EP1481106B1 - Al-mg alloy sheet or strip for the production of bent parts having a small bend radius - Google Patents

Abstract

The invention relates to an aluminium alloy sheet or strip with a thickness of between 1 and 5 mm which is intended for the production of stamped and bent parts having a small bend radius, the composition ( % by weight) of said alloy comprising: Si<0.4, Fe<0.4, Cu<0.4, Mn + Cr:0.3-0.7, Mg:4-5.5, Zn<1, other elements <0.05 each and <0.15 in total, and the remainder aluminium. Moreover, the inventive sheet or strip presents the following properties: an elastic limit R0.2 in direction T>215 MPa, elongation A80>15 % and a difference Rm - R0,2 >80 MPa. Said sheets and strips are used, for example, for the production of parts for motor vehicles, such as reinforcements for openable panels or jacks.

Description

Field of the invention

The invention relates to the manufacture of folded parts with a small bending radius, and most often stamped, intended in particular for the automotive industry, from aluminum alloy sheet or strip of the aluminum-magnesium type, that is to say of the 5000 series according to the nomenclature of the Aluminum Association.

State of the art

Aluminum-magnesium alloys with more than 4% magnesium are widely used in automobile construction for parts other than body skins, for example reinforcements or structural parts, possibly shaped by stamping or bending. They allow a good mechanical resistance without requiring, like the alloys of skin of the series 6000, heat treatment of putting in solution and quenching. Mention may be made, for example, of the alloys 5019, 5182 and 5083, whose composition (% by weight) recorded at the Aluminum Association is shown in Table 1: Table 1 Alloy Yes Fe Cu mn mg Cr Zn 5019 <0.40 <0.50 <0.10 0.1-0.6 4.5-5.6 <0.20 <0.20 5182 <0.20 <0.35 <0.15 0.2-0.5 4.0-5.0 <0.10 <0.25 5083 <0.40 <0.40 <0.10 0.4-1.0 4.0 to 4.9 0.05-0.25 <0.25

Problem

The manufacture of stamped and folded parts requires a material having sufficient formability to make the stamped parts of the parts, and a folding ability all the better that it is desired to obtain very small bending radii, typically of the order of thickness of the sheet. This aptitude must be as good in the direction of rolling as in the perpendicular direction. The sheet or strip must have as high a mechanical strength as possible so as to minimize the thickness, and thus obtain the optimal lightening effect resulting from the use of aluminum with respect to steel.
In addition, the motor vehicle parts are subjected to a heat treatment during the baking of the paints of the bodywork, which is done at a temperature between 150 and 200 ° C for 15 to 30 minutes. It is therefore necessary to take into account the possible loss of mechanical strength during this operation, and it is desirable that this degradation be as low as possible. The aim of the invention is Al-Mg alloy sheets and strips making it possible to satisfy these requirements.

Object of the invention

• Si < 0,3 Fe : 0,2 - 0,4 Mn : 0,3 - 0,45 Mg : 4,5 - 5,5 Cr < 0,04 - 0,1 Cu < 0,1 Zn < 0,1 autres éléments < 0,05 chacun et < 0,15 au total, reste aluminium, et, pour cette composition particulière, R_0,2 sens T > 240 MPa, A₈₀ > 15% et R_m - R_0,2 > 90 MPa.

Elle a également pour objet un procédé de fabrication d'une telle tôle ou bande, comportant la coulée d'une plaque de la composition ci-dessus, son laminage à chaud jusqu'à une épaisseur e_c, le laminage à froid jusqu'à une épaisseur finale e_f comprise entre 70 et 40% de e_c, et un recuit de restauration à une température comprise entre 180 et 280°C sans écrouissage ultérieur.The subject of the invention is an aluminum alloy sheet or strip having a thickness of between 1 and 5 mm, intended for the manufacture of stamped and folded parts with a small bending radius, of composition (% by weight):

• If <0.3 Fe: 0.2 - 0.4 Mn: 0.3 - 0.45 Mg: 4.5 - 5.5 Cr <0.04 - 0.1 Cu <0.1 Zn <0, 1 other elements <0.05 each and <0.15 in total, remains aluminum, and for this particular composition, R _0.2 direction T> 240 MPa, A ₈₀ > 15% and R _m - R _0.2 > 90 MPa.

