NL1006511C2 - Production and heat treatment of ductile aluminium@ plate - Google Patents

Abstract

An Aluminium (Al) plate is produced from an Al-Magnesium (Mg)-Silicon (Si)alloy of the AA6000 type and contains 0.4-1.7 wt.% Si and 0.2-0.9 wt.% Mg. It is suitable for forming into automobile outer panels by hot and/or cold rolling and heat treatment, e.g. by homogenisation and solution treatments, so that gross Si and Mg2Si particles and/or precipitates are prevented from forming.

Description

METHOD FOR MANUFACTURING A GOOD FOLDABLE ALUMINUM PLATE

The invention relates to a process for manufacturing an aluminum sheet of an AA6000 type AlMgSi alloy comprising Si in a range from 0.4 to 1.7 weight percent and Mg in a range from 0.2 to 0.9 weight percent. for conversion into automotive exterior parts.

Aluminum plate of the indicated type is used, inter alia after deformation by, for example, pressing, in automotive outer parts which are coupled to automotive inner parts. This coupling is usually effected by seaming an edge part of the aluminum plate. Such a coupling must be of sufficient quality and durability. The flaring behavior of an aluminum sheet for use in automotive exterior parts is therefore an important feature of the aluminum sheet. It has now been found that upon flaring, for example to a flat seam (flat hem), of aluminum sheet 15 of the indicated type and manufactured by a method comprising homogenizing, hot rolling, cold rolling, solution annealing and cooling to below 100 ° C surface defects on the standing seam can occur. These undesirable surface defects are characterized, among other things, by visually observable cracks. In addition to good flaring behavior, the maintenance of a good surface structure after deformation is essential, because an automotive exterior part is also assessed on the visual aspects of the outer surface after applying, among other things, a paint layer, for which an even light reflection is required. This means that the aluminum plate must at least also be free from roping or ridging lines.

An object of the invention is to provide a method for manufacturing an aluminum plate of the indicated type, wherein the aluminum plate has an improved flaring behavior while retaining the good surface structure after deformation by, for example, pressing.

To this end, the method according to the invention is characterized by the fact that for the deformation process of hot and / or cold rolling and heat treatments, such as homogenization and / or solution annealing, it is chosen that coarse Si and Mg2Si particles and / or precipitates in the aluminum sheet are substantially prevented.

This achieves that the aluminum plate is well-seamable and has a texture stable for deformation. The method according to the invention is based on the insight that in order to improve the flaring behavior of the aluminum plate while retaining the good surface structure after deformation of the aluminum plate to, for example, an automotive outer part, the prevention of coarse Si * and Mg2Si particles and / or precipitates in the final microstructure of the aluminum sheet is important. The coarse Si and Mg2Si particles are believed to be largely responsible for the moderate flaring behavior, while such coarse Si and Mg2Si-15 particles and / or precipitates do not directly adversely affect the desired good surface texture. Coarse Si and Mg2Si particles in this context are understood to mean that these particles and / or precipitates are incoherent with the matrix and are therefore no longer in solution. This means in this case that particles and / or precipitates larger than about 1 micron are no longer coherent with the matrix.

A particularly favorable embodiment of the method according to the invention is characterized in that the method successively comprises the steps of: (i) homogenizing; (ii) cooling the aluminum sheet to a temperature range of 450-525 ° C; (iii) hot rolling, the aluminum sheet having a temperature in a range of 230-300 ° C immediately after hot rolling; (Iv) heat treating by keeping the aluminum sheet in a temperature range of 300 - 450 ° C for a residence time of at least 30 min; (v) cold rolling with a total cold rolling reduction of at least 70%; (Vi) solution annealing; (vii) cooling to below 100 ° C at a cooling rate of at least 300 ° C / min.

