FR2740144A1 - ALMG ALLOY FOR WELDED CONSTRUCTIONS WITH IMPROVED MECHANICAL CHARACTERISTICS - Google Patents

Abstract

The invention relates to sheets for welded constructions of Almg alloy having a tensile strength Rm> 275 MPa, an elongation A> 17.5% and a product A x Rm> 6000 of composition (% by weight): Mg: 4 , 3 - 4.8 Mn: <0.5 Zn <0.4 Fe <0.45 Si <0.30 and optionally: Cr <0.15 Cu <0.25 Ti <0.20 Zr <0.20 other elements <0.05 each and <0.15 in total, with the relationships: Mn + Zn <0.7 and Fe> 0.5Mn. The sheets according to the invention are intended in particular for the manufacture of rail or road tanks.

Description

AlMg ALLOY FOR WELDED CONSTRUCTIONS

IMPROVED MECHANICAL CHARACTERISTICS

Technical field The invention relates to the technical field of aluminum alloy sheets of the AlMg type and more particularly of 5083 or 5086 alloy according to standard EN 573-3, intended for welded constructions such as fixed tanks or

mobile and in particular road or rail transport tanks for solid or liquid materials.

Problem meposed To increase the mechanical resistance of welded constructions while reducing their weight, it is advantageous to have available alloys which, compared to the 5083 or 20 5086 alloys currently used, have improved mechanical characteristics without losing anything in other use properties such as as weldability, corrosion resistance or formability. The two mechanical characteristics which, according to the principles of mechanical construction known to those skilled in the art, must be optimized to ensure a suitable plastic behavior of aluminum alloy structures, are the elongation at break A and the tensile strength. Rm rupture. For AlMg alloys, these two characteristics have opposite evolutionary tendencies when the composition of the alloy is modified and a compromise must be found for

each type of application. This is why, to calculate the behavior of structures under rapid plastic deformation, for example in the event of damage, the product A x Rm is generally used for these alloys, provided that A and Rm each have suitable minimum values.

The aim of the present invention is therefore to improve this compromise between elongation and tensile strength.

while ensuring satisfactory corrosion resistance and a manufacturing range as simple and reliable as possible.

PRIOR ART Japanese patent application JP 06-212373 shows examples of AlMgMn alloy sheets exhibiting a good compromise between elongation and strength, but the manufacture by hot rolling requires a minimum temperature at the exit of the rolling mill of 450 ° C. which requires a rapid production rate and minimal lubrication and therefore does not allow reliable and economical production of bands. Japanese patent application JP 06-93365 also shows AlMgMn alloy sheets having mechanical characteristics meeting the intended objective, but their manufacture involves a complicated and expensive range, comprising hot rolling followed by intermediate annealing, lukewarm rolling and final annealing. OBJECT OF THE INVENTION The Applicant has demonstrated a narrow range of composition within the composition ranges of alloys 5083 and 5086 making it possible to meet the objectives sought for the mechanical characteristics and to use a reliable manufacturing range and economic. The sheets for welded constructions according to the invention are made of AlMg alloy of the following composition (% by weight): Mg: 4.3 - 4.8 Mn: <0.5 Zn: <0.4 Fe <0.45 Si <0.30 and possibly Cr: <0.15 Cu <0.25 Ti <0, 20 Zr <0.20 other elements <0.05 each and 0.20 in total35 with the relations Mn + Zn <0.7 (preferably <0.6) and Fe> 0.5Mn, and have a tensile strength Rm> 275 MPa, an elongation A> 17.5% and a product Rm x A> 6000 and preferably> 6500. The zinc content is preferably between 0.07 and 0.2%. The sheets according to the invention are preferably manufactured without final annealing and by hot rolling with an outlet temperature of the rolling mill of between 300 and 3707C, and of

preferably between 320 and 360 C.

Description of the invention

The role of magnesium and manganese as addition elements is well known. Magnesium provides good mechanical strength, but too high a content reduces corrosion resistance, which would limit the use of tanks manufactured with such alloys. Manganese improves tensile strength, but too high a content leads to a decrease in elongation. It is also known that zinc, in the presence of manganese, improves the breaking strength, but the Applicant has found, surprisingly, that, for the chosen contents of magnesium and manganese, the product A x Rm depended on the sum Mn + Zn rather than individual Mn and Zn contents, and that this product was markedly improved when the sum of Mn + Zn was less than 0.7, and preferably 0.6%. In the range of composition adopted for Mg, Mn and Zn, an addition of chromium, provided that it does not exceed 0.15%, makes it possible to increase both the elongation A and the corrosion resistance, and an addition of copper less than 0.25% leads to an increase in Rm. The iron content must be below 0.45% in order to avoid the formation of primary phases, the presence of which leads to an unacceptable deterioration of the mechanical characteristics sheet metal. However, in the field of composition chosen for the elements Mg, Mn and Zn, the Applicant has surprisingly demonstrated that it is advantageous to choose an iron content close to 0.45%, because during casting, almost all of the iron forms eutectic precipitates of the AlMnFe type. It is found, which is contrary to what is usually observed, that a large fraction of these eutectic precipitates improves the ductility of the sheet and that it is desirable that this fraction be at least 0.7%. At the same time, still to have high ductility, the fraction of manganese dispersoids in the final sheet should remain low, preferably below 1.5 times the fraction of eutectics, which is expressed by the relation Fe > 0.5Mn. The volume fractions of eutectic precipitates and

dispersoids are measured by area fractions calculated on micrographs by well known metallographic techniques, for example by scanning electron microscopy and image analysis on a polished section of a sheet sample.