It also relates to a method of manufacturing such a sheet or strip, comprising the casting of a plate of the above composition, its hot rolling to a thickness e _c, the cold rolling to a final thickness e _f between 70 and 40% of e _c, and a recovery annealing at a temperature between 180 and 280 ° C without subsequent strain hardening.

Description of figures

The single figure illustrates the results of Example 1 in yield strength R _0.2 and radius of bending.

Description of the invention

The invention is based on the combination of the narrow selection of an Al-Mg alloy composition with more than 4% Mg, and a particular manufacturing range to obtain a compromise of properties, in particular between the limit of elasticity, elongation and folding ability, particularly favorable for producing stamped and folded parts with a small bending radius.

The alloys according to the invention are alloys with more than 4.5% Mg as the alloys 5182, 5019 or 5083 mentioned above.

Magnesium contributes to the mechanical strength and its content can be adjusted as a function of the desired mechanical strength. Beyond 5.5% Mg, the alloy becomes more difficult to cast and to implement.

The control of the total content of manganese and chromium is an important point to obtain all the desired properties. A content of less than 0.3% improves the elongation, but decreases the yield strength without improving the folding ability. A content greater than 0.7% improves the yield strength without greatly reducing elongation, but surprisingly gives a wrong bend radius.

The method of manufacturing the strips according to the invention comprises casting a plate of the alloy in question, its hot rolling to obtain a strip of thickness e _c , then its cold rolling to the final thickness e _f , between 1 and 5 mm. To obtain the desired properties, the final thickness e _f must be between 40 and 70% of the thickness of the hot-rolled strip e _c . If the cold rolling rate is insufficient, the desired yield strength can not be reached. If it is too important, the coefficient of hardening n becomes too low and the formability is insufficient.

The cold-rolled strip then undergoes a restoring anneal at a temperature between 180 and 280 ° C. The control of this temperature is important: a lack of restoration or a too low temperature impairs the elongation. Conversely, an annealing temperature greater than 280 ° C leads to a recrystallized state, with insufficient mechanical strength.

An essential characteristic of the method of manufacturing strips and sheets according to the invention is the absence of re-curing after the restoration annealing, either by cold rolling or by planing under tension. Admittedly, such work hardening would increase the yield strength, but reduce the elongation and the coefficient of hardening too much, which would be unfavorable to the formability and the folding ability. In addition, the gain in elastic limit is lost very quickly during the baking treatment of paints, while for the restored products and not re-locked, the loss of mechanical resistance to baking paints is reduced.

Another advantage of the absence of hardening, especially by planing, after restoration is to obtain sheets and strips with low anisotropy, with a difference between the elasticity limits in the L and T directions of less than 15 MPa , and most often less than 10 MPa ..

To avoid re-hardening the metal after restoration, it is necessary to control the flatness of the band during cold rolling, and especially during the finishing, especially when slitting relatively narrow strips of fairly thick, where it avoid deformations of the "sword blade" type.

The sheets and strips according to the invention are particularly well suited to the manufacture of stamped and folded parts with a small bending radius, particularly for the automotive industry. 180 ° bending radii are obtained less than the thickness of the sheet or strip, or even less than 80% of this thickness. The coefficient of hardening n is greater than 0.10, which contributes to the rapid increase in the mechanical strength of the parts during their shaping, and therefore to the use of lower thicknesses.

As an example of use can be mentioned anti-intrusion opening reinforcements which comprise stamped and folded parts, and which are subjected to the treatment of baking paints, including cataphoresis layers. For a treatment of 20 minutes at 200 ° C., the loss of elastic limit remains less than 20 MPa. Another interesting use of the sheets and strips according to the invention is the manufacture of jacks, which allow a significant weight gain compared to steel jacks.

