EP1488018B1 - Al-mg alloy products for a welded construction - Google Patents

Description

Technical field of the invention

The present invention relates to alloys of Al-Mg type with high mechanical strength, and more particularly alloys for welded constructions such as automobile bodies, industrial vehicles and fixed or mobile tanks.

State of the art

In order to increase the mechanical strength of the welded constructions while reducing their weight, it is advantageous to have improved mechanical characteristics compared to the currently used alloys 5083, 5086, 5182, 5186 or 5383, without losing any of the other properties of use such as as weldability, corrosion resistance or formability, especially in the low work-hardened states such as the state O and the state H111. The designation of these alloys follows the rules of The Aluminum Association, and that of the metallurgical states is defined in the European standard EN 515.

For the dimensioning of a structure, the parameters that govern the user's choice are essentially the static mechanical characteristics: the tensile strength R _m , the elastic limit R _p0 , ₂ , and the elongation at break A. Other parameters that come into play, depending on the specific needs of the intended application, are the mechanical characteristics of the welded joint, the corrosion resistance of the sheet and the welded joint, the fatigue strength of the sheet and of the welded joint, resistance to crack propagation, toughness, bendability, weldability, propensity for the formation of residual stresses under conditions of manufacture and use of the specified sheets, and the ease of producing regular quality sheet metal with a production cost as low as possible.
The state of the art offers several ways to improve the mechanical characteristics of Al-Mg type alloys.

The European patent application EP 769 564 A1 (Pechiney Rhenalu) discloses an alloy of composition (in percent by mass): Mg 4.2 - 4.8 Mn <0.5 Zn <0.4 Fe <0.45 If <0.30 with Mn + Zn <0.7 and Fe> 0.5 Mn
which may also contain certain other elements, which makes it possible to manufacture sheets having in a low-wrought state a value of R _m > 275 MPa, a value of A> 17.5% and a product R _m × A>6500; a better controlled composition makes it possible to bring this product R _m x A to a value greater than 7000 and even greater than 7500.
Alloys of this type are used under the designation 5186 in construction of welded road tanks. For this application, the product R _m x A is used as a parameter to estimate the behavior of the structures under a large plastic deformation, for example in case of damage. The person skilled in the art knows how to increase in one of the known Al-Mg alloys one of the two parameters R _m and A to the detriment of the other; said patent application teaches that sheets with a better compromise between these two parameters can be obtained if the sheet has a very particular microstructure. The sheets 5186 alloy are characterized not only by a product R _m x A high, but also by a high value of A, which promotes the folding of said sheets and facilitated their use in mechanical engineering.

Another way is proposed by the patent application JP 62-207850 (Sky ) which discloses alloys of composition (in percent by weight): Mg 2 - 6 Mn 0.05 - 1.0 Cr 0.03 - 0.3 Zr 0.03 - 0.3 V 0.03 - 0.3 which may additionally contain Cu 0.05 - 2.0 and / or Zn 0.1 - 2.0
produced by continuous casting and whose size of intermetallic particles is less than or equal to 5 μm. These alloys would be suitable for the manufacture of sheets for automobile bodies, because they would make it possible to develop, by means of very specific thermo-mechanical treatment ranges, sheets of a thickness of 1 mm which do not show Lüders lines. .

Another way is proposed by the patent EP 0 892 858 B1 (Hoogovens Aluminum Walzprodukte GmbH) which discloses compositional alloys Mg 5 - 6 Mn 0.6 - 1.2 Zn 0.4 - 1.5 Zr 0.05 - 0.25 which may also contain certain other elements, which make it possible to manufacture very hard alloys, especially with a zinc content of the order of 0.8%. These products show an elongation at break not exceeding a value of the order of 10% in the H321 state and 20% in the O state.

The patent EP 823 489 B1 (Pechiney Rhenalu) discloses compositional products 3.0 <Mg <6.5 0.2 <Mn <1.0 Fe <0.8 0.05 <Si <0.6 Zn <1.3 may also contain some other elements, and characterized by a very particular microstructure; these products have not been designed for use in the construction of tanks but for welded constructions used in contact with seawater or in the maritime environment.

Problem

The problem addressed by the present invention is to improve the mechanical characteristics of Al-Mg alloy products, in particular with a view to their use for producing welded constructions, such as tanks for road or rail transport of hazardous materials, while keeping the other characteristics of the material at a level at least comparable to that of existing materials.

