EP1558778B1 - Simplified method for making rolled al-zn-mg alloy products, and resulting products - Google Patents

Abstract

The production of an aluminum alloy intermediate product comprises: (a) the semi-continuous casting of plate of an Al-Zn-Mg alloy; (b) subjecting the plate to a homogenization or reheating at a temperature (T1), where 500 degrees C at most T1 at most (Ts-20 degrees C) where Ts is the burning temperature; (c) effecting a first stage of hot rolling with one or more passes at an inlet temperature (T2) where (T1-30 degrees C) at most T2 at most(T1-5 degrees C) and an outlet temperature (T3) where (T1-100 degrees C) at most T3 at most (T1-30 degrees C); (d) rapidly cooling the strip from the hot rolling stage to a temperature (T4); (e) effecting a second hot rolling stage in a tandem mill at an inlet temperature (T5), where T5 at most T4 and 200 degrees C at most T5 at most 300 degrees C and rolling so that the coiling temperature (T) is such that (T5-150 degrees C) at most T6 at most (T5-50 degrees C). Independent claims are also included for: (a) an aluminum alloy product obtained by this method; (b) welded constructions incorporating this product

Description

Technical field of the invention

The present invention relates to Al-Zn-Mg type alloys with high mechanical strength, and more particularly alloys intended for welded constructions such as structures used in the field of shipbuilding, automobile bodywork, industrial vehicle and fixed or mobile tanks.

State of the art

For the manufacture of welded structures, aluminum alloys of the 5xxx series (5056, 5083, 5383, 5086, 5186, 5182, 5054 ...) and 6xxx (6082, 6005A ...) are usually used. The weldable low-copper 7xxx alloys (such as 7020, 7108 ...) are also suitable for producing welded parts as they have very good mechanical properties, even after welding. These alloys, however, are subject to problems of laminar corrosion (in the T4 state and in the affected area of the welds) and corrosion corrosion under stress (in the T6 state).

Alloys in the 5xxx family (Al-Mg) are usually used in H1x (hardened), H2x (hardened and then restored), H3x (hardened and stabilized) or O (annealed) states. The choice of the metallurgical state depends on the compromise between mechanical strength, corrosion resistance and formability that is aimed for a given use.

The 7xxx alloys (Al-Zn-Mg) are said to be "structurally hardened", which means that they acquire their mechanical properties by precipitation of the additive elements (Zn, Mg). The person skilled in the art knows that in order to obtain these mechanical properties, the hot transformation by rolling or spinning is followed by dissolution in solution, quenching and an income. These operations, carried out in the majority of cases separately, are respectively intended to dissolve the alloying elements, to maintain them in the form of super-saturated solid solution at room temperature, and finally to precipitate them in a controlled manner.

The alloys of the 6xxx (Al-Mg-Si) and 7xxx (Al-Zn-Mg) families are generally used in the reclaimed state. In the case of products in the form of sheets or strips, the income giving the maximum mechanical strength is designated T6, when the shaping by rolling or spinning is followed by a separate dissolution and quenching.

For the dimensioning of a structure, the parameters that govern the user's choice are essentially the static mechanical characteristics, that is to say the breaking strength R _m , the elastic limit R _p0,2 , and the The 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 (laminating and stress) of the sheet and welded joint, fatigue strength of sheet and welded joint, resistance to crack propagation, toughness, dimensional stability after cutting or welding, resistance to abrasion. For each intended use, it is necessary to find a suitable compromise between these different properties.

The ability to industrially produce regular quality rolled products with as simple a manufacturing process as possible and a production cost as low as possible is also an important factor in the choice of material.

For 7xxx alloys (Al-Zn-Mg), the state of the art offers several ways to improve the property compromise.

The patent GB 1 419 491 (British Aluminum) discloses a weldable alloy containing 3.5 - 5.5% zinc, 0.7 - 3.0% magnesium, 0.05 - 0.30% zirconium, optionally up to 0.05% each of chromium and manganese, up to 0.10% iron, up to 0.075% silicon, and up to 0.25% copper.

BJ Young's article "New weldable AlZnMg alloys", published in Light Metals Industry, November 1963, mentions two compositional alloys: Zn 5.0% Mg 1.25% Mn 0.5% Cr 0.15% Cu 0.4% and Zn 4.5% 1.2% Mg Mn 0.3% Cr 0.2%.

The article mentions the use of this type of alloys for truck tippers and marine construction.

The patent FR 1 501 662 (Vereinigte Aluminum-Werke Aktiengesellschaft) discloses a weldable alloy of composition Zn 5.78% Mg 1.62% Mn 0.24% Cr 0.13% Cu 0.02% Zr 0.17% used in the form of sheets with a thickness of 4 mm, after being dissolved for one hour at 480 ° C., quenched with water and returned in two stages (24 hours at 120 ° C. and then 2 hours at 180 ° C.) , for the manufacture of shields.

The patent US 5,061,327 (Aluminum Company of America) discloses a method of manufacturing an aluminum alloy rolled product comprising casting a plate, homogenizing, hot rolling, reheating the blank to a temperature of between 260 ° C and 582 ° C, its rapid cooling, a precipitation treatment at a temperature between 93 ° C and 288 ° C, and then cold rolling or hot rolling at a temperature not exceeding 288 ° C.

• une homogénéisation à une température comprise entre 400 et 600°C;
• un laminage à chaud à une température comprise entre 350 et 600°C, ledit laminage à chaud comprenant un premier et un deuxième laminage à chaud jusqu'à environ 10 mm; et
• un laminage à froid.

The document US-B-6,302,973 discloses a process for producing an Al-Zn-Mg aluminum alloy intermediate rolled product with a weight percent composition: Mg 0.5 - 1.5%; Zn 0.1 - 3.8%; If 0.05 - 1.5%; Mn 0.2 - 0.8%; Zr 0.05 - 0.25%; Cr 0.3% max .; Cu <0.3%; Fe 0.5% max .; Ag 0.4% max .; Ti 0.2% max .; Al remains and unavoidable impurities, the method comprising:

• homogenization at a temperature between 400 and 600 ° C;
• hot rolling at a temperature between 350 and 600 ° C, said hot rolling comprising first and second hot rolling to about 10 mm; and
• cold rolling.

Problem

The problem to which the present invention tries to respond is first of all to improve the compromise of certain properties of Al-Zn-Mg alloys in the form of sheets or strips, namely the compromise between the mechanical characteristics (determined on the metal base and welded joint), and corrosion resistance (laminar corrosion and stress corrosion). Moreover, we seek to produce these products with a manufacturing range that is as simple and reliable as possible, allowing them to be manufactured with as little manufacturing cost as possible.

Object of the invention

The first object of the present invention defined in claim 1 is a process for producing an intermediate laminated product of Al-Zn-Mg type aluminum alloy, comprising the following steps:
a) a plate containing (in mass percents) is produced by semi-continuous casting Mg 0.5-2.0 Mn <1.0 Zn 3.0 - 9.0 If <0.50 Fe <0.50 Cu <0.50 Ti <0.15 Zr <0.20 Cr <0.50 the rest of the aluminum with its inevitable impurities, in which Zn / Mg>1.7;
b) the said plate is subjected to homogenization and / or reheating at a temperature T ₁ , chosen such that 500 ° C ≤ T ₁ ≤ (T _S -20 ° C), where T _S represents the burning temperature of l 'alloy,
c) performing a first hot rolling step comprising one or more rolling passes on a hot rolling mill, the inlet temperature T ₂ being chosen such that (T ₁ - 60 ° C) ≤ T ₂ ≤ (T ₁ - 5 ° C), and the rolling process being conducted in such a way that the outlet temperature T ₃ is such that (T ₁ - 150 ° C) ≤ T ₃ ≤ (T ₁ - 30 ° C) and T ₃ <T ₂ ;
d) cooling the strip from said first hot rolling step by appropriate means at a temperature T ₄ ;
e) a second step of hot rolling of said strip is carried out on a tandem mill, the inlet temperature T ₅ being chosen such that T ₅ ≤ T ₄ and 200 ° C ≤ T ₅ ≤ 300 ° C, and the process rolling mill being conducted so that the winding temperature T ₆ is such that (T ₅ -150 ° C) ≤ T ₆ ≤ (T ₅ - 20 ° C).