Examples Example 1

The plates were cast in 7 different alloys A to G, the alloys A to E having a composition according to the invention, and the alloys F and G a composition outside the invention. The compositions (% by weight) are indicated in Table 1: Table 1 Alloy mg mn Cu Yes Fe AT 4.62 0.37 0.06 0.13 0.30 B 4.76 0.36 0.05 0.10 0.31 VS 4.61 0.35 0.05 0.14 0.37 D 4.53 0.36 0.05 0.09 0.29 E 5.18 0.35 0.06 0.11 0.18 F 4.58 0.27 0.02 0.04 0.17 BOY WUT 5.10 0.81 0.05 0.11 0.24

The plates were hot-rolled to obtain 5 mm thick strips and then cold rolled to 3 mm, ie 60% of the thickness of the hot strip. The strips were annealed at 200 ° C. The yield strength R _0.2 direction L was measured, elongation at break A ₈₀ according to standard NF EN 10002-1 relating to tensile tests on metallic materials, and the bending radius at 180 °. The results are shown in Table 2: Table 2 Alloy R _0.2 (MPa) A ₈₀ (%) Bending radius (mm) AT 239 15.5 1.8 B 223 16.2 1.5 VS 225 17.5 1.4 D 220 16.5 1.7 E 258 16.8 2.0 F 235 16.9 2.5 BOY WUT 279 12.2 2.8

It is found that the high Mn alloy H has an elongation <15% and a relatively high limiting bending radius, greater than 80% of the thickness. The alloy F at low Mn also has a fairly high bending radius. FIG. 1 shows the compromise between the elastic limit R _0.2 and the bending radius. A radius of 1.5 mm is considered acceptable for a R _0.2 of 200 MPa and 2.5 mm for a R _0.2 of 280 MPa. The points corresponding to the 5 alloys according to the invention are on the left of the right, and show a good compromise between the two properties. The points corresponding to the alloys F and G are on the right of the right and therefore do not present an acceptable compromise.

Example 2

A heat treatment of 20 minutes at 170 ° C., 185 ° C. and 200 ° C., respectively, simulating paint baking treatments of a motor vehicle, was carried out on sheet metal samples of Example 1 in alloy C and E, and on a sample of alloy C which has also undergone work hardening by traction planing. The mechanical characteristics in the long direction have been measured, namely the breaking strength R _m , the elastic limit R _0.2 and the elongation A ₈₀ , before and after heat treatment. The results are shown in Tables 3 (for restored alloy C), 4 (for E) and 5 (for C hardened). Table 3 (C not hardened) baking Without 20 minutes 170 ° C 20 minutes 185 ° C 20 minutes at 200 ° C. R _m (MPa) 325 316 314 313 R _0.2 (MPa) 225 212 210 208 A ₈₀ (%) 17.5 16.6 18.5 19.0 baking Without 20 minutes 170 ° C 20 minutes 185 ° C 20 minutes at 200 ° C. R _m (MPa) 355 351 353 351 R _0.2 (MPa) 258 254 256 254 A ₈₀ (%) 16.8 16.2 16.6 16.7 baking Without 20 minutes 170 ° C 20 minutes 185 ° C 20 minutes at 200 ° C. R _m (MPa) 328 320 315 313 R _0.2 (MPa) 242 214 210 207 A ₈₀ (%) 14.9 16.0 17.2 18.7

It is found that the drop in R _0.2 due to the heat treatment is much lower for the uncooled samples than for the hardened sample.

Example 3

On samples C and E of Example 1, the mechanical characteristics Rm, R _0.2 and A ₈₀ were measured in the long direction, at 45 ° and in the cross direction. The results are shown in Table 6: Table 6 C long C 45 ° Through E long E 45 ° Through R _m (MPa) 324 325 324 357 347 352 R _0.2 (MPa) 225 229 230 258 247 255 A ₈₀ (%) 17.5 19.1 18.2 16.8 16.6 16.1

It is found that the mechanical characteristics, in particular the yield strength, vary very little according to the direction of the measurement.