Object of the invention

The object of the invention is a wrought product of Al-Mg alloy, characterized in that it contains (in mass percents) Mw 4.85 - 5.35 Mn 0.20 - 0.50 Zn 0.20 - 0.45 If <0.20 Fe <0.30 Cu <0.25 Cr <0.15 Ti <0.15 Zr <0.15 the rest of the aluminum with its inevitable impurities.

Another subject of the invention is a road or rail tank made at least partially with sheet metal (in percent by mass) Mg 4.90 - 5.35 Mn 0.20 - 0.50 Zn 0.25 - 0.45 If 0.05 - 0.20 Fe 0.10 - 0.30 Cu <0.25 Cr <0.15 Ti <0.15 Zr <0.10 the rest of the aluminum with its inevitable impurities,
said sheets having a product R _{m (TL)} x A _(TL) of at least 8500, and preferably at least 9000.

Detailed description of invention

The designation of the alloys follows the rules of The Aluminum Association. Unless otherwise stated, the chemical compositions are indicated in percent by weight. The metallurgical states are defined in the European standard EN 515. Unless otherwise stated, the static mechanical characteristics, that is to say the breaking strength R _m , the yield point R _p0 , ₂ and the elongation at break A, are determined by a tensile test according to EN 10002-1, on proportional test pieces (and characterized by an initial length between marks L _o = 5.65 √S _o where S _o represents the area of the initial section ) taken in the TL (through-long) sense.

The Applicant has surprisingly found that in order to solve the problem, it is necessary to select a very narrow Al-Mg-Mn-Zn composition range which is distinctly different from that of alloy 5186. In particular, it is necessary to increase the content of magnesium, add a small amount of zinc, and reduce the levels of minor addition elements, Fe, Si, and Mn, while keeping them above a minimum level.

Indeed, magnesium is well known to increase the mechanical characteristics (R ₀ , ₂ and R _m ) of certain types of aluminum alloys; the Applicant has found that a magnesium content of at least 4.85%, preferably greater than 4.90% and even more preferably greater than 4.95% or even 5.00%, makes it possible to obtain the level of characteristics mechanical requirements. However, beyond 5.35% magnesium, the corrosion resistance begins to degrade; a maximum value of 5.30% is preferred.

The addition of zinc in sufficient quantity (minimum 0.20%, preferably at least 0.25% and even more preferably at least 0.30%) is found to have a beneficial effect on the mechanical characteristics of the sheets and on the limit of elasticity at the welded joints. Moreover, it improves the resistance to corrosion. In the context of the present invention, it is preferred not to exceed a content of 0.45%. A content of between 0.25% and 0.40% is preferred.

The Applicant has found that a minimum content of 0.20% of manganese must be maintained to control the granular structure, but it must remain below 0.50% and preferably 0.40%, in order to avoid the formation of coarse intermetallic phases and to facilitate recrystallization in the final state. The preferred range is from 0.25 to 0.35%. The presence of manganese in sufficient quantity also contributes to obtaining the mechanical characteristics.

In 5xxx alloys, copper is known to degrade the general corrosion behavior. The applicant has found that it is preferable to maintain the copper content below 0.25%; a content of less than 0.20%, less than 0.15% or even less than 0.10% is preferred.

Iron and silicon are the usual impurities of aluminum. In the context of the present invention, the iron content must not exceed 0.30% and the silicon content 0.20%. However, the Applicant has surprisingly found that the presence of a certain amount of iron and silicon contributes to achieving the objective of the present invention: by way of example, a content of at least 0.05% of Silicon promotes a finely recrystallized granular microstructure. For iron, a content of at least 0.10% is preferred.

The product according to the invention may contain a small amount of chromium, titanium and zirconium. The content of each of these elements must not exceed 0.15%, and more preferably 0.10%, because a too high content of these elements limits the recrystallization and leads to a fall in the value of A.

