WO2003074747A1

WO2003074747A1 - Al-mg alloy sheet or strip for the production of bent parts having a small bend radius

Info

Publication number: WO2003074747A1
Application number: PCT/FR2003/000691
Authority: WO
Inventors: Jean-Luc Hoffmann; Bruce Morere; Fabrice Helfer
Original assignee: Pechiney Rhenalu
Priority date: 2002-03-07
Filing date: 2003-03-04
Publication date: 2003-09-12
Also published as: EP1481106B1; ATE348199T1; RS78404A; DE60310381D1; RS50328B; FR2836929A1; ES2279123T3; PL369769A1; FR2836929B1; DE60310381T2; EP1481106A1; AU2003238148A1; PL198511B1

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

Al-M alloy sheet or strip for the production of bent parts with a small bend radius

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 automobile construction, from sheets or strips of aluminum alloy of the aluminum-magnesium type, that is to say - 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 good mechanical resistance without requiring, like the skin alloys of the 6000 series, heat treatment for dissolution and quenching. Mention may be made, for example, of alloys 5019, 5182 and 5083, the composition of which (% by weight) registered with the Aluminum Association is indicated in Table 1:

Table 1

Problem The production of stamped and folded parts requires a material having sufficient formability to produce the stamped parts of the parts, and a folding ability that is all the better when one wishes to obtain very small folding radii, typically of the order of 1. 'sheet thickness. This ability must be as good in the rolling direction as in the peφendicular direction. The sheet or strip must have as high a mechanical resistance as possible so as to reduce the thickness as much as possible, and thus obtain the optimal lightening effect resulting from the use of aluminum compared to the steel.

Furthermore, the parts of motor vehicles are subjected to a heat treatment during the curing of bodywork paints, 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 small as possible. The object of the invention is to provide sheets and strips of Al-Mg alloy enabling these requirements to be satisfied.

Subject of the invention

The subject of the invention is a sheet or strip of aluminum alloy with a thickness of between 1 and 5 mm, intended for the manufacture of stamped and bent parts with a small bending radius, of composition (% by weight):

If <0.4 Fe <0.4 Cu <0.4 Mn + Cr: 0.3 - 0.7 Mg: 4.0 - 5.5 Zn <1 other elements <0.05 each and <0.15 in total, aluminum remains, having a yield strength Ro _{, 2} directions T> 215 MPa, an elongation A o> 15%, and a difference R _m - R _0, > 80 MPa. Preferably, we have Si <0.3 Fe: 0.2 - 0.4 Mn: 0.3 - 0.45 Mg: 4.5 - 5.5 Cr <0, l Cu <0, l Zn <0 , l.

The invention also relates to a sheet or strip of preferential composition: 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 other elements <0.05 each and <0.15 in total, remains aluminum, and, for this particular composition, Ro _{> 2} directions T> 240 MPa, At ₈ o> 15% and R _m - R _0.2 > 90 MPa.

It also relates to a process for manufacturing such a sheet or strip, comprising the casting of a plate of the above composition, its hot rolling up to a thickness e _c , cold rolling to a final thickness ef between 70 and 40% of e _c , and a restorative annealing at a temperature between

180 and 280 ° C without subsequent work hardening.

Description of the figures

The single figure illustrates the results of Example 1 in yield strength Ro _{> 2} and bending radius.

Description of the invention

The invention is based on the combination of the close 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 the production of stamped and bent parts with a small bending radius.

The alloys according to the invention are alloys with more than 4% of Mg like the alloys 5182, 5019 or 5083 mentioned above, and preferably with more than 4.5%. Magnesium contributes to mechanical resistance and its content can be adjusted according to the mechanical resistance desired. Above 5.5% of Mg, the alloy becomes more difficult to cast and to process. A particularly well-suited composition is as follows: Si <0.3 Fe: 0.2 - 0.4 Mn -. 0.3 - 0.45 Mg: 4.5 - 5.5 Cu <0.1 l Cr: 0.04 - 0.1 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 reduces the elastic limit without improving the ability to fold. A content greater than 0.7% improves the elastic limit without reducing the elongation too much, but, surprisingly, gives a poor bending radius.

The method of manufacturing the strips according to the invention comprises the casting of a plate of the alloy considered, 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 ef must be between 40 and 10% of the thickness of the hot-rolled strip e _c . If the cold rolling rate is insufficient, the desired yield strength cannot be reached. If it is too large, the work hardening coefficient n becomes too low and the formability is insufficient.

The cold rolled strip is then subjected to a restorative annealing at a temperature between 180 and 280 ° C. The control of this temperature is important: an absence of restoration or a too low temperature harms the elongation. Conversely, an annealing temperature above 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-hardening after the restoration annealing, either by cold rolling or by tensioning under tension. Admittedly, such a work hardening would increase the elastic limit, but would reduce too much the elongation and the coefficient of work hardening, which would be unfavorable to formability and to the ability to bend. In addition, the gain in elastic limit is lost very quickly during the curing treatment of the paints, while for restored and not rewashed products, the loss of mechanical resistance to the curing of the paints is more reduced. Another advantage of the absence of work hardening, in particular by leveling, after restoration is to obtain sheets and strips having a low anisotropy, with a difference between the elastic limits in the directions L and T of less than 15 MPa , and most often less than 10 MPa ..