A second object defined in claim 11 is a product obtainable by the method according to the invention.

A third object defined in claims 14 to 19 is the use of the product obtained by the process according to the invention for the manufacture of welded constructions.

Another object defined in claims 22 and 24 is the welded construction made with at least two products obtainable by the process according to the invention, characterized in that its elastic limit R _p0,2 in the welded joint between two of said products is at least 200 MPa.

Description of figures

• The figure 1 presents a typical manufacturing range in a time - temperature diagram. The cipher marks correspond to the different process steps:
  1. (1) First stage of hot rolling
  2. (2) Cooling
  3. (3) Second stage of hot rolling
  4. (4) Coil winding and cooling
• The figure 2 presents the specimens used for the flaky corrosion tests.
• The figure 3 presents the test pieces used for the stress corrosion tests. The dimensions are given in millimeters.
• The figure 4 gives the principle of the slow tensile test (stress corrosion).
• The figure 5 compares the limit of elasticity in the direction L (black points connected by the black curve) and the loss of mass during a flaky corrosion test (bars) for an intermediate product according to the invention and five different heat treatments of said intermediate product .
• The figure 6 compares the Vickers microhardness in the weld zone for three different welded samples.
• The figure 7 compares the tear strength Kr as a function of the extension of the crack ("delta a", which means Δ a) for six different sheets.
• The figure 8 compares the speed of crack propagation da / dn of a sheet according to the invention with a sheet according to the state of the art.

Detailed description of the invention

Unless stated otherwise, all the information relating to the chemical composition of the alloys is expressed in percent by weight. Therefore, in a mathematical expression, "0.4 Zn" means: 0.4 times the zinc content, expressed in mass percent; this applies mutatis mutandis to other chemical elements. The designation of the alloys follows the rules of The Aluminum Association, known to those skilled in the art. The metallurgical states are defined in the European standard EN 515. The chemical composition of standardized aluminum alloys is defined for example in the standard EN 573-3. Unless otherwise stated, the static mechanical characteristics, ie the tensile strength R _m , the yield strength R _p0,2 , and the elongation at break A, of the metal sheets are determined by means of a tensile test. traction in accordance with EN 10002-1, the location and direction of specimen collection being defined in EN 485-1.

The crack propagation rate da / dN is determined according to the ASTM E647 standard, the damage tolerance K _R according to the ASTM E 561 standard, the resistance to exfoliating corrosion (also called laminar corrosion) is determined according to the ASTM G34 standard ( Exco test) or ASTM G85-A3 (Swaat test); for these tests, as well as for even more specific tests, additional information is given below in the description and in the examples.

The Applicant has surprisingly found that it is possible to manufacture 7xxx alloy rolled products which show a very good compromise of properties, in particular in the welded state, using a simplified process, in which the dissolution, quenching and tempering are carried out during hot rolling by rolling.

The process according to the invention can be carried out on Al-Zn-Mg alloys in a wide range of chemical composition: Zn 3.0 - 9.0%, Mg 0.5 - 2.0%, the alloy may also contain Mn <1.0%, Si <0.50%, Fe <0.50%, Cu <0.50%, Cr <0.50%, Ti <0.15%, Zr <0.20 %, as well as the inevitable impurities.

The magnesium content must be between 0.5 and 2.0% and preferably between 0.7 and 1.5%. Below 0.5%, mechanical properties are obtained which are unsatisfactory for many applications, and above 2.0% there is a deterioration in the corrosion resistance of the alloy. Moreover, above 2.0% magnesium, the quenchability of the alloy is no longer satisfactory, which affects the efficiency of the process according to the invention.

The manganese content must be less than 1.0% and preferably less than 0.60%, to limit the sensitivity to flaky corrosion and to maintain good quenchability. A content not exceeding 0.20% is preferred.

The zinc content must be between 3.0 and 9.0%, and preferably between 4.0 and 6.0%. Below 3.0%, the mechanical characteristics are too low to be of technical interest, and above 9.0% there is a deterioration of the corrosion resistance of the alloy, as well as degradation of the quenchability.

The ratio Zn / Mg must be greater than 1.7 to allow to remain in the composition field which benefits from the structural hardening.

The silicon content must be less than 0.50% in order not to deteriorate the corrosion behavior or the tear resistance. For these same reasons, the iron content must also be less than 0.50%.

The copper content must be less than 0.50% and preferably less than 0.25%, which makes it possible to limit the sensitivity to pitting corrosion and to maintain good quenchability. The chromium content must be less than 0.50%, which makes it possible to limit the sensitivity to flaky corrosion and to maintain good quenchability. The titanium content must be less than 0.15% and that in zirconium less than 0.20%, in order to avoid the formation of harmful primary phases; for Zr it is preferred not to exceed 0.15%.

• Sc < 0,50 % et préférentiellement < 0,20 %
• Y < 0,34 % et préférentiellement < 0,17 %
• La < 0,10 % et préférentiellement < 0,05 %
• Dy < 0,10 % et préférentiellement < 0,05 %
• Ho < 0,10 % et préférentiellement < 0,05 %
• Er < 0,10 % et préférentiellement < 0,05 %
• Tm < 0,10 % et préférentiellement < 0,05 %
• Lu < 0,10 % et préférentiellement < 0,05 %
• Hf < 1,20 % et préférentiellement < 0,50 %
• Yb < 0,50 % et préférentiellement < 0,25 %

The addition of one or more elements selected from the group consisting of Sc, Y, La, Dy, Ho, Er, Tm, Lu, Hf, Yb is advantageous; their concentration should not exceed the following values:

• Sc <0.50% and preferentially <0.20%
• Y <0.34% and preferentially <0.17%
• <0.10% and preferentially <0.05%
• Dy <0.10% and preferentially <0.05%
• Ho <0.10% and preferentially <0.05%
• Er <0.10% and preferentially <0.05%
• Tm <0.10% and preferentially <0.05%
• <0.10% and preferentially <0.05%
• Hf <1.20% and preferentially <0.50%
• Yb <0.50% and preferentially <0.25%

"Hardenability" is understood here to mean the ability of an alloy to be quenched in a fairly wide range of quenching speeds. An alloy said to be easily quenchable is therefore an alloy for which the cooling rate during quenching does not have a strong influence on the properties of use (such as strength or corrosion resistance).

1. (a) La coulée d'une plaque de laminage en alliage d'aluminium selon l'une des méthodes connues, ledit alliage ayant la composition indiqué ci-dessus ;
2. (b) L'homogénéisation ou le réchauffage de cette plaque de laminage à une température T₁ comprise entre 500°C et (T_S - 20°C), où T_S représente la température de brûlure de l'alliage, pour une durée suffisante pour homogénéiser l'alliage et l'amener à une température convenable pour la suite du procédé ;
3. (c) Une première étape de laminage à chaud de ladite plaque, typiquement à l'aide d'un laminoir réversible, à une température d'entrée T₂ telle que (T₁ - 60°C) ≤ T₂ ≤ (T₁ - 5 °C), et le procédé de laminage étant conduit d'une façon à ce que la température de sortie T₃ soit telle que (T₁ - 150°C) ≤ T₃ ≤ (T₁ - 30 °C) et T₃ < T₂ ;
4. (d) Le refroidissement de la bande issue de ladite première étape de laminage par un moyen approprié à une température T₄ ;
5. (e) Une seconde étape de laminage à chaud de ladite bande, typiquement à l'aide d'un laminoir tandem, la température d'entrée T₅ étant choisie telle que T₅ ≤ T₄ et 200°C ≤ T₅ ≤ 300°C, et le procédé de laminage étant conduit de façon à ce que la température de bobinage T₆ soit telle que (T₅ - 150°C) ≤ T₆ ≤ (T₅ - 20 °C).