The products according to the invention are always produced by semi-continuous casting, followed by the conversion steps which correspond to the desired product shape: spinning for spun or drawn products (bars, tubes, profiles, wires); rolling for rolled products (sheets, strips, thick plates). In the case of rolled products, the rolling plates produced by semi-continuous casting are hot-rolled, then possibly cold-rolled. The strips are then glued and cut into sheets. In this manufacturing process, the hot mill outlet temperature and the winding temperature as well as the rate of work hardening which affect the mechanical characteristics of the product must be carefully adjusted. The preferred final thickness is between 3 and 12 mm. In a preferred embodiment of the invention, the sheet is obtained directly at the final thickness by hot rolling. In this case, an outlet temperature of the hot rolling mill of between 260 ° C. and 330 ° C., and preferably between 290 ° C. and 330 ° C., will advantageously be chosen. Below 260 ° C, the resulting microstructure is not well suited to the intended application, and above 330 ° C, there is sometimes a magnification of the grain that degrades the desired mechanical characteristics. This particular embodiment of the invention, namely the direct obtaining of the sheets at the final thickness not hot rolling, also facilitates the manufacture of sheets of very large width, for example greater than 3000 mm, and preferably higher at 3300 mm and even more preferably above 3500 mm.

In a preferred embodiment, the product according to the invention is characterized by an elongation at rupture A of at least 24%, and preferably at least 27%. This feature facilitates the implementation of the product. For example, it gives the rolled sheets an excellent ability to bend and shape.

In another preferred embodiment, it is sought to optimize the three parameters R _{p0.2 (TL)} , R _{m (TL)} and A _(TL) . The index "TL" indicates that these mechanical characteristics are measured on tensile specimens taken in the Travers-Long direction (perpendicular to the rolling direction) of the sheets. By appropriately adjusting the chemical composition within the indicated areas, a product is obtained which has a yield strength R _p0 , _{2 (TL)} of at least 145 MPa, preferably at least 150 MPa and even more preferably at least 170 MPa, a breaking strength R _{m (TL)} of at least 290 MPa and preferably at least 300 MPa, and an elongation at break A _(TL) of at least 24% and preferably at least 27%.

By way of example, it is advantageous to choose Mn 0.20-0.40, Zn> 0.25 and preferably> 0.30, an iron content of at least 0.10% iron, and a content of silicon of at least 0.10%.

In another preferred embodiment, it is sought essentially to optimize the product R _{m (TL)} x A _(TL) . By suitably adjusting the chemical composition within the indicated areas, a product having a product R _{m (TL)} x A _(TL) in which R _{m (TL)} is expressed as MPa and A _(TL) is obtained. in percent, measured on specimens sampled in the TL sense, is greater than 8200, preferably greater than 8500 and even more preferably greater than 9000, while keeping a sufficient level of R _p0 , _{2 (TL)} . This product, especially in the form of sheets, is particularly suitable for the manufacture of tanks, particularly for the road or rail transport of hazardous materials.

The products according to the invention show a corrosion resistance at least as good as the comparable Al-Mg alloy products which are known, and this, despite a significantly higher magnesium content. In the context of the present invention, this corrosion resistance is preferably characterized, either by the loss of mass and by the maximum depth of metal having defects due to intergranular corrosion after an intergranular corrosion test ( Official Journal of the European Communities, 19/11/1984, N ° L300-35 to 43 ), or by a stress corrosion test performed according to ASTM G 30, G39, G44 and G49. The stress corrosion test can advantageously be carried out with reference to the ASTM G 129 standard, the applicant having established in the past the good correlation between these standards and ASTM G 129 (cf. R. Dif et al., Proceedings of the 6th International Conference on Aluminum Alloys, 1998, Toyohashi, Japan, pp. 1615-1620 , as well as R. Dif et al., Proceedings of the Eurocorr Conference 1997, Trondheil, Norway, pp. 259 - 264 ).

The selected intergranular corrosion test is considered representative of a natural exposure in a marine atmosphere ( R.Dif et al., Proceedings of the EUROCORR Conference, 1999, Aachen, Germany ).

The corrosion behavior is evaluated in the initial state but also after artificial aging treatments whose conditions may vary. A treatment of 7 days at 100 ° C is conventionally used on the alloys of the 5xxx series in order to reproduce the natural aging at room temperature during about twenty years ( EHDix et al., Proceedings of the 4th Annual Conference of NACE, San Francisco, USA, 1958 ).