To avoid re-hardening the metal after restoration, it is necessary to properly control the flatness of the strip during cold rolling, and especially during finishing, in particular during the slitting of relatively narrow strips of fairly large thickness, where it deformation of the "saber blade" type must be avoided. The sheets and strips according to the invention are particularly well suited to the production of stamped and folded parts with a small bending radius, in particular for the automotive industry. 180 ° bending radii are obtained which are less than the thickness of the sheet or strip, or even less than 80% of this thickness. The work hardening coefficient n is greater than 0.10, which contributes to the rapid increase in the mechanical resistance of the parts during their shaping, and therefore to the use of smaller thicknesses. By way of example of use, the reinforcements of anti-intrusion sashes which include stamped and folded parts and which are subjected to the baking treatment of paints, in particular cataphoresis layers, may be mentioned. For a treatment of 20 min at 200 ° C, the loss of yield strength remains below 20 MPa. Another advantageous 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

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

The plates were hot rolled to obtain strips 5 mm thick, then cold rolled to 3 mm, ie 60% of the thickness of the hot strip. The strips underwent restorative annealing at 200 ° C. The elastic limit Ro _j2 sense L, the elongation at break A ₈ o according to standard NF EN 10002-1 relating to tensile tests on metallic materials, and the radius of folding at 180 ° were measured. The results are shown in Table 2: Table 2

It is found that the alloy with a high Mn H has an elongation <15% "and a fairly high limit bending radius, greater than 80%" of the thickness. The low Mn alloy F also has a fairly large bending radius. FIG. 1 shows the compromise between the elastic limit Ro, ₂ and the folding radius. A radius of 1.5 mm is considered acceptable for an R _0j2 of 200 MPa and 2.5 mm for a Ro _{; 2} of 280 MPa. The points corresponding to the 5 alloys according to the invention are to the left of the right, and show a good compromise between the two properties. The points corresponding to alloys F and G are to the right of the right and therefore do not present an acceptable compromise.

Example 2

A 20 min heat treatment was carried out respectively at 170 ° C, 185 ° C and 200 ° C, simulating baking treatments of the paints of a motor vehicle, on sheet metal samples of Example 1 made of alloy C and E, and on a sample of the alloy C having undergone in addition a work hardening by planing by traction. The mechanical characteristics were measured in the long direction, namely the tensile strength R _m , the elastic limit R _0j2 and the elongation Ago, before and after heat treatment. The results are shown in Tables 3 (for alloy C restored), 4 (for E) and 5 (for C hardened). Table 3 (C not work hardened)

Table 4 (E not work hardened)

Table 5 (C hardened)

It can be seen that the drop in Ro, ₂ due to the heat treatment is much lower for the non-hardened samples than for the hardened sample.

Example 3

On samples C and E of Example 1, the mechanical characteristics Rm, Ro _{> 2} and A ₈ o were measured in the long direction, at 45 ° and in the transverse direction. The results are shown in Table 6: Table 6

It can be seen that the mechanical characteristics, in particular the elastic limit, vary very little as a function of the direction of the measurement.

Example 4

We poured alloy plates of composition:

The thickness of the hot rolling outlet was varied, the final thickness remaining at 3 mm, so that the ratio β _f / e _c was varied between 70%> and 40%. The final annealing temperature was also varied between 200 and 320 ° C. No work hardening after the final annealing was carried out. In each case, the tensile strength Rm, the elastic limit Ro _{, 2} , the elongation A and the work hardening coefficient n were measured. The results, corresponding to the average of 5 measurements, are shown in Table 7:

Table 7

There is a significant drop in R _m and especially in Ro _{, 2} when the temperature of the final annealing goes from 260 to 290 ° C, which corresponds to the transition to the recrystallization temperature. Furthermore, at a given restoration 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 hardening n decrease.

Claims

claims

1. Sheet or strip of aluminum alloy with a thickness between 1 and 5 mm, intended for the production of stamped and bent parts with a small bending radius, of composition (%> by weight):

If <0.4 Fe <0.4 Cu <0.4 Mn + Cr: 0.3 - 0.7 Mg: 4.0 - 5.5 Zn <l other elements <0.05 each and <0.15 in total, aluminum remains, having an elastic limit in the direction TR _{0> 2} > 215 MPa, an elongation A ₈ o> 15%, and a difference R _m - R _{0> 2} > 80 MPa.

2. Sheet or strip according to claim 1, characterized in that the iron content is between 0.2 and 0.4%.

3. Sheet or strip according to one of claims 1 or 2, characterized in that the manganese content is between 0.3 and 0.45% o, and the chromium content less than 0.1%.

4. Sheet or strip according to one of claims 1 to 3, characterized in that the magnesium content is between 4.5 and 5.5%>.

5. Sheet or strip according to one of claims 1 to 4, characterized in that the composition of the alloy is: 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, l, other elements <0.05 each and <0.15 in total, aluminum remains.

6. Sheet or strip according to claim 5, characterized in that Ro _{, 2} directions T> 240 MPa, A ₈₀ > 15% and R _m - R _{0> 2} > 90 MPa.

7. Sheet or strip according to one of claims 1 to 6, characterized in that its 180 ° folding radius is less than its thickness.

8. Sheet or strip according to claim 7, characterized in that its folding radius is less than 80%> of its thickness.

9. Sheet or strip according to one of claims 1 to 8, characterized in that the difference between the elastic limit direction L and direction T is less than 15 MPa.

10. Sheet or strip according to claim 9, characterized in that the difference between the elastic limit direction L and direction T is less than 10 MPa.

11. Sheet or strip according to one of claims 1 to 10, characterized in that the difference between the elastic limit before and after a paint curing treatment of 20 min at 185 ° C is less than 20 MPa.

12. A method of manufacturing a sheet or strip according to one of claims 1 to 11, comprising the casting of a plate, its hot rolling to a thickness e _c , the cold rolling to a thickness final ef between 70 and 40% "e _c , and a restorative annealing at a temperature between 180 and 280 ° C without subsequent work hardening.

13. Use of sheets or strips according to one of claims 1 to 11 for the manufacture of motor vehicle window reinforcements.

14. Use of sheets or strips according to one of claims 1 to 11 for the manufacture of jacks.