The process according to the invention comprises the following steps:

1. (a) casting an aluminum alloy rolling plate according to one of the known methods, said alloy having the composition indicated above;
2. (b) homogenizing or reheating said rolling plate at a temperature T ₁ of between 500 ° C. and (T _S -20 ° C.), where T _S represents the burning temperature of the alloy, for a period of time sufficient to homogenize the alloy and bring it to a suitable temperature for the rest of the process;
3. (c) A first step of hot rolling said plate, typically using a reversible mill, at an inlet temperature T ₂ such that (T ₁ - 60 ° C) ≤ T ₂ ≤ (T ₁ - 5 ° C), and the rolling process being conducted in such a way that the outlet temperature T ₃ is such that (T ₁ - 150 ° C) ≤ T ₃ ≤ (T ₁ - 30 ° C) and T ₃ <T ₂ ;
4. (d) cooling the strip from said first rolling step by appropriate means at a temperature T ₄ ;
5. (e) A second step of hot rolling said strip, typically using a tandem mill, the inlet temperature T ₅ being chosen such that T ₅ ≤ T ₄ and 200 ° C ≤ T ₅ ≤ 300 ° C, and the rolling method being conducted so that the winding temperature T ₆ is such that (T ₅ - 150 ° C) ≤ T ₆ ≤ (T ₅ - 20 ° C).

The burn temperature T _s is a quantity known to those skilled in the art, which determines it for example directly by calorimetry on a raw sample of casting, or by thermodynamic calculation taking into account the phase diagrams. The temperatures T ₂ and T ₅ correspond to the temperature of the surface (usually the upper surface) of the plate or strip measured just before entering the hot rolling mill; the execution of this measurement can be done according to the methods known to those skilled in the art.

In an advantageous embodiment, the temperature T ₃ is chosen such that (T ₁ -100 ° C) ≤ T ₃ ≤ (T ₁ - 30 ° C). In another advantageous embodiment, T ₂ is chosen such that (T ₁ - 30 ° C) ≤ T ₂ ≤ (T ₁ - 5 ° C). In another advantageous embodiment, T ₆ is chosen such that (T ₅ - 150 ° C) ≤ T ₆ ≤ (T ₅ - 50 ° C).

It is preferable to choose the temperature T ₃ so that it is greater than the solvus temperature of the alloy. The solvus temperature is determined by those skilled in the art using differential calorimetry. Keeping T ₃ above the solvus temperature makes it possible to minimize the coarse precipitation of MgZn ₂ phases. It is preferred that these phases are formed in a controlled manner in the form of fine precipitated during winding or after winding. The control of the temperature T ₃ is therefore particularly critical. The temperature T ₄ is also a critical parameter of the process.

Between steps b) and c), c) and d), and d) and e), the temperature must not fall below the specified value. In particular, it is desirable that the inlet temperature to the hot rolling mill during step (e), which is advantageously carried out on a tandem mill, be substantially equal to the temperature of the strip after cooling, which requires either a sufficiently fast transfer of the strip from one mill to another, or, preferably, an in-line process. In a preferred embodiment of the method according to the invention, the steps b), c) d) and e) are carried out in line, that is to say a given volume of metal element (in the form of a plate of rolling or rolled strip) goes from one stage to another without intermediate storage likely to lead to an uncontrolled drop in temperature which would require intermediate heating. Indeed, the process according to the invention is based on a precise evolution of the temperature during steps b), c), d) and e); the figure 1 illustrates an embodiment of the invention.

The cooling in step (d) can be done by any means ensuring sufficiently rapid cooling, such as: immersion, sprinkling, forced convection, or a combination of these means. For example, passage of the web through a quenching cell, followed by passage through a quench box by natural or forced convection, followed by passage through a second quenching cell by spraying good results. On the other hand, natural convection cooling as the only means is not fast enough, whether in tape or coil. In general, at this stage of the process, coil cooling does not give satisfactory results.

After winding (step e)), the coil can be allowed to cool. The product from step (e) can be subjected to other operations such as cold rolling, income, or cutting. In an advantageous embodiment of the invention, the intermediate rolled product according to the invention with a cold work hardening of between 1% and 9%, and / or with a complementary heat treatment comprising one or more steps at temperatures of between 80 ° C. and 250 ° C., said complementary heat treatment which can intervene before, after or during said cold work-hardening.

The process according to the invention is designed so as to be able to carry out in line three heat treatment operations which are usually carried out separately: the dissolving (carried out according to the invention during the first hot rolling step), quenching (performed according to the invention during the cooling of the strip), the income (made according to the invention during cooling of the coil). More particularly, the method according to the invention can be conducted so that it is not necessary to heat the product once it has entered the reversible hot rolling mill, each step of said process being at a temperature lower than the previous one. This saves energy. The intermediate rolled product obtained by the process according to the invention can be used as it is, that is to say without subjecting it to other process steps which modify its metallurgical state; this is preferable. If necessary, it may be subjected to other process steps that change its metallurgical state, such as cold rolling.

Compared to a process that performs these three steps separately, the method according to the invention can sometimes lead, for a given alloy, to static mechanical characteristics slightly worse. On the other hand, in some cases, it leads to an improvement of the damage tolerance, as well as an improvement of the resistance to corrosion, especially after welding. This has been found in particular for a restricted composition domain, as will be explained later. The compromise of properties that is obtained with the process according to the invention is at least as interesting as that obtained by a conventional manufacturing process, in which solution, quenching and tempering are carried out separately. and which leads to the T6 state. On the other hand, the process according to the invention is much simpler and less expensive than the known processes. It advantageously leads to an intermediate product whose thickness is between 3 mm and 12 mm; above 12 mm, the winding becomes technically difficult, and below 3 mm, besides the technical difficulties of hot rolling in this zone of thickness, the band is likely to cool too much.

As will be explained below, a preferred composition range for carrying out the process according to the invention is characterized by Zn 4.0 - 6.0, Mg 0.7 - 1.5, Mn <0.60 and preferably Cu <0.25. Alloys exhibiting good quenchability are preferred, and among these alloys, alloys 7020, 7003, 7004, 7005, 7008, 7011, 7018, 7022 and 7108 are preferred.

A particularly advantageous implementation of the process according to the invention is carried out on a type 7108 alloy with: T ₁ = 550 ° C, T ₂ = 540 ° C, T ₃ = 490 ° C, T ₄ = 270 ° C, T ₅ = 270 ° C, T ₆ = 150 ° C.

The Al-Zn-Mg alloy products according to the invention can be welded by all known welding processes, such as MIG or TIG welding, friction welding, laser welding, electron beam welding. Welding tests were carried out on X-chamfered plates welded by semi-automatic smooth-flow MIG welding with a 5183 alloy filler wire. The welding was carried out in the direction perpendicular to the rolling. The mechanical tests on the welded specimens were carried out according to a method recommended by Det Norske Veritas (DNV) in their document "Rules for classification of Ships - Newbuildings - Materials and Welding - Part 2 Chapter 3: Welding" of January 1996. According to this method, the width of the tensile test piece is 25 mm, the cord is symmetrically leveled and the useful length of the specimen and the length of the extensometer used is given by (W + 2.e) where the parameter W designates the width of the bead and the parameter e designates the thickness of the specimen.

More particularly, the Applicant has found that the MIG welding of the products according to the invention leads to welded joints characterized by a greater yield strength and rupture limit than with an alloy manufactured according to a conventional range. (T6). This result, which is reflected in a clear advantage for mechanically welded constructions, that is to say the constructions in which the welded zone has a structural role, is surprising insofar as the static properties of the unwelded metal are rather weaker than at T6.

