 1o)).

Figure 1 represents the evolutions of the coefficient of work hardening n with the matured state as a function of the elastic limit in the hardened state with and without pre-income, under the conditions shown in Example 2.

The invention will be better understood using the following examples:

Example 1

The alloys, the composition of which is given in Table I, were produced in ingots, 1.25 × 0.6 m ² in section, scalped, reheated (rising speed: 46 ° C / h; holding temperature: 480 ° C) and hot rolled with an inlet temperature of 480 ° C and an outlet temperature of 310 ° C to a thickness of 4mm, then cold rolled to a thickness of 1.2mm.

The dissolution in a passing oven was carried out under the conditions given in Table II, fog cooling then the sheets have undergone 15 days aging at room temperature before testing.

Mechanical characteristics (long sense) and breaking strains ε f in biaxial expansion obtained are given in Table III.

The biaxial expansion test consists of deforming a sheet 300x300x1.2 mm maintained by a circular blank with a diameter of 250 mm by pressure hydraulic. The deformation is measured at the top of the dome formed. It can be seen that the alloy according to the invention exhibits improved formability characteristics compared to those of alloys obtained according to the prior art. There is also a slight hardening which is not specifically sought in the invention.

The Mn precipitates have a median size of 0.06 µm with a maximum dimension of 0.18 µm. Alloy Composition (weight%) Mn Mg Yes Cu Fe AT 0.1 0.35 0.9 0.1 0.25 B 0.4 0.45 0.9 0.1 0.25 VS 0.4 0.4 1.1 0.4 0.25 Range Reheating Dissolution State (T4) Speed Maintenance 1 (witness) 46 ° C / h 1h at 580 ° C 30 sec at 550 ° C 15 days at 20 ° C 2 (claimed) 46 ° C / h 2h at 480 ° C 30 min at 550 ° C 15 days at 20 ° C Alloy Range R 0.2, (MPa) Rm (MPa) AT % A% distributed ε f (%) AT 1 105 220 25 19 52 B 1 115 230 29 22 59 B 2 120 235 31 23 62 VS 1 135 250 28 21 53 VS* 2 140 255 30 23 60

Example 2

An alloy with the following weight composition (in%)
If: 1.08 Fe: 0.10 Cu: 0.05 Mn: 0.38 Mg: 0.40 was poured into 1.25 x 0.6 m ² trays, homogenized at 520 ° C for 33 h, hot rolled up to 4 mm thick between 494 and 304 ° C, cold rolled up to 1.2 mm thick, dissolved in an air oven with rise in 30 min at 560 ° C and maintaining 5 min at this temperature and quenching with water at 20 ° C.
10 min after quenching, samples were pre-annealed for 2 h at 100 ° C, other comparison samples were not treated.
The tensile tests were carried out 14 days after quenching and certain samples were checked after 30 min tempering at 180 ° C, simulating the firing conditions of the coatings.

The results obtained are reported in Table IV below and shown graphically in Figure 1.
We can see the beneficial effects of pre-tempering on the coefficient n in the matured state (T4) and on the mechanical characteristics after curing of the coatings.

Example 3

Products were treated in accordance with Example 2, except for that concerns different cooling rates during quenching.

The results obtained are reported in Table V. Quenching speed (° C / sec) STATUS T4 30 min at 180 ° CR 0.2 R 0.2 (MPa) Rm (MPa) AT (%) At (%) (MPa) 6 111 247 30.7 22.2 120 20 117 251 30.9 23.4 136 580 140 271 32.4 24.4 161

It can be seen that the high quenching speeds are clearly favorable obtaining high mechanical characteristics in the hardened state, with an increase in elongation distributed in the T4 state.

Claims (11)

1. An aluminium alloy for strips and sheets for deep drawing, characterised in that it contains, in weight % :

   0.15 to 0.65 Mn

   0.3 to 0.6 Mg

   0.7 to 1.2 Si

   0.1 to 0.5 Cu

   up to 0.4 Fe

   up to 0,05 (each et 0.15 (in total) of other elements,

   remainder : Al
2. An alloy according to claim 1, characterised in that it contains :

   0.25 to 0.45 Mn

   0.3 to 0.5 Mg

   0.85 to 1,10 Si

   0.1 to 0.3 Cu

   up to 0.3 Fe.
3. An alloy according to any one of claims 1 to 2, characterised in that it contains Mn precipitates of the type Al (Mn, Fe) Si, in which the average size is less than 0.07 µm and the maximum size is less than 0.20 µm.
4. A process for the production of sheets or bands of an Al alloy in accordance with any one of claims 1 to 3, comprising the steps of reheating without homogenization and hot rolling (and optionally cold rolling), solution heat treating and quenching, and ageing at room temperature, characterised in that the reheating temperature and the temperature at the start of the hot rolling step is between 460 and 520° C.
5. A process according to claim 4, characterised in that the temperature is held for between 30 min and 24 h.
6. A process according to claims 4 or 5, characterised in that the heating rate is between 10° C/h and 150° C/h.
7. A process acording to any one of claims 4 to 6, characterised in that the hot rolling end temperature is less than 400° C, preferably less than 350° C.
8. A process according to any one of claims 4 to 7, characterised in that the solution heat treatment step is carried out at between 520° C and 570° C, for 5 min to 1 h.
9. A process according to any one of claims 4 to 8, characterised in that the room temperature ageing period is at least 15 days.
10. A process for the production of sheets or strips of Al alloy in accordance with any of claims 1 to 3, comprising at least one ingot homogenization or reheating step, a hot rolling step (and optionally a cold rolling step), a solution heat treatment step and a quenching step, an ageing step, a forming step and a firing treatment step for any coating, characterised in that pre-tempering between 70 and 150°C of a period of 0.5 to 5 h is carried out between the quenching and ageing steps.
11. A process for the production of a sheet or strip of Al alloy in accordance with claim 10, characterised in taht the average cooling rate during the quenching step is greater than 100° C/sec.

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