EP1178125A2 - AlMg alloy with improved mechanical properties for welded constructions - Google Patents

Abstract

An Al-Mg alloy sheet used for the production of welded structures of thickness above 2 mm and manufactured by semi-continuous casting has breaking strength Rm greater than 275 MPa, elongation A above 17.5, and product A x Rm above 6500. Preferred Features: The Al-Mg alloy comprises (in weight %): Mg 4.2-4.8; Mn less than 0.5; Zn less than 0.4, preferably 0.07-0.2; Fe 0.20-0.45; Si less than 0.3; optionally, Cr less than 0.15, Cu less than 0.25, Ti less than 0.2, and other elements less than 0.05 each and less than 0.15 in total; and Al the remainder. Mn+Zn is less than 0.7, preferably less than 0.2; and %Fe is greater than 0.5%Mn. The volumetric fractions of eutectic precipitates and dispersoids are above 0.7% and less than 1.5 times the fraction of eutectics, respectively. The sheet is fabricated without final annealing and the fabrication process includes hot rolling at a rolling exit temperature of 300-370 degrees C, preferably 320-360 degrees C. The width of the sheet is above 2200 mm. Independent claims are given for use of the Al-Mg alloy sheet for the fabrication of road and rail tanks, for road, rail and/or marine transport containers, for welded and/or forged car and lorry wheels, and for welded structures obtained by MIG or TIG welding with a bevel of around 45 degrees over about two-thirds of the thickness.

Description

Technical area

The invention relates to the technical field of alloy sheets aluminum of the AlMg type and more particularly an alloy 5083 or 5086 according to standard EN 573-3, intended for welded constructions such as fixed tanks or mobile and in particular road transport tanks or railways of solid or liquid materials.

Problem

To increase the mechanical resistance of constructions welded while reducing their weight, it is interesting to have alloys offering compared to alloys 5083 or 5086 currently used, mechanical characteristics improved without losing anything on the other properties of use such 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 proper plastic behavior of the aluminum alloy structures, are the elongation at break A and the resistance to rupture R _m . For AlMg alloys, these two characteristics have opposite trends 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 R _m is generally used for these alloys, since A and R _m each have minimum values suitable.

The aim of the present invention is therefore to improve this trade-off between elongation and breaking strength while ensuring satisfactory corrosion resistance and manufacturing range as simple and reliable as possible.

Prior art

Japanese patent application JP 06-212373 shows examples of AlMgMn alloy sheets presenting a good compromise between elongation and strength but manufacturing by hot rolling requires minimum exit temperature of the rolling mill at 450 ° C., which requires a rate of fast manufacturing and minimal lubrication and does not allow therefore not a reliable and economical manufacture of bands.

Japanese patent application JP 06-93365 also shows AlMgMn alloy sheets with mechanical characteristics meeting the intended objective, but their manufacture intervene a complicated and expensive range, comprising a hot rolling followed by intermediate annealing, lukewarm rolling and final annealing.

Subject of the invention

The applicant has highlighted an area of composition narrow inside the composition ranges of alloys 5083 and 5086 to meet the objectives referred for mechanical characteristics and to use a range of reliable and economical manufacturing.

Mg: 4,2 - 4,8 Mn: < 0,5 Zn: < 0,4

Fe < 0,45 Si < 0,30 et éventuellement

Cr: < 0,15 Cu < 0,25 Ti < 0,20 Zr < 0,20

autres éléments < 0,05 chacun et 0,20 au total, balance Al,

avec la relation Mn + Zn < 0,7 (de préférence < 0,6),

et présentent une résistance à la rupture Rm > 275 MPa, un allongement A > 17,5% et un produit R_m x A > 6500 (R_m exprimé en MPa et A en %).