où les températures sont exprimées en Kelvin. Q représente l'énergie d'activation thermique de la diffusion du magnésium (en J / mol). R est la constante des gaz parfaits.
La valeur du rapport $\frac{Q}{R}$

issue de la littérature est de l'ordre de 10 000K à 13 500K.In very specific use cases, the structures can be subjected to relatively high temperatures (above 60 ° C). Those skilled in the art know that under these conditions, certain alloys of the 5xxx series can develop, beyond a certain duration of exposure, a certain sensitivity to corrosion. In order to study this so-called sensitization phenomenon, it is advisable to carry out thermal treatments more advanced than 7 days at 100 ° C. The concept of equivalent time is usually used to limit the number and duration of treatments to be performed. More precisely, a treatment of duration t ₁ carried out at a temperature T ₁ will be equivalent to a treatment of duration t ₂ carried out at a temperature T ₂ , given by the equation ( R.Dif et al., Proceedings of the 6th International Conference on Aluminum Alloys, 1998, Toyohashi, Japan, pp. 1489-1494 ): $$t_1\mathrm{.}\exp \left(-\frac{Q}{R\mathrm{.}{T}_1}\right)=t_2\mathrm{.}\exp \left(-\frac{Q}{R\mathrm{.}{T}_2}\right)$$

where the temperatures are expressed in Kelvin. Q represents the thermal activation energy of the magnesium diffusion (in J / mol). R is the constant of perfect gases.
The value of the report $\frac{Q}{R}$

from the literature is of the order of 10,000K to 13,500K.

In a particular embodiment of the present invention, the products according to the invention show, during the intergranular test, an intergranular corrosion resistance which is characterized by at least a loss of mass of less than 20 mg / cm ² after an aging of 7. days at 100 ° C., and with a maximum attack depth of less than 130 μm, and preferably less than 70 μm.

Preferably, said products also show, after aging for 20 days at 100 ° C., a mass loss of less than 50 mg / cm ² and preferably less than 30 mg / cm ² , and a maximum attack depth of less 250 microns, and preferably less than 100 microns. The most preferred products in the context of the present invention show, after an aging of 20 days at 120 ° C., a loss of mass of less than 95 mg / cm ² , and preferably less than 80 mg / cm ² , and even more preferentially less at 60 mg / cm ² , with a maximum attack depth of less than 450 μm and preferentially less than 400 μm, it being understood that this characteristic is added to at least one of the characteristics mentioned above, namely after aging for 20 days at 100 ° C or 20 days at 120 ° C. These products, if they possess in addition to excellent mechanical characteristics (for example a product R _m x A of at least 8500 or even 9000) are particularly suitable for the manufacture of welded constructions, such as road or rail tanks, as explained below.

As regards the study of stress corrosion resistance, the Applicant prefers the Slow Strain Rate Testing method described for example in the ASTM G129 standard. This test is faster and has been shown to be more discriminating than the conventional methods of determining the non-breaking stress stress corrosion stress, provided that the experimental conditions are well controlled.

Les aspects critiques de l'essai de traction lente concernent le choix de l'éprouvette de traction, de la vitesse de déformation et de la solution corrosive. La demanderesse a utilisé une éprouvette (prélevée dans le sens Travers-Long), présentant une forme échancrée avec un rayon de courbure de 100 mm, ce qui permet de localiser la déformation et de rendre l'essai encore plus sévère.
Concernant la vitesse de sollicitation, une vitesse trop rapide ne permet pas aux phénomènes de corrosion sous contrainte de se développer, mais une vitesse trop lente masque la corrosion sous contrainte. La demanderesse a utilisé une vitesse de déformation de 5.10^-5 s^-1 (correspondant à une vitesse de déplacement de la traverse de 4,5.10^-2 mm/min) qui permet de maximiser les effets de la corrosion sous contrainte ( R.Dif et al., Proceedings of the 6th International Conference on Aluminium Alloys, 1998, Toyohashi, Japon, pp. 1615-1620 ).
Concernant l'environnement agressif à utiliser, le même type de problème se pose dans la mesure où un milieu trop agressif masque la corrosion sous contrainte, mais où un environnement trop peu sévère ne permet pas de mettre en évidence de phénomène de corrosion. Une solution de 3%NaCl+0.3%H₂O₂ a été utilisé avec succès dans le cadre de la présente invention.The principle of the slow tensile test consists in comparing the tensile properties in an inert medium (laboratory air) and in an aggressive medium. The decrease in static mechanical properties in a corrosive environment corresponds to the sensitivity to stress corrosion. The features of the most sensitive tensile test are elongation at break A and maximum stress (at necking) R _m . The Applicant has found that the elongation at break is a parameter much more discriminating than the maximum stress. It is necessary to ensure that the reduction of the static mechanical characteristics corresponds effectively to stress corrosion, defined as synergistic and simultaneous action of the mechanical stress and the environment. The Applicant therefore also carried out tensile tests in an inert medium (laboratory air), after a preliminary pre-exposure of the test specimen, without constraint, to the aggressive medium, for the same duration as the tensile test performed in this test. middle. If the tensile characteristics obtained do not differ from those obtained in an inert medium, the sensitivity to stress corrosion can then be defined using an index 1 of "sensitivity to SCC" defined as: $$I=\frac{\mathrm{AT}{\%}_{\mathit{MilieuInerte}}-\mathrm{AT}{\%}_{\mathit{MilieuAgressif}}}{\mathrm{AT}{\%}_{\mathit{MilieuInerte}}}x100$$