The corrosion resistance of the base metal and welded joints was evaluated using SWAAT and EXCO tests. The SWAAT test makes it possible to evaluate the resistance to corrosion (especially in flaky corrosion) of aluminum alloys in general. Since the method according to the present invention leads to a product with a highly fiber-reinforced structure, it is important to ensure that said product is resistant to exfoliating corrosion, which develops mainly on products showing a fiber structure. The SWAAT test is described in Annex A3 of ASTM G85. This is a cyclic test. Each cycle, lasting two hours, consists of a humidification phase of 90 minutes (relative humidity of 98%) and a 30-minute spraying period, of a compound solution (for one liter) of salt. for artificial seawater (see Table 1 for composition, which complies with ASTM D1141) and 10ml of glacial acetic acid. The pH of this solution is between 2.8 and 3.0. The temperature throughout the cycle is between 48 ° C and 50 ° C. In this test, the samples to be tested are inclined at 15 ° to 30 ° with respect to the vertical. The test was carried out with a duration of 100 cycles. Table 1: Salt Composition for Artificial Seawater NaCl MgCl ₂ Na ₂ SO ₄ CaCl ₂ KCI NaHCO ₃ KBr H ₃ BO ₃ SrCl ₂ NaF g / l 24.53 5.20 4.09 1.16 0.69 0.20 0.10 0,027 0,025 0,003

The EXCO test, which lasts 96 hours, is described in ASTM G34. It is primarily intended to establish the laminar corrosion resistance of aluminum alloys containing copper, but may also be suitable for AI-Zn-Mg alloys (see J.Marthinussen, S.Grjotheim, "Qualification of new aluminum alloys"). , 3 ^rd International Forum on Aluminum Ships, Haugesund, Norway, May 1998).

For these two types of test, rectangular test pieces were used, one side of which was protected by an adhesive aluminum strip (in order to attack only the other side) and the face to be attacked was either left as it was it is machined to half thickness on half the surface of the sample, and left full thickness on the other half. The diagrams of the test pieces used for each test are given to the figures 2 (laminar corrosion) and 3 (stress corrosion).

The Applicant has found that the product according to the invention has a resistance in flaky corrosion equivalent to that obtained for the standard product (alloy identical or neighboring T6 state).

A particularly preferred product according to the invention contains between 4.0 and 6.0% of zinc, between 0.7 and 1.5% of magnesium, less than 0.60%, and even more preferably less than 0.20%. of manganese, and less than 0.25% of copper. Such a product shows a loss of mass of less than 1 g / dm ² in the 100-day SWAT test (100 cycles) and less than 5.5 g / dm ² in the 96-hour EXCO test before income or after income. corresponding at most to 15 h at 140 ° C.

The resistance to stress corrosion has been characterized using the Slow Strain Rate Testing method, described for example in the ASTM G129 standard. This test is faster and more discriminating than the methods of determining the stress of the non-breaking stress corrosion stress. The principle of the slow traction test, schematized in figure 4 , 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 most sensitive tensile test characteristics are elongation at break A and maximum stress (at necking) R _m . The elongation at break, which is a much more discriminating quantity than the maximum stress, was used. It is however 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. It has therefore been suggested to also carry out tensile tests in an inert medium (laboratory air), after a preliminary pre-exposure of the specimen, without stress, to the aggressive medium, for the same duration as the tensile test. performed in this medium. The sensitivity to stress corrosion is then defined using an index I defined as: $$I=\frac{\mathrm{AT}{\%}_{\mathrm{Pr}e-\mathit{Expo}}-\mathrm{AT}{\%}_{\mathit{MilieuAgressif}}}{\mathrm{AT}{\%}_{\mathit{MilieuInerte}}}$$

The critical aspects of the slow tensile test are the selection of the tensile specimen, the rate of deformation and the corrosive solution. A scalloped specimen with a radius of curvature of 100 mm, which makes it possible to locate the deformation and make the test even more severe, was used. It was taken in the direction Long or Travers-Long. Regarding the speed of stress, it is recognized, especially on Al-Zn-Mg alloys (see the article "Stress Corrosion of Al-5Zn-1.2Mg Crystals in 30g / l NaCl Medium" by T. Magnin and C. Dubessy, published in the Mémoires et Etudes Scientifiques Revue de Metallurgie, October 1985, pages 559-567 ), that too fast a speed does not allow the phenomena of corrosion under stress to develop, but that a speed too slow mask the corrosion under stress. In a preliminary test, the Applicant has determined the deformation rate of 5.10 ^-7 s ^-1 (corresponding to a traversing speed of the crossbar of 4.5 × 10 ^-4 mm / min) which makes it possible to maximize the effects of corrosion under constraint; it was this speed which was then chosen for the test. 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. In order to get closer to the real conditions of use, but also to maximize the effects of stress corrosion, a synthetic seawater solution was used for this test (see specification ASTM D1141, with composition recalled in Table 1). ). For each case, at least three test pieces were tested.

The applicant has found that the process according to the invention makes it possible to obtain products which, for a domain of restricted composition with respect to the composition domain in which the process according to the invention can be implemented, namely Zn 4, 0 - 6.0%, Mg 0.7-1.5%, Mn <0.60%, and Cu <0.25%, have new microstructural characteristics. These microstructural characteristics lead to particularly advantageous use properties, and in particular to better resistance to corrosion.

In these products according to the invention, the width of the zone free of precipitates (PFZ = precipitation-free zone) at the grain boundaries is greater than 100 nm, preferably between 100 to 150 nm, and even more preferably from 120 to 140 nm. nm; this width is much greater than that of comparable products according to the state of the art (that is to say of the same composition, same thickness and obtained according to a standard method T6), for which this value does not exceed 60 nm. It is also noted that the MgZn ₂ precipitates at the grain boundaries have an average size greater than 150 nm, and preferably between 200 and 400 nm, whereas this size does not exceed 80 nm in the products according to the state of the technical. Moreover, the MgZn ₂ type hardening precipitates are much coarser in a product according to the invention than in a comparable product according to the prior art. This indicates that in the process according to the invention the quenching is not as fast as in a conventional method with solution in a furnace followed by a separate quenching. It is clear that the method according to the invention makes it possible to avoid a certain precipitation of coarse phases from the temperature T ₄ . However, care should be taken in carrying out the process according to the invention that the quenching rate is sufficiently high, and to obtain the precipitation at a temperature as low as possible. Said phases must not precipitate massively at a temperature between T ₄ and T ₅ .

These quantitative microstructural analyzes were carried out by transmission electron microscopy with an acceleration voltage of 120 kV on samples taken at mid-thickness in the L-TL direction and electrolytically quenched by double jet in a 30% HNO ₃ + methanol mixture. at -35 ° C under a voltage of 20 V.

It is also noted that the product obtained by the process according to the invention has a fibered granular structure, that is to say grains whose thickness or whose thickness / length ratio is significantly lower than for the products according to the state. of the technique. As an indication, for a product according to the invention, the grains have a size in the direction of the thickness (short-through) of less than 30 μm, preferably less than 15 μm and even more preferably less than 10 μm, and a length / thickness ratio of more than 60, and preferably of more than 100, whereas for a comparable product according to the state of the art, the grains have a size in the direction of the thickness (transverse-short) greater than 60 μm and a length / thickness ratio much less than 40.

The sheets and strips resulting from the process according to the present invention, and in particular those based on the restricted area of composition defined by Zn 4.0 - 6.0%, Mg 0.7 - 1.5%, Mn <0.60% , and preferably Cu <0.25%, can be advantageously used for the construction of automobile parts, industrial vehicles, road or rail tanks, and for construction in the maritime environment.

All sheets and strips resulting from the process according to the present invention are particularly suitable for welded construction; they can be welded by all known welding processes which are suitable for this type of alloys. Sheet metal according to the invention can be welded to one another, or to other sheets of aluminum or aluminum alloy, using a suitable filler wire. By welding two or more sheets according to the invention, it is possible to obtain constructions having, after welding, a yield strength (measured as described above) of at least 200 MPa. In a preferred embodiment, this value is at least 220 MPa. The breaking strength of the welded joint is at least 250 MPa, and in a preferred embodiment of at least 280 MPa, and preferably at least 300 MPa, measured after a maturation of at least one month. In a preferred embodiment, a thermally affected zone is obtained which exhibits a hardness of at least 100 HV, preferably from minus 110 HV, and even more preferably at least 115 HV; this hardness is at least as great as that of the base plates which has the least hardness.