This possibility of choosing an iron content that is not too low makes it possible to choose a base metal which is less pure and therefore less expensive, while having good mechanical characteristics. With the composition according to the invention, it is possible to obtain sheets. of thickness> 2 mm having a tensile strength Rm> 275 MPa, an elongation A> 17.5% and a product A x Rm> 6000, by rolling without final annealing at a temperature> 250 C, and, more particularly , by hot rolling. For reasons of industrial reliability, it is preferable for the temperature at the outlet of the hot rolling mill to be less than 4000 ° C., and, preferably, 3700 ° C., or even 350 C. The sheets according to the invention can be used for welded constructions. , such as fixed tanks or

mobile, for example rail or road tanks.

These sheets can be welded by all the means usually used for this type of alloy, in particular by butt welding using a MIG or TIG process and

with a chamfer of the order of 45 over approximately 2/3 of the thickness.

The advantage of having sheets with improved mechanical characteristics is particularly great in the case of road tanks intended for the transport of dangerous materials, which must exhibit suitable plastic behavior in the event of an accident. indicated in Table 1, by semi-continuous casting of plates. After reheating for 20 h at a temperature above 500 ° C, these were hot rolled to a final thickness of 6 mm. The outlet temperature of the rolling mill was 340 C. Alloys 0 to 4 have a composition outside the invention.

(alloy 0 representing a composition of 5083), alloys 5 to 17 a composition according to the invention.

The tensile strength Rm and the elongation A were measured on these sheets. On micrographs produced by optical microscopy, the surface fractions of eutectic precipitates and dispersoids were also measured. These results are collated in Table 1 and show that, for the compositions according to the invention, there is always Rm> 275 MPa, A> 17.5% and their product> 6000. MIG welding tests carried out by butt welding MIG25 with a chamfer of 45 over 2/3 of the thickness showed weldability similar to that of 5083 alloy sheets

and 5086 of usual composition.

TABLE 1

Ref. Mg Cu Mn Fe Si Cr Zn TI Fraction Fraction Rn A RnA%% %%%%%% Eutecti- Disper- Mpa% que soides

0 4.5 0.02 0.65 0.21 0.2 0.03 0.08 0.01 0.78 0.86 298 17 5066

1 4.52 <0.01 0.72 0.12 0.16 0.05 <0.01 0.01 0.63 1.1 296 17 5032

2 4.55 <0.01 0.83 0.12 0.18 0.13 0.01 0.01 0.74 1.4 310 15 4650

3 4.6 <0.01 0.85 0.17 0.25 0.1 0.16 0.01 0.95 1.4 313 16,5008

4 4.5 0.05 0.28 0.16 0.18 0.02 0.02 0.01 0.34 0.29 266 24 6384

4.5 <0.01 0.46 0.1 0.21 0.05 0.02 0.01 0.37 0.65 283 22 6226

6 4.7 <0.01 0.47 0.13 0.21 0.03 0.02 0.01 0.44 0.66 295 22 6490

7 4.7 0.06 0.54 0.08 0.18 0.02 0.01 0.01 0.33 0.82 305 20 6100

8 4.5 <0.01 0.5 0.11 0.22 0.3 0.02 0.01 0.42 0.72 295 22 6490

9 4.6 <0.01 0.42 0.15 0.2 0.05 0.16 0.01 0.46 0.56 298 22 6556

4.6 <0.01 0.45 0.21 0.17 0.07 0.3 0.01 0.54 0.61 295 21 6195

Il 4.5 <0.01 0.55 0.22 0.23 0.02 0.01 0.01 0.77 0.77 284 23 6532

12 4.5 0.02 0.3 0.2 0.2 0.03 0.08 0.01 0.44 0.29 277 24 6648

13 4.38 <0.01 0.5 0.27 0.2 0.09 0.01 0.01 0.83 0.67 278 24 6672

14 4.5 0.02 0.31 0.45 0.2 0.03 0.08 0.01 0.85 0.31 277 27 7479

4.5 0.02 0.58 0.45 0.19 0.0 0.08 0.01 1.41 0.72 296 22 6512

16 4.3 0.18 0.4 0.37 0.22 0.03 0.12 0.01 0.89 0.52 285 25 7125

17 4.8 0.1 0.58 0.43 0.2 0.03 0.08 0.01 1.4 0.78 295 23 6785

Claims (9)

1) Sheet for welded construction in AlMg alloy having a tensile strength Rm> 275 MPa, an elongation A> 17.5% and a product A x Rm> 6000 and, preferably> 6500, of composition (% by weight) : Mg: 4.3 4.8 Mn: <0.5 Zn: <0.4 Fe <0.45 Si <0.30 and optionally: Cr <0.15 Cu <0.25 Ti <0.20 Zr <0.20 other elements <0.05 each and <0.15 in total with the relationships: Mn + Zn <0.7 and Fe> 0.5Mn 2) Sheet according to claim 1 wherein Mn + Zn <0.6. 3) Sheet according to one of claims 1 and 2 wherein Zn: 0.07 - 0.2 4) Sheet according to one of claims 1 to 3 for which the volume fraction of the eutectic precipitates is greater than 0.7% 5) Sheet according to one of claims 1 to 4 such that volume fraction of dispersoids is less than 1.5 times the fraction of eutectics. 6) Sheet according to any one of claims 1 to 5, characterized in that it is manufactured without final annealing. 7) Sheet according to claim 6, characterized in that it is manufactured by hot rolling with a temperature output of the rolling mill between 300 and 370 C. 8) Sheet according to claim 7, characterized in that the * - 2740144 hot rolling mill outlet temperature is included between 320 C and 3600C. 9) Use of sheets according to any one of the preceding claims for the manufacture of road or rail tanks.