The critical aspects of the slow tensile test are the selection of the tensile specimen, the rate of deformation and the corrosive solution. The Applicant has used a specimen (taken in the Travers-Long direction), having an indented shape with a radius of curvature of 100 mm, which makes it possible to locate the deformation and make the test even more severe.
Concerning the speed of stress, too fast a speed does not allow the phenomena of stress corrosion to develop, but a speed too slow mask the corrosion under stress. The Applicant has used a deformation speed of 5.10 ^-5 s ^-1 (corresponding to a traverse displacement speed of 4.5 × 10 ^-2 mm / min) which makes it possible to maximize the effects of stress corrosion cracking ( R.Dif et al., Proceedings of the 6th International Conference on Aluminum Alloys, 1998, Toyohashi, Japan, pp. 1615-1620 ).
Regarding the aggressive environment to use, the same type of problem arises insofar as a too aggressive environment masks the corrosion under stress, but where a too mild environment does not allow to highlight corrosion phenomenon. A solution of 3% NaCl + 0.3% H ₂ O ₂ has been used successfully in the context of the present invention.

The products according to the invention can advantageously be used for welded constructions, for the construction of road or rail tanks or for the construction of industrial vehicles. They can also be used for the construction of automobile bodies, especially as reinforcements. They show good fitness skills.

In a preferred use, the products according to the invention are used in the form of rolled sheets in a slightly hardened metallurgical state, such as the O state or the H111 state, with a thickness of between 3 mm and 12 mm, and preferably between 4.5 mm and 10 mm, for the construction of road or rail tanks, said sheets being characterized by a product R _{m (TL)} x A _(TL) greater than 8200, preferably greater than 8500 and even more preferably greater than 9000 and good corrosion resistance. For this use, in a preferred manner, the weight loss during an intergranular corrosion resistance test is less than 30 mg / cm ² after aging for 20 days at 100 ° C., and the CSC index in Slow traction is less than 50% after 20 days aging at 100 ° C.

The products according to the invention can be welded by all the welding processes that can be used for Al-Mg type alloys, such as MIG or TIG welding, friction welding, laser welding, electron beam welding. . More particularly, the Applicant has found that the MIG welding of the products according to the invention leads to welded joints characterized by a breaking limit at least as high as with known alloys such as 5186. These welding tests were carried out in the Travers-Long direction on H111 butt-welded metal plates with a V-chamfer by semi-automatic smooth-flow MIG welding with a 5183 alloy filler wire. The mechanical tests were carried out on tensile specimens taken in the Long direction (perpendicular to the weld seam) with symmetrically flared cord and unsealed cord, or in the TL direction. On a specimen taken in the long direction, a value of R _m of at least 275 MPa is found, which underlines the excellent aptitude of the material for use in welded constructions.

The invention will be better understood with the aid of the examples, which are however not limiting in nature.

examples Example 1

Semicontinuous casting was developed by rolling into different alloys. Their composition is shown in Table 1. The chemical analysis of the elements was carried out by spark spectroscopy on a spectrometry counter obtained from liquid metal taken from the casting channel.