Surprisingly, the Applicant has found that the product obtained by the process according to the invention, in the field of preferred composition (Zn 4.0 - 6.0%, Mg 0.7 - 1.5%, Mn <0 , 60%), shows a higher resistance to abrasion by sand than comparable products. It notes that this resistance to abrasion does not depend in a simple way on the mechanical characteristics of the product, nor its hardness nor its ductility. The fiber structure in the TC direction seems to favor the resistance to abrasion by sand. For this property of use, the superiority of the product resulting from the process according to the invention is due to the combination between a particular fiber structure, inaccessible with the known processes, and the level of mechanical characteristics that its composition confers on it. The Applicant has found that the sand abrasion resistance of the product obtainable by the process according to the invention, expressed in the form of mass loss during a test described in Example 10 below. is less than 0.20 g, and preferably less than 0.19 g for an exposed flat surface of dimensions 15 x 10 mm.

The product according to the invention has good properties of damage tolerance. It can be used as structural element in aeronautical construction. In a preferred embodiment of the invention, the product shows a tensile toughness K _R in the TL sense, measured according to ASTM E561 standard on CCT type specimens of width w = 760 mm and initial crack length 2a ₀ = 253 mm, of at least 165 MPa√m for a Δa _eff of 60 mm, and preferably of at least 175 MPa√m. Its resistance to the propagation of fatigue cracks is comparable to that of sheet metal currently used as a fuselage coating.

The product according to the invention, and in particular that which belongs to the restricted composition range defined by Zn 4.0 - 6.0%, Mg 0.7 - 1.5%, Mn <0.60%, is thus suitable. to be used as a structural element to meet specific requirements for damage tolerance (toughness, resistance to crack propagation) in fatigue). Here is called "structural element" or "structural element" of a mechanical construction a mechanical part whose failure is likely to endanger the safety of said construction, its users, its users or others. For an aircraft, these structural elements include the elements that make up the fuselage (such as fuselage skin (fuselage skin in English), stiffeners or stringers, bulkheads, fuselage (circumferential frames)), the wings (such as the wing skin), the stiffeners (stringers or stiffeners), the ribs (ribs) and spars) and the empennage, as well as the floor beams, seat rails and doors. Of course, the present invention relates only to structural elements that can be made from rolled sheets. More particularly, the product according to the invention is suitable for use as a fuselage coating sheet, in conventional assembly (especially riveted) or welded assembly.

The process according to the invention thus makes it possible to obtain a novel product having an advantageous combination of properties, such as mechanical strength, damage tolerance, weldability, resistance to exfoliating corrosion and stress corrosion, abrasion resistance, which is particularly suitable for use as a structural element in mechanical engineering. In particular, it is suitable for use in industrial vehicles, as well as in equipment for storing, transporting or handling granular products, such as skips, tanks or conveyors.

Moreover, the method according to the invention is particularly simple and fast; its operating cost is lower than that of the processes according to the state of the art likely to lead to products with comparable properties of use.

The invention will be better understood with the aid of the examples, which are however not limiting in nature. Examples 1 and 2 belong to the state of the art. Examples 3, 4, 8 and 9 correspond to the invention. Each of Examples 5, 6, 7, 9 and 10 compares the invention with the state of the art.

examples Example 1

This example corresponds to a transformation range according to the state of the art. Two plates A and B were prepared by semi-continuous casting. Their composition is indicated in Table 2. The chemical analysis of the elements was carried out by X-ray fluorescence (for Zn and Mg elements) and by spark spectroscopy (other elements) on a pawn obtained from liquid metal taken from the casting channel.

The rolling plates were reheated for 22 hours at 530 ° C and hot rolled as soon as they reached a temperature of 515 ° C at the oven exit. The hot-rolled strips were wound to a thickness of 6 mm, the process being conducted so that the temperature, measured on the banks of the coil after the complete winding (at mid-thickness of the winding) is between 265 ° C and 275 ° C, this value being the average between 2 measurements made at both sides of the coil. After hot rolling, the coils were cut and a portion of the sheets obtained was cold rolled to a thickness of 4 mm. Table 2 Alloy mg Zn mn Yes Fe Cu Zr Ti Cr AT 1.20 4.48 0.12 0.12 0.21 0.10 0.12 0,036 0.25 B 1.15 4.95 0.006 0.04 0.10 0.13 0.11 0,011 0.05

After rolling, all the sheets were dissolved in an air oven for 40 minutes at temperatures between 460 ° C and 560 ° C, quenched with water and fractionated by about 2%. Part of the products thus obtained was characterized as such, in the T4 state, which corresponds to the heat-affected zone of the welds. The other part was subjected to a T6 tempering treatment comprising a 4 hour stage at 100.degree. C. followed by a 24 hour stage at 140.degree.

The products in the T4 state have been characterized only in flaky corrosion (EXCO and SWAAT tests) because it is known (see in particular the article "Stress corrosion corrosion susceptibility of aluminum alloy 7020 welded sheets" by MC Reboul, B. Dubost and M. Lashermes, published in Corrosion Science, Vol 25, No. 11, p. 999-1018, 1985 ) that it is the most sensitive state to flaky corrosion for Al-Zn-Mg alloys. On products in the T6 state, the yield strength was measured in the Longitudinal direction and the resistance to flaky corrosion (loss of mass after SWAAT test on full thickness test specimen or machined test on half of its surface ) has been evaluated. The sensitivity to stress corrosion has been determined in both directions only in the T6 state because it is known (see the article by Reboul et al., Cited above) that this is the most sensitive state. stress corrosion. The results are given in Tables 3 and 4. The first letter of the reference of the sheet designates the composition, the second the rolling range (C = hot to 6 mm, F = hot + cold to 4 mm) and the last the solution temperature (B = low at 500 ° C, H = high at 560 ° C). Table 3 Sheet metal Thickness
[Mm] Dissolution R _{p0,2 (TL)} State T6
[MPa] SWAAT Test Half Machined
[Δm in g / dm ² ] Full thickness SWAAT test
[Δm in g / dm ² ] T4 T6 T4 T6 CBA 6mm 500 ° C 359 1.15 1.08 1.44 0.52 ACH 560 ° C 362 0.80 0.76 1.24 0.56 AFB 4mm 500 ° C 362 Not characterized 1.14 0.30 AFH 560 ° C 362 1.10 0.58 BCB 6mm 500 ° C 362 0.65 0.68 1.10 0.36 BCH 560 ° C 375 0.47 0.48 0.66 0.30 BFB 4mm 500 ° C 362 Not characterized 0.74 0.32 BFH 560 ° C 365 0.52 0.32

It is observed that the sensitivity to the flaky corrosion is lower for the alloy according to the composition B (process of elaboration and identical test conditions). This sensitivity is much stronger in the T4 state than in the T6 state. It decreases when the dissolution temperature increases or when the alloy undergoes a cold rolling step. Table 4 sheet metal Thickness
[Mm] Implementation
solution Direction of
solicitation AT%
Air Labo AT%
Sea water AT%
Pre-Expo I = Index
of CSC CBA 6mm 500 ° C Long 16.2 14.9 15.8 5.5% Travers 15.1 14.7 15.1 2.6% ACH 560 ° C Long 16.7 15.1 16.3 7.2% Travers 14.7 13.4 14.5 7.5% AFB 4mm 500 ° C Long 17.0 15.3 16.1 4.7% AFH 560 ° C Long 16.2 15.5 16.4 5.5% BCB 6mm 500 ° C Long 16.1 14.2 16.1 11.8% Travers 17.0 15.6 16.8 7.0% BCH 560 ° C Long 15.2 13.1 15.1 13.1% Travers 16. 0 12.8 16.0 20.0% BFB 4mm 500 ° C Long 15.2 13.7 15.3 10.5% BFH 560 ° C Long 15.2 12.2 15.2 19.7%

It is observed that the sensitivity to stress corrosion (SCC) is higher for the alloy according to the composition B. This sensitivity increases with the dissolution temperature.

Example 2

The sheets from Example 1, rolled to 6 mm and put into solution at 560 ° C., designated ACH and BCH, were welded in the T6 state. The weld was made in the Travers-Long direction, with an X chamfer, by a smooth-running semi-automatic MIG process, with a 5183 alloy filler wire (Mg 4.81%, Mn 0.651%, Ti 0.120%, Si 0.035%, Fe 0.130%, Zn 0.001%, Cu 0.001%, Cr 0.075%) of 1.2mm diameter, supplied by Soudure Autogène Française.