The sheets for welded constructions according to the invention are made of an AlMg alloy of the following composition (% by weight):

Mg: 4.2 - 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 total, Al balance,

with the relation Mn + Zn <0.7 (preferably <0.6),

and have a tensile strength Rm> 275 MPa, an elongation A> 17.5% and a product R _m x A> 6500 (R _m expressed in MPa and A in%).

The zinc content is preferably between 0.07 and 0.2%. The iron content is preferably between 0.20 and 0.45% and more than half the manganese content.

The sheets according to the invention are produced by semi-continuous casting preferably without final annealing and by rolling to hot with an exit temperature from the rolling mill between 300 and 370 ° C, and preferably between 320 and 360 ° C.

Mg: 4,2 - 4,7 Mn: 0,20 - 0,40 Zn: 0,07 - 0,20

Fe: 0,20 - 0,45 Si < 0,25 Cr < 0,15 Cu < 0,15

Ti < 0,10 Zr < 0,10 autres éléments < 0,05 chacun et < 0,15 au total, balance Al.

A particularly advantageous composition of the alloy according to the invention, leading to sheets having a breaking strength R _m > 275 MPa, an elongation A> 22% and a product A x R _m > 7000, is as follows:

Mg: 4.2 - 4.7 Mn: 0.20 - 0.40 Zn: 0.07 - 0.20

Fe: 0.20 - 0.45 Si <0.25 Cr <0.15 Cu <0.15

Ti <0.10 Zr <0.10 other elements <0.05 each and <0.15 in total, Al balance.

Description of the invention

The role of magnesium and manganese as elements of addition is well known. Magnesium provides good resistance mechanical, but too high a content decreases the resistance to corrosion, which would limit the use of tanks made with such alloys.

Manganese improves tensile strength, but a too high 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 R _m depended on the sum Mn + Zn rather than individual contents of Mn and Zn, and that this product was markedly improved when the sum Mn + Zn was less than 0.7, and preferably 0.6%.

In the composition range chosen 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 resistance to corrosion, and an addition of copper of less than 0.25% leads to an increase in R _m .

The iron content must be below 0.45% in order to to avoid the formation of primary phases whose presence results in an unacceptable deterioration of the characteristics sheet metal mechanics. However, in the area of composition used for the elements Mg, Mn and Zn, the Applicant has surprisingly highlighted that it is advantageous to choose an iron content close to 0.45%, because during casting, almost all of the iron forms AlMnFe-type eutectic precipitates. We see, which is contrary to what we usually observe, that a strong fraction of these eutectic precipitates improves ductility of sheet metal and it is desirable that this fraction be at least 0.7%. At the same time, always to have a high ductility, the fraction of manganese dispersoids in the final sheet must 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 the surface fractions calculated on micrographs by techniques well known metallography, for example by microscopy scanning electronics and image analysis on a section polished from a sheet sample.

This possibility of choosing an iron content that is not too low allows to choose a less pure base metal 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 breaking strength R _m > 275 MPa, an elongation A> 17.5% and a product A x R _m > 6500, by rolling without final annealing at a temperature> 250 ° C, and, more particularly, by hot rolling and in large width, for example> 2200 mm. For reasons of industrial reliability, it is preferable that the temperature at the outlet of the hot rolling mill is less than 400 ° C., and preferably at 370 ° C. or even at 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, but also road and rail transport containers and / or maritime, as well as welded and / or forged wheels for automobile or truck. These sheets can be welded by all the means usually used for this type of alloy, in particular by butt welding using a MIG process or TIG and with a chamfer of the order of 45 ° on approximately the 2/3 of the thickness. It is beneficial for all of these applications, to be able to have very wide sheets, in particular of width greater than 2200 mm.

The advantage of having sheets with mechanical characteristics improved is especially great in the case of tanks Roads intended for the transport of dangerous goods, which must exhibit proper plastic behavior in the event accident.

Examples

Alloys having the compositions indicated in Table 1, by semi-continuous casting of plates. After a reheating for 20 h at a temperature above 500 ° C, these have been hot rolled to a thickness final 6 mm. The exit temperature of the rolling mill was 340 ° C.