The rolling plates were heated and then hot rolled. For example, the plate corresponding to Example H1 was heated in three stages: 10 h at 490 ° C, 10 h at 510 ° C, 3 h 45 min at 490 ° C and then hot rolled with a temperature of inlet at 490 ° C and a winding temperature of 310 ° C. For the plates corresponding to examples H2, I1, I2, I3 and I4, the heating was done in two stages (21h at 510 ° C + 2h at 490 ° C), the rolling inlet temperatures were 477 respectively. ° C, 480 ° C, 479 ° C, 474 ° C and 478 ° C while the coil temperatures were 290 ° C, 300 ° C, 270 ° C, 310 ° C and 300 ° C, respectively. After the winding, all the sheets were glued and cut. Table 1 Alloy mg Zn mn Yes Fe Cu Zr Ti Cr AT 4.28 0.06 0.31 0.11 0.26 0.04 <0.01 0.02 0.08 B 4.45 0.12 0.43 0.14 0.28 0.06 <0.01 0.02 0.09 VS 4.68 0.02 0.26 0.09 0.25 0.06 <0.01 0.03 0.01 D 4.54 0.03 0.27 0.10 0.23 0.04 <0.01 0.01 0.01 E 4.42 0.07 0.28 0.13 0.25 0.07 <0.01 0.02 0.03 F 4.31 0.04 0.32 0.13 0.27 0.05 <0.01 0.02 0.07 BOY WUT 5.05 0.38 0.29 0.12 0.22 <0.01 <0.01 0.02 0.01 H1, H2 5.19 0.38 0.31 0.08 0.15 0.01 <0.01 0.02 0.01 I1 to I4 5.30 0.26 0.33 0.10 0.16 0.05 <0.02 0.02 0.02

Alloys A, B, C, D, E and F are alloys according to the state of the art. Alloys G, H and I are alloys according to the invention.
The properties of the sheets made from these alloys are shown in Table 2. The sheets bear the same reference letter as the alloy in which they were made. Table 2 Plate properties sheet metal State Thickness [mm] R _{m (TL)} [MPa] R _p0 , _{2 (TL)} [MPa] A _(TL) [%] R _{m (TL)} x A _(TL) AT H111 6.5 278 170 23 6394 B H111 5.1 300 177 23 6900 VS O 5.4 290 149 26.5 7685 D H111 6.2 274 138 28 7672 E O 4.9 287 147 27 7749 F H111 5.3 294 170 23.5 6909 BOY WUT H 111 4.7 300 180 27.7 8310 H1 H111 5.0 308 154 28.5 8778 H2 H111 5.0 309 176 29 8961 I1 H111 6.1 301 148 28.1 8458 I2 H111 8.1 321 182 26.8 9602 I3 H111 6.1 300 149 29.6 8880 I4 H111 5.1 310 164 28.3 8773

Example 2

Two sheets corresponding to example H1 of thickness 5.0 mm in the state H111 were welded end-to-end in the Travers-Long direction with a chamfer in V (angle 45 °) by semi-automatic MIG welding. smooth current. An alloy wire 5183 (Mg 4.81%, Mn 0.651%, Ti 0.120%, Si 0.035%, Fe 0.130%, Zn 0.001%, Cu 0.001%, Cr 0.075%) 1.2 mm thick supplied by French Autogenous Welding was used .

The test piece was taken in the long direction through the welded joint so that the seal is in the middle. With the cord trimmed symmetrically, we found a value of R _m of 285 MPa, and with an unstressed cord a value of 311 MPa.

The same test was carried out on two sheets corresponding to the sheet H2. With the weld bead symmetrically trimmed, a value of R _m of 290 MPa was found. With an unshaped cord, there is a value of 318 MPa. By way of comparison, we obtain 283 MPa with a flat bead on sheets according to the prior art of comparable thickness (see L. Cottignies et al., "AA 5186: a new aluminum alloy for welded constructions", Journal of Light Metal Welding and Construction, 1999 ).

The same test was carried out on two sheets corresponding to sheets I2 and I4; for this test, the specimens were taken in the TL direction through the welded joint. The following results are found: sheet metal Solicitation direction Welding direction Cord trimmed or not Rp0.2 [MPa] Rm [MPa] AT [%] I4 TL The arasé 153 291 13.0 I2 TL The arasé 156 293 16.8 I4 TL The Not leveled 155 312 18.4 I2 TL The Not leveled 163 323 21.3

Example 3

On sheets made as described in Example 1, tests were made of LDH (Limit Dome Height). LDH is a peripherally locked blank coining test ( R. Thompson, "The LDH test to evaluate the sheet metal formability-Final report of the LDH Committee of the North American Deep Drawing Research Group, SAE Conference, Detroit, 1993, SAE Paper No. 93-0815 ). The blank size 490 mm x 490 mm, is solicited in equiaxed bi-expansion. Lubrication between the punch (diameter 250 mm) and the sheet is ensured by a plastic film and grease. The value LDH is the displacement of the punch at break, the limit depth of the stamping.