Example 4

Composite alloy plates were cast: Yes Fe mn mg Cu Cr 0.12 0.18 0.33 4.57 0.04 0.06

The exit thickness of the hot rolling was varied, the final thickness remaining at 3 mm, so that the ratio e _f / e _c was varied between 70% and 40%. The final annealing temperature was also varied between 200 and 320 ° C. No hardening subsequent to the final annealing was performed. In each case the breaking strength Rm, the yield strength R _0.2 , the elongation A and the coefficient of work hardening n were measured. The results, corresponding to the average of 5 measurements, are shown in Table 7: Table 7 e _f / e _c (%) 200 ° C 230 ° C 260 ° C 290 ° C. 320 ° C 70 R _m (MPa) 305 304 296 289 266 R _0.2 (MPa) 209 207 197 179 126 A ₈₀ (%) 16.4 17.8 18.7 21.4 25.5 not 0.168 0.172 0.178 0.203 0.309 60 R _m (MPa) 317 313 307 285 267 R _0.2 (MPa) 228 222 216 166 132 A ₈₀ (%) 15.9 17.5 18.7 23.6 25.9 not 0.155 0.157 0,165 0.242 0.312 50 R _m (MPa) 339 332 333 283 268 R _0.2 (MPa) 261 253 244 161 138 A ₈₀ (%) 15.2 17.1 18.6 24.6 27.0 not 0,135 0.141 0.155 0.262 0.307 40 R _m (MPa) 338 330 337 278 268 R _0.2 (MPa) 260 251 248 156 142 A ₈₀ (%) 14.5 16.1 18.9 25.0 25.9 not 0.133 0,137 0.156 0.274 0.304

There is a significant drop of R _m and especially of R _0.2 when the final annealing temperature increases from 260 to 290 ° C, which corresponds to the transition to the recrystallization temperature. Moreover, at a given restoring temperature, it can be seen that, when the ef / ec ratio decreases, that is to say when the cold rolling is greater, R _0.2 increases, but the elongation and the coefficient Work hardening n decrease.

Claims (9)

1. Aluminium alloy sheet or strip having a thickness of between 1 and 5 mm, intended for the production of stamped or bent parts having a small bend radius, with the composition (weight %): Si < 0.3, Fe: 0.2-0.4, Mn: 0.3-0.45, Cr: 0.04-0.1, Mg: 4.5-5.5, Cu < 0.1, Zn < 0.1, other elements < 0.05 each and < 0.15 in total, remainder aluminium, characterised in that it has, in its recovered temper, a yield strength R_0.2 in direction T> 240 MPa, an elongation A80> 15% and a difference R_m-R_0.2 > 90 MPa.
2. Sheet or strip according to claim 1, characterised in that its bend radius at 180° is less than its thickness.
3. Sheet or strip according to claim 2, characterised in that its bend radius is less than 80 % of its thickness.
4. Sheet or strip according to any one of claims 1 to 3, characterised in that the difference between the yield strength in direction L and direction T is less than 15 MPa.
5. Sheet or strip according to claim 4, characterised in that the difference between the yield strength in direction L and direction T is less than 10 MPa.
6. Sheet or strip according to one of claims 1 to 5, characterised in that the difference between the yield strength before and after a 20-mn paint baking treatment at 185 °C is less than 20 MPa.
7. Method for producing a sheet or strip according to one of claims 1 to 6, comprising the casting of a plate, the hot rolling thereof to a thickness e_c, the cold rolling thereof to a final thickness e_f between 70 and 40% of e_c, and a recovery annealing at a temperature between 180 and 280°C without subsequent strain hardening.
8. Use of sheets or strips according to one of claims 1 to 7 for the production of motor vehicle door leaf reinforcements.
9. Use of sheets or strips according to one of claims 1 to 7 for the production of jacks.

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