Tensile specimens (width 25 mm, symmetrically trimmed bead, effective length of test piece and length of extensometer equal to (W + 2 e) where W is the width of the bead and the thickness of the test piece) were taken in the long direction, perpendicular to the weld, so that the seal is in the middle. The Characterization was made 19, 31 and 90 days after welding, because the skilled person knows that for this type of alloys, the mechanical properties after welding increase sharply during the first weeks of maturation. Test specimens machined at mid-thickness on half of their surface were also subjected to SWAAT and EXCO tests. The results are shown in Tables 5 (for the properties on the base metal in the T6 state) and 6 (properties on the welded metal). Table 5 sheet metal R _{p0.2 (L)}
[MPa] R _{m (L)}
[MPa] A% _(L)
[%] Mass loss Δm
[g / dm ² ] Corrosion rating
exfoliation SWAAT
100 cycles EXCO
96h SWAAT
100 cycles EXCO
96h ACH 351 378 17 0.76 4.68 EA EA BCH 351 376 16.9 0.48 3.25 pc pc sheet metal R _p0,2
[MPa] R _m
[MPa] R _p0,2
[MPa] R _m
[MPa] R _p0,2
[MPa] R _m
[MPa] Dimensioning of the welded zone 19 days after welding 31 days after welding 90 days after welding SWAAT 100 cycles EXCO 96h ACH 216 346 219 354 236 358 EB EB BCH 194 321 197 325 218 328 EB EB

It can be seen that the alloy according to composition B has less advantageous mechanical properties after welding than the alloy according to composition A. After welding, the resistance in flaky corrosion of the two alloys is degraded with respect to the behavior of the base metal.

Example 3

This example corresponds to the present invention. A plate C was prepared by semicontinuous casting. Its composition is identical to that of the plate B resulting from example 1. The plate was hot rolled after reheating for 13 hours at 550 ° C. (time at the stage) followed by a rolling bearing at 540 ° C. The first step, reversible mill, brought the plate to a thickness of 15.5 mm, the exit temperature of the mill being about 490 ° C. The rolled plate was then cooled by spraying and natural convection to a temperature of about 260 ° C. At this temperature, it was fed into a tandem mill (3 cages), rolled to the final thickness of 6 mm, and wound. The winding temperature of the coil, measured as in Example 1, is about 150 ° C. Once cooled naturally, the coil was discharged into sheets. These were hovered and did not undergo any other deformation operation.

As in Examples 1 and 2, the sheets obtained (reference "C") were characterized in terms of manufacturing (static mechanical characteristics Long and Travers-Long direction, stress corrosion and stress) and after welding (static mechanical characteristics, flaking corrosion). . The welding was carried out simultaneously with the welding of Example 2, and according to the same method. Test specimens machined at half thickness on half of their surface were subjected to SWAAT and EXCO tests. The results are collated in Tables 7 and 8 (non-welded sheets) and in Table 9 (welded sheets). Table 7 Sheet metal R _p0,2
[MPa] R _m
[MPa] AT%
[%] Mass loss Δm in
g / dm ² Rating in flaky corrosion SWAAT
100 cycles EXCO
96h SWAAT
100 cycles EXCO
96h VS 305 _(L) 344 _(L) 14.4 _(L) 0.85 5.1 EA EA / EB 330 _(TL) 356 _(TL) 13.3 _(TL) Sheet metal Thickness
[Mm] Solicitation direction AT%
Air Labo AT%
Sea water AT%
Pre-Expo I = Index
of CSC VS 6 mm Travers 13.1 10.8 13.5 20% sheet metal R _p0,2
[MPa] R _m
[MPa] R _p0,2
[MPa] R _m
[MPa] R _p0,2
[MPa] R _m
[MPa] Dimensioning of the welded zone 19 days after welding 31 days after welding 90 days after welding SWAAT
100 cycles EXCO
96h VS 223 338 235 338 245 340 EB EB

The raw sheet (not welded) according to the invention has a lower resistance to flake corrosion than that of the sheet BCH, manufactured from the same composition but with a much more complex manufacturing process. On the other hand, its corrosion resistance under stress is equivalent.

After welding, the sheet according to the invention has a mechanical strength very much greater than that of ACH and BCH sheets developed with a method according to the prior art. Its resistance to flaky corrosion on welded joint is equivalent.

It is found that the method according to the invention performs the winding at a temperature of about 120 ° C lower than the method according to the state of the art of Example 1.

Example 4

The sheet marked "C" derived from Example 3 was subjected to additional heat treatments of the tempered type at a temperature of 140 ° C. The samples thus obtained were then characterized as in Example 3 (static mechanical characteristics L direction and laminating corrosion). The results are summarized in Table 10 and on figure 5 (the black dots and the black line correspond to the elastic limit, and the bars to mass loss during the SWAAT test). Table 10 Heat treatment R _{p0.2 (L)}
[MPa] R _{m (L)}
[MPa] A% _(L)
[%] Mass loss Δm in
g / dm ² Rating in flaky corrosion SWAAT
100 cycles EXCO
96h SWAAT
100 cycles No
(" VS ") 305 344 14.4 0.85 5.1 EA 3h 140 ° C. 299 336 15.1 0.97 5.0 EA 6h 140 ° C. 294 332 15.3 0.89 5.2 Pc / EA 9h 140 ° C 297 335 15.3 0.69 4.0 Pc / EA 12h 140 ° C 293 332 15.3 0.71 4.1 Pc / EA 15h 140 ° C 289 330 15.5 0.67 3.8 pc

This result shows that the flaky corrosion behavior of the product according to the invention can be very substantially improved by a simple supplementary treatment of income or by a slightly higher winding temperature, and this probably without degradation of the mechanical properties after welding.

Example 5

The microstructure of samples ACH, BCH, BFH and C of Examples 1, 2 and 3 was characterized by scanning electron microscopy with field emission cannon (FEG-SEM, in BSE (backscattered electron) mode, acceleration voltage 15 kV, diaphragm 30 μm, working distance 10 mm, polished to L-TC sampling direction with Pt / Pd conductive deposition) and transmission electron microscopy (TEM, L-TL sampling direction, slide preparation by double jet electrochemical thinning with 30% HNO ₃ in methanol at -35 ° C with a potential of 20 V). All samples were taken at mid-thickness of the plate.

• La largeur de la zone exempte de précipités (PFZ = precipitation-free zone) aux joints de grains est de l'ordre de 25 à 35 nm dans les échantillons ACH, BCH et BFH, alors qu'elle est de l'ordre de 120 à 140 nm dans l'échantillon C.
• Les précipités de type MgZn₂ aux joints de grains ont une taille moyenne de l'ordre de 30 à 60 nm dans les échantillons ACH, BCH et BFH, alors qu'ils ont une taille moyenne comprise entre 200 et 400 nm dans l'échantillon C.

There are significant differences between the ACH, BCH and BFH samples on the one hand, and the C sample on the other hand:

• The width of the zone free from precipitates (PFZ = precipitation-free zone) at the grain boundaries is of the order of 25 to 35 nm in the samples ACH, BCH and BFH, whereas it is of the order of 120 at 140 nm in sample C.
• The MgZn ₂ precipitates at the grain boundaries have an average size of the order of 30 to 60 nm in the ACH, BCH and BFH samples, whereas they have an average size of between 200 and 400 nm in the sample vs.