A value of 101 mm is obtained for the sheet H1, and a value of 94.1 mm for the sheet H2. By way of comparison, an alloy of the prior art with a comparable thickness had obtained the LDH value of 94.3 mm (cf. L. Cottignies et al., "AA 5186: a new aluminum alloy for welded constructions", Journal of Light Metal Welding and Construction, 1999 ).

Example 4

On a sheet of the prior art and the sheet corresponding to Example H1, we carried out slow tensile tests according to the method and with the parameters described in the paragraph "Detailed description of the invention". The elongation values obtained for the two alloys and the different aging conditions are given in Table 3. Table 3 Slow Traction Results Alloy Aging A% Air A% NaCl + H ₂ O ₂ A% Pre-Exhibition I% CSC index Previous Art No 22.8 22.8 Not tested 0% 7 to 100 ° C. 24.2 24.0 Not tested 1% 20 to 100 ° C. 25.0 10.5 24.4 58% 20 to 120 ° C. 24.6 5.4 24.4 78% Invention (eg H1) No 28.9 29.8 Not tested 0% 7 to 100 ° C. 30.4 30.5 Not tested 0% 20 to 100 ° C. 30.7 21.3 30.8 31% 20 to 120 ° C. 30.3 7.7 30.6 75%

It is observed that the alloy according to the invention has a better resistance to corrosion under stress after aging, especially for intermediate levels of aging, despite a higher magnesium content.

Intergranular corrosion tests were carried out on the sheets H1, H2, I2 and I4, corresponding to the invention, as well as on a sheet of alloy 5186 according to the state of the art, according to the recommendations of the Official Journal of the European Communities. ; 19/11/84, No. L300, 35 to 43, using solution B (NaCl 30 g / l + HCl 5 g / l), on samples of size 30 mm * 30 mm * 5 mm. The results obtained during these tests are reported in Table 4, with reference to the results of the prior art. Table 4 sheet metal Loss of mass [mg / cm ² ] Maximum depth of puncture [μm] Not aged 7 days at 100 ° C. 20 days at 100 ° C. 20 days at 120 ° C. 40 to 120 ° C Not aged 7 days at 100 ° C. 20 days at 100 ° C. 20 days at 120 ° C. 40 to 120 ° C 5186 20 47 77 101.5 122.5 100 220 400 550 650 H1 3.5 19 17.5 66 94 40 50 90 280 420 H2 3.5 6 12 54 75.5 30 130 110 350 450 I4 9.5 18.5 35.5 93.5 60 120 250 450 I2 7.5 9.5 11 31 50 50 50 150

The alloy according to the invention has a level of resistance to intergranular corrosion comparable to or better than that of the prior art.

Example 5

A rolling plate with a composition of: Mg 5.0%, Zn 0.30%, Mn 0.35%, Si 0.01%, Fe 0.15%, Cu 0.03% was produced by semi-continuous casting. , Zr 0.02%, Cr 0.03%, Ni <0.01%, Ti 0.02%. After homogenization for 19 h at 505 ° C, the plate was hot rolled to a thickness of 7 mm. After a slight leveling, the sheets were annealed with a rise in temperature at 378 ° C for 8 h, followed by a hold for 30 minutes at a temperature between 378 ° C and 390 ° C.

The sheets thus obtained have the following mean mechanical characteristics (TL direction):
R _m = 297 MPa, R _p0.2 = 139 MPa, A = 28.9%.

Claims (32)