Alloys 0 to 4, 10 and 13 have a composition outside invention (alloy 0 representing a composition of 5083), the other alloys a composition according to the invention.

The tensile strength R _m and the elongation A were measured on these sheets. On surface micrographs, 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 R _m > 275 MPa, A> 17.5% and their product> 6500.

Mg: 4,2 - 4,7 Mn: 0,20 - 0,40 Zn: 0,07 - 0,20

Fe: 0,20 - 0,45 Si < 0,25 Cr < 0,15 Cu < 0,15

Ti < 0,10 Zr < 0,10

on obtient des tôles présentant un produit A x R_m toujours supérieur à 7000 et, le plus souvent, supérieur à 7400 (cf. références 14, 18 à 23).

It is further noted that with the narrower composition mentioned above, namely:

Mg: 4.2 - 4.7 Mn: 0.20 - 0.40 Zn: 0.07 - 0.20

Fe: 0.20 - 0.45 Si <0.25 Cr <0.15 Cu <0.15

Ti <0.10 Zr <0.10

sheets are obtained having a product A x R _m always greater than 7000 and, more often than not, 7400 (cf. references 14, 18 to 23).

MIG welding tests carried out by MIG butt welding with a 45 ° chamfer on 2/3 of the thickness have shown a weldability similar to that of sheets of alloys 5083 and 5086 of usual composition.

Claims (16)

Sheet for welded construction in AlMg alloy thickness> 2 mm, produced by semi-continuous casting, with a breaking strength R _m > 275 MPa, an elongation A> 17.5% and a product A x R _m > 6500 , of composition (% by weight): Mg: 4.2 - 4.8 Mn: <0.5 Zn: <0.4 0.20 <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 <0.15 in total, balance Al, with the relation: Mn + Zn <0.7. Sheet according to claim 1 wherein the alloy has a composition such that Mn + Zn <0.6. Sheet according to one of claims 1 and 2 in which the alloy has a composition such as Zn: 0.07 - 0.2 Sheet according to one of claims 1 to 3 in which the alloy has a composition such as 0.20 <Fe <0.45. Sheet metal for welded construction according to claim 4 having a breaking strength R _m > 275 MPa, an elongation> 22% and a product A x R _m > 7000, and preferably> 7400, of composition (% by weight): Mg: 4.2 - 4.7 Mn: 0.20 - 0.40 Zn: 0.07 - 0.20 Fe: 0.20 - 0.45 Si <0.25 Cr <0.15 Cu <0.15 Ti <0.10 Zr <0.10 other elements <0.05 each and 0.15 in total, balance Al. Sheet according to one of claims 1 to 5 in which % Fe> 0.5% Mn. Sheet according to one of claims 1 to 6 for which the volume fraction of eutectic precipitates is greater than 0.7% Sheet according to one of claims 1 to 7 such that the volume fraction of dispersoids is less than 1.5 times the fraction of eutectics. Sheet according to any one of Claims 1 to 8, characterized in that the sheet is produced without final annealing. Sheet according to Claim 9, characterized in that the sheet is produced by hot rolling with an exit temperature from the rolling mill of between 300 and 370 ° C. Sheet according to claim 10, characterized in that the exit temperature from the hot rolling mill is between 320 ° C and 360 ° C. Sheet according to one of claims 10 or 11, characterized in that the width of the sheet is greater than 2200 mm. Use of a sheet according to any one of the Claims 1 to 12 for the manufacture of tanks road or rail. Use of sheets according to any one of Claims 1 to 12 for the manufacture of containers road, rail and / or sea transport. Use of sheets according to any one of Claims 1 to 12 for the manufacture of welded and / or forged car or truck. Use of sheets according to any one of Claims 1 to 12 for constructions welded by MIG or TIG process with a chamfer of the order of 45 ° about 2/3 of the thickness.