Example 6

An ACH sheet, a BCH sheet (made as described in Example 1) and a sheet C (made according to the invention as described in Example 3) were welded in the TL (Travers-Long) direction as described in FIG. Examples 2 and 3. On a polished section through the welded joint (TC-L plane), the microhardness of the joint was then determined by successive measurements arranged on a straight line perpendicular to the joint. The values shown in Table 11 and the figure 6 . The parameter Dist [mm] indicates the distance from the measuring point to the core of the weld seam. The hardness values are given in Hv (Hardness Vickers). Table 11 dist -19 -18 -17 -16 -15 -14 -12 -11 -10 -9 -8 -7 -6.5 ACH 128 125 129 128 125 124 127 113 120 114 115 111 113 BCH 125 123 130 126 131 124 123 121 107 109 111 104 114 VS 107 114 113 116 109 110 104 104 107 105 102 103 104 dist -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 ACH 112 110 110 109 109 107 113 112 111 118 111 110 107 BCH 109 109 109 112 110 108 106 109 107 111 105 75 74 VS 112 121 119 118 118 119 118 111 110 115 118 94 87 dist 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 7 ACH 110 108 113 113 117 120 125 114 112 111 115 119 118 BCH 81 77 109 105 106 99 109 109 115 107 104 108 112 VS 88 89 115 111 112 115 116 119 120 123 122 117 101 dist 8 9 10 11 12 13 14 15 16 17 18 ACH 123 127 133 125 139 140 135 134 BCH 111 117 107 128 124 134 131 135 129 130 135 VS 102 104 103 108 105 109 104 109 105 106 109

An influence of the method of manufacture of the base plate on the characteristics of the welded joint obtained with this base sheet: a welded joint made with a sheet C, manufactured by the method according to the invention, shows a significantly higher hardness in the heat-affected zone (HAZ = heat-affected zone) of the weld joint (Dist = [-5.5, -1.5] and [+1.5, +5.5]) that an elaborate welded joint with a BCH sheet, of the same composition but manufactured according to a known method. Furthermore, the heat-affected zone has a hardness greater than that of the base metal for sheet C produced by the process according to the invention, which is quite unusual.

Example 7

6056 sheets coated on both sides with alloy 1300 were prepared according to the method described in example 3 of the patent application. EP 1 170 118 A1 . The chemical composition of the core 6056 is given in Table 12. These products are compared with the sheet C of Example 3 of the present patent application.

The plane TL stress toughness according to ASTM E561 was determined on CCT type specimens of width w = 760 mm and initial crack length 2a ₀ = 253 mm. The thickness of the test pieces is indicated in Table 12. The test makes it possible to define the curve R of the material, giving the tear resistance K _R as a function of the extension of the crack Δa. The results are summarized in Table 13 and on Figure 7 .

The crack propagation rate da / dn according to ASTM standard E 647 in the TL sense was also determined for R = 0.1 on a CCT test piece of width w = 400 mm with an initial crack length 2a0 = 4 mm. , at a frequency f = 3 Hz. The specimens were cut in the full thickness of the sheets. The results are gathered on the Figure 8 . Table 12 sheet metal Fe
[%] Yes
[%] Cu
[%] mn
[%] Sheet thickness
plated [mm] Thickness
curve R [mm] 6056-1 0.14 1.01 0.61 0.55 4.5 4.5 6056-2 0.07 0.83 0.66 0.60 3.2 3.2 6056-3 0.07 0.83 0.66 0.60 3.2 3.2 6056-4 0.12 0.85 0.67 0.59 7 5.5 (*) 6056-5 0.12 0.85 0.67 0.59 7 5.5 (*) NOTE: 0.1% Zr content and 0.7% Mg content for all five sheets.
(*) Obtained by symmetrical machining sheet metal VS 6056-1 6056-2 6056-3 6056-4 6056-5 Δa _eff [mm] Toughness in plane stress K _R [MPa√m] 10 87 90 81 88 86 82 20 117 109 106 111 105 99 30 138 121 124 128 117 110 40 156 130 139 141 124 118 50 170 137 152 153 129 125 60 182 163 164 133 131 70 193 173 173 135 136 80 203 183 182 136 140

It can be seen that the product according to the invention shows a better toughness in plane stress K _R than a known reference product, whereas the crack propagation speed da / dN (TL) at the high ΔK values is substantially comparable.

Example 8

According to the process of the present invention, an alloy whose composition is indicated in Table 14 was produced. Table 14 Alloy mg Zn mn Yes Fe Cu Zr Ti Cr S 1.23 5.00 0.01 0.03 0.09 0.01 0.14 0.03 0,002

• T₁ = 550°C, T₂ = 520 °C, T₄ = 267 °C, T₅ = 267 °C, T₆ = 210 °C

The essential parameters of the process, called here S1, were:

• T ₁ = 550 ° C, T ₂ = 520 ° C, T ₄ = 267 ° C, T ₅ = 267 ° C, T ₆ = 210 ° C

The temperature T _s was 603 ° C. (value obtained by numerical calculation). The final thickness of the strip was 6 mm, its width 2400 mm.

It can be seen that the final product shows no recrystallization. In the L / TC plane, a fibered microstructure is observed at mid-thickness, with a grain thickness of the order of 10 μm.

Representative sheets, delivered at full width in the middle of the coil, showed at mid-width the mechanical characteristics indicated in Table 15: Table 15 R _{p0.2 (L)}
[MPa] R _{m (L)}
[MPa] A% _(L)
[%] R _{p0.2 (TL)}
[MPa] R _{m (TL)}
[MPa] A% _(TL)
[%] 275 236 15.9 279 249 16.4

Corrosion resistance, evaluated by the EXCO test, was EA at the surface and at mid-thickness. The corrosion resistance, evaluated by the SWAAT test, was P at the surface and at mid-thickness, and the mass loss was 0.52 g / dm ² at the surface and 0.17 g / dm ² at mid-thickness.

Example 9

According to the process of the present invention, an alloy whose composition is indicated in Table 16 was prepared. Table 16 Alloy mg Zn mn Yes Fe Cu Zr Ti Cr U 1.23 5.07 0.19 0.05 0.12 0.07 0.10 0.03 0,002

Four coils (width 2415 mm) were prepared with different processing conditions. In addition, a coil of composition S (here called S2) according to Example 8 has been transformed (width 1500 mm).

The essential parameters of the process were (all temperatures in ° C): Table 17 coil T ₁ T ₂ T ₃ T ₄ T ₅ T ₆ U1 550 528 435 277 277 240 U2 550 508 445 256 256 220 U3 550 517 405 289 289 200 U4 550 499 430 264 264 200 S2 550 535 460 272 272 155

The temperature T _S for the alloy U was 600 ° C. (value obtained by numerical calculation). The thickness of the strips U3 and U4 was 6 mm, that of the strips U1, U2 and S2 8 mm.

Representative sheets, cut at full width in the middle of the coil, showed at mid-width the mechanical characteristics indicated in Table 18: Table 18 coil R _{p0.2 (L)}
[MPa] R _{m (L)}
[MPa] A% _(L)
[%] U1 298 265 13.5 U2 358 335 11.4 U3 317 294 13.2 U4 352 334 13.4 S2 332 307 11.9

Example 10

The microstructure and the abrasion resistance of different sheets obtained by the process according to the invention (reference 7108 F7) and according to the state of the art (marks 5086 H24, 5186 H24, 5383 H34, 7020 T6, 7075 T6 and 7108 T6). Table 19 gathers results concerning the mechanical characteristics and the microstructure of these sheets. Table 19 landmark R _{p0.2 (L)} R _{m (L)} A% _(L) Hardness Average grain length
[.Mu.m] [MPa] [MPa] [%] (H V) TC sense Meaning L TL direction 5086 H24 254 327 17 92 10 300 150 5186 H24 270 335 17 94 19 200 110 5383 H34 279 374 18 105 8 190 165 7020 T6 335 371 15 132 33 200 220 7075 T6 541 607 11 191 24 220 155 7108 T6 360 395 17.5 125 100 390 320 7108 F7 305 344 14.5 112 8 500 290

The material 7108 T6 had the composition of the alloy B of Example 2, and was close to the BCH material. The material 7108 F7 has the same composition B of Example 2.

The abrasion resistance has been characterized using an original device that reproduces the conditions such as they may occur for example when loading, transporting and unloading sand in a bucket. This test consists in measuring the loss of mass of a sample subjected to a vertical movement back and forth in a tank filled with sand. The diameter of the tank is about 30 cm, the height of the sand about 30 cm. The sample holder is fixed on a vertical rod connected to a double-acting jack which ensures the vertical movement of the rod back and forth. The sample holder is in the form of a pyramid with a 45 ° angle. This is the tip of the pyramid that plunges into the sand. The samples to be tested, of dimension 15 x 10 x 5 mm, are embedded in the faces of the pyramid so that their surface is tangent to that of the corresponding face of the pyramid; it is the face corresponding to the L-TL plane (dimension 15 x 10 mm) which is exposed to the sand. The depth of penetration of the sample into the sand was 200 mm.