1. Al-Mg alloy wrought product, characterised in that contains (percentage by weight) Mg 4.85-5.35 Mn 0.20-0.50 Zn 0.20-0.45 Si < 0.20 Fe < 0.30 Cu < 0.25 Cr < 0.15 Ti < 0.15 Zr < 0.15 the remainder being aluminium with its inevitable impurities.
2. Product according to claim 1, characterised in that Mg 4.90-5.30%.
3. Product according to any of claims 1 or 2, characterised in that Mn 0.20-0.40% and preferentially 0.25-0.35%.
4. Product according to any of claims 1 to 3, characterised in that Zn 0.25-0.40%.
5. Product according to any of claims 1 to 4, characterised in that Cu < 0.20, preferentially < 0.15 and more preferentially < 0.10%.
6. Product according to any of claims 1 to 5, characterised in that it contains at least 0.10% iron.
7. Product according to any of claims 1 to 6, characterised in that it contains at least 0.05% silicon.
8. Product according to any of claims 1 to 7, characterised in that it contains at least 4.95% magnesium.
9. Product according to any of claims 1 to 8, characterised in that it contains at least 5.0% magnesium.
10. Product according to any of claims 1 to 9, characterised in that its elongation at fracture A is at least 24% and preferentially at least 27%.
11. Product according to any of claims 1 to 10, characterised in that its tensile yield strength R_p0._2(LT) is at least 145 MPa, its ultimate tensile strength R_m(LT) is at least 290 MPa and its elongation at fracture A_(LT) is at least 24%.
12. Product according to claim 11, characterised in that its tensile yield strength R_p0._2(LT) is at least 150 MPa and preferentially at least 170 MPa.
13. Product according to any of claims 11 or 12, characterised in that the elongation at fracture A(_LT) is at least 27%.
14. Product according to any of claims 10 to 13, characterised in that its ultimate tensile strength R_m(LT) is at least 300 MPa.
15. Product according to any of claims 1 to 14, characterised in that the R_m(LT) x A_(LT) product, wherein R_m(LT) is expresses in MPa and A_(LT) as a percentage, is greater than 8200, preferentially greater than 8500 and more preferentially greater than 9000.
16. Product according to any of claims 1 to 15, characterised in that the loss of mass after the intergranular corrosion test after ageing for 7 days at 100°C is less than 20 mg/cm².
17. Product according to any of claims 1 to 15, characterised in that the loss of mass after the intergranular corrosion test after ageing for 20 days at 100°C is less than 50 mg/cm² and preferentially less than 30 mg/cm².
18. Product according to any of claims 1 to 15, characterised in that the loss of mass after the intergranular corrosion test after ageing for 20 days at 120°C is less than 95 mg/cm², preferentially less than 80 mg/cm2, and more preferentially less than 60 mg/cm².
19. Product according to any of claims 1 to 18, characterised in that it consists of a rolled sheet.
20. Sheet according to claim 19, characterised in that its thickness is between 3 mm and 12 mm.
21. Sheet according to claim 20, characterised in that its thickness is between 4.5 mm and 10 mm.
22. Sheet according to any of claims 19 to 21, characterised in that it has been produced by hot rolling from an ingot obtained by means of semi-continuous casting.
23. Sheet according to claim 22, characterised in that the hot rolling mill output temperature is between 260°C and 330°C and preferentially between 290°C and 330°C.
24. Use of a sheet according to any of claims 1 to 23 for welded constructions.
25. Use of a sheet according to any of claims 1 to 23 for road or rail tankers.
26. Use of a sheet according to any of claims 1 to 23 for industrial vehicle construction.
27. Use of a sheet according to any of claims 1 to 23 for motor car bodywork construction.
28. Road or rail tanker produced at least partially with sheets of the following composition (percentage by weight) : Mg 4.95-5.35 Mn 0.20-0.50 Zn 0.25-0.45 Si 0.05-0.20 Fe 0.10-0.30 Cu < 0.25 Cr < 0.15 Ti < 0.15 Zr < 0.10 the remainder being aluminium with its inevitable impurities,
    said sheets having an R_m(LT) x A_(LT) product of at least 8500, and preferentially of at least 9000.
29. Tanker according to claim 28, characterised in that said sheets show a corrosion resistance characterised by a loss of mass during the intergranular corrosion test of less than 50 mg/cm² after ageing for 20 days at 100°C, and preferentially less than 30 mg/cm².
30. Tanker according to any of claims 28 or 29, characterised in that said sheets show a stress corrosion resistance characterised by an SC index of less than 50% after ageing for 20 days at 100°C.
31. Welded construction produced at least partially with sheets according to any of claims 1 to 23.
32. Welded construction according to claim 31, characterised in that the weld seam, obtained by butt-welding in the long transverse direction with a V-shaped chamfer (45° angle) by MIG welding with a 5183 alloy filler wire, shows a value of R_m of at least 275 MPa, measured on a test piece sampled in the longitudinal direction through the welded seam and arranged such that said welded seam is located at the centre of the length of the test piece, after symmetric levelling of the weld seam.