The same procedure was used for all samples. It involves the acetone degreasing of the sample, the filling of the tank with the same amount of the same standardized sand (sand according to NF EN 196-1), the stopping of the machine every 1000 cycles and replacement of the used sand by new sand, weighing samples every 2000 cycles (preceded by cleaning with acetone and compressed air), stopping the test after 10,000 cycles. The results are given in Table 20: Table 20 landmark Face tested Loss of mass [g] at 10,000 cycles 5086 H24 Brute 0.198 5186 H24 Brute 0.233 5383 H34 Brute 0,193 7020 T6 Brute 0.252 7075 T6 Brute 0,225 7108 T6 machined 0.199 7108 F7 machined 0,175

The weight loss values given are the average of three tests; the confidence interval is in the range of ± 0.01 to 0.02 g; this underlines the good repeatability of this test.

Table 19 shows the very particular microstructure of the product obtained by the process according to the present invention, by comparing the two alloy products 7108, one (reference T6) obtained according to a known method, the other (reference F7) according to the process which is the subject of the present invention. Table 20 shows the effect of this microstructure on abrasion resistance. It is immediately apparent that the product according to the invention is more resistant to abrasion than the standard product 5086 H24. This highlights its good suitability for use in industrial vehicles, as well as storage and handling equipment for granular products, such as skips, tanks, or conveyors.

Claims (27)

1. Process for generating an intermediate laminated product in an aluminium alloy of the Al-Zn-Mg type, including the following steps:

   a) by semi-continuous casting a plate is generated containing (in percentages per unit mass)
   Mg 0.5 - 2.0 Mn < 1.0 Zn 3.0 - 9.0 Si < 0.50
   Fe< 0.50 Cu < 0.50 Ti < 0.15 Zr < 0.20
   Cr < 0.50
   the remainder of the aluminium with its inevitable impurities, in which Zn/Mg > 1.7;

   b) said plate is subjected to homogenisation or reheating to a temperature T₁, selected so that 500°C ≤ T₁ ≤ (T_s - 20°C), where T_s is the alloy incipient melting temperature,

   c) an initial hot-rolling step is carried out including one or more roll runs on a hot rolling mill, the input temperature T₂ being selected such that (T₁ - 60°C) ≤ T₂ ≤ (T₁ - 5°C), and the rolling process being conducted in such a way that the output temperature T₃ is such that (T₁ - 150°C) ≤ T₃ ≤ (T₁ - 30°C) and T₃ ≤ T₂;

   d) the strip emerging from said initial hot-rolling step is cooled to a temperature T₄;

   e) a second step of hot-rolling said strip is carried out, the input temperature T₅ being selected such that T₅ ≤ T₄ and 200°C ≤ T₅ ≤ 300°C, and the rolling process being conducted in such a way that the coiling temperature T₆ is such that (T₅ - 150°C) ≤ T₆ ≤ (T₅ - 20°C).
2. Process according to claim 1, characterised in that the zinc content of the alloy is between 4.0 and 6.0%, the Mg content is between 0.7 and 1.5%, and the Mn content is less than 0.60%.
3. Process according to claim 2, characterised in that Cu < 0.25%.
4. Process according to claim 2, characterised in that the alloy is chosen from the group formed by the alloys 7020, 7108, 7003, 7004, 7005, 7008, 7011, 7022.
5. Process according to any one of claims 1 to 3, characterised in that the alloy additionally contains one or more elements chosen from the group formed by Sc, Y, La, Dy, Ho, Er, Tm, Lu, Hf, Yb with a concentration not exceeding the following values:

   Sc < 0.50% and preferably < 0.20%,

   Y < 0.34% and preferably < 0.17%,

   La, Dy, Ho, Er, Tm, Lu < 0.10% each and preferably < 0.05% each,

   Hf < 1.20% and preferably < 0.50%,

   Yb < 0.50% and preferably < 0.25%.
6. Process according to any one of claims 1 to 5, characterised in that said intermediate laminated product has a thickness between 3 mm and 12 mm.
7. Process according to any one of claims 1 to 6, characterised in that said intermediate laminated product is subjected to cold working between 1% and 9%, and/or to an additional heat treatment including one or more points at temperatures between 80°C and 250°C, said additional heat treatment being able to occur before, after or during said cold working.
8. Process according to any one of claims 1 to 7, characterised in that the temperature T₃ is such that (T₁ - 100°C) ≤ T₃ ≤ (T₁ - 30°C) and/or in that the temperature T₂ is such that (T₁ - 30°C) ≤ T₂ ≤ (T₁ - 5°C).
9. Process according to any one of claims 1 to 8, characterised in that the temperature T₃ is greater than the solvus temperature of the alloy.
10. Process according to any one of claims 1 to 9, characterised in that the alloy is 7108 alloy and the temperatures T₁ to T₆ are respectively T₁ = 550°C, T₂ = 540°C, T₃ = 490°C, T₄ = 270°C, T₅ = 270°C, T₆ = 150°C.
11. Sheet or plate having a thickness comprised between 3 mm and 12 mm which can be obtained via the process according to any one of claims 1 to 10, characterised in that its yield strength R_p0,2 is at least 250 Mpa, its fracture strength R_m is at least 280 MPa, and its elongation at fracture is at least 8%, in that its zinc content is comprised between 4.0 and 6.0%, its Mg content is comprised between 0.7 and 1.5%, its Mn content is less than 0.60% (and preferentially less than 0.25%), its copper content is less than 0.25%, in that grain boundary precipitates of the type MgZn₂ have an average size higher than 150 nm, and preferentially comprised between 200 nm and 400 nm, and in that it has a fibred structure characterized by a ratio length / thickness of the grains of more than 60, and preferentially of more than 100 with grains having in the short-transverse direction a thickness of less than 30 µm, preferentially of less than 15 µm, and even more preferentially of less than 10 µm.
12. Sheet or plate according to claim 11, characterised in that its yield strength R_p0,2 is at least 290 MPa and that its fracture strength R_m is at least 330 MPa.
13. Sheet or plate according to any one of claims 11 or 12, characterised in that the width of the precipitation-free zones at the grain boundaries is more than 100 nm, preferably between 100 nm and 150 nm, and even more preferably from 120 nm to 140 nm.
14. Use of a sheet or plate according to any one of claims 11 to 13 to manufacture welded constructions.
15. Use of a sheet or plate according to any one of claims 11 to 13 to build road or rail tankers.
16. Use of a sheet or plate according to any one of claims 11 to 13 to build industrial vehicles.
17. Use of a laminated product according to any one of claims 11 to 13 to build equipment for storage, transport or handling of granulous products, such as buckets, tanks or conveyors.
18. Use of a sheet or plate according to any one of claims 11 to 13 to manufacture motor vehicle parts.
19. Use of a sheet or plate according to any one of claims 11 to 13 as a structural component in aeronautical construction.
20. Use according to claim 19, wherein said structural component is a fuselage facing sheet.
21. Use according to any one of claims 14 to 20, wherein at least two of said structural components are assembled by welding.
22. Welded construction made with at least two sheets or plates according to any one of claims 11 to 13, characterised in that its yield strength R_p0,2 in the welded joint between two of said products is at least 200 MPa.
23. Welded construction according to claim 22, wherein the yield strength R_p0,2 in the welded joint between two of said products is at least 220 MPa.
24. Welded construction made with at least two sheets or plates according to any one of claims 11 to 13, characterised in that its fracture strength R_m in the welded joint between two of said products is at least 250 MPa.
25. Welded construction according to claim 24, wherein the fracture strength R_m in the welded joint between two of said products is at least 300 MPa.
26. Welded construction according to any one of claims 22 to 25, wherein the hardness in the heat-affected zone is greater than or equal to 100 HV, preferably greater than or equal to 110 HV, and even more preferably greater than or equal to 115 HV.
27. Welded construction according to claim 26, wherein the hardness in the heat-affected zone is at least as high as the hardness of that of the base sheet with the lowest hardness.

Images