This combination of process steps ensures that the aluminum sheet is free from roping and also free from coarse Si and 40 Mg2Si particles and / or precipitates, so that the aluminum sheet is well-seamable. It is noted that from international patent application WO 95/22634 a method is known for manufacturing aluminum plate of the indicated type, which aluminum plate is free from roping. The known method consists of successively homogenizing according to steps (a) at a temperature greater than 500 ° C; (b) cooling the aluminum sheet to a temperature range of 350-450 ° C; (c) hot rolling, the aluminum sheet having a temperature in the range 200-300 ° C immediately after the hot rolling; (d) cold rolling with a total cold rolling reduction of at least 50%; (e) solution annealing for up to 10 min in a temperature range of 500-580 ° C, the aluminum sheet being heated at a heating rate of at least 2 ° C / s; (f) cooling the aluminum sheet to below 100 ° C at a cooling rate of at least 5 ° C / s. With the method according to the present invention, a further improvement with regard to the roping is achieved and further the flaring behavior is improved.

Preferably, the method according to the invention is characterized in that in (iv) the aluminum plate is kept in a temperature range of 350-400 ° C. Hereby it is achieved that complete recrystallization can take place.

Preferably, in (v), the total cold roll reduction is at least 80%. By increasing the degree of deformation during cold rolling, more energy is stored in the sheet. This increase in stored energy accelerates the nucleation process during solution annealing. A total degree of deformation of 80% or more is sufficient to obtain the desired grain size after recrystallization during solution annealing.

Preferably, the temperature of the aluminum sheet immediately after the hot rolling, the so-called exit temperature, is in the range of 250-280 ° C. Because the aluminum sheet has an exit temperature in the indicated range, it is achieved that less dynamic recrystallization can occur during hot rolling. As a result, more energy is present in the aluminum sheet for the nucleation of various texture components so that better texture development can take place during recrystallization by inter-annealing during step (iv).

A further embodiment of the method according to the invention is characterized in that the aluminum sheet has a temperature immediately before the hot rolling, the so-called entry temperature, in the range of 450 - 525 ° C and more preferably in a 1 0 0 6 5 1 1 - 4 - range of 450 - 500 ° C, while the known method was carried out with an entry temperature in a range of 350 - 450 ° C. This makes a further contribution to the mechanism as described at a reduced exit temperature. In the temperature range indicated, the formation of coarse Mg2Si particles is prevented, while the surplus of Si remains in solution and does not have sufficient time to precipitate.

A further embodiment of the method according to the invention is characterized in that step (i) comprises a two-step homogenization in which the temperature in the second step is in the range 470 - 530 ° C. The homogenization cycle is an important process parameter in preventing the formation of coarse Mg2Si particles in the microstructure of the aluminum sheet. During the homogenization cycle, the temperature of the aluminum plate 15 should rise above a certain value, so that all soluble structural components are in solution. At a temperature above about 480 - 485 ° C, β particles (Mg2Si) and Si remain in solution. Particularly good results are obtained with a two-step homogenization cycle in which the aluminum sheet is kept in a temperature range of 540 - 570 ° C for 4-6 hours in the first step, and in the second step for 0-1 hours in a temperature range of 470-530 ° C, while the known homogenization treatment consists of 6-12 hours treatment at 540-550 ° C.

The solution annealing should be done at a temperature high enough to dissolve all soluble structural components. The cooling should be done at a sufficiently high cooling rate of at least 300 ° C / min to prevent structural components, Si and Mg2Si in particular, from being secreted. More preferably, the cooling rate is equal to or greater than 50 ° C / s. This ensures that all Si and Mg2Si remain in solution. Secreted Si and Mg2Si particles and / or precipitates tend to preferentially locate at the grain boundaries, thereby adversely affecting the flaring behavior of the aluminum sheet. In order to suppress the formation of Si and Mg2Si-35 particles and / or precipitates, it is desirable that heating also takes place at a sufficiently high heating rate during solution annealing. To this end, the aluminum sheet according to the invention is preferably annealed to final thickness in a continuous annealing furnace with a heating rate of the aluminum sheet in the heating section of the continuous annealing furnace of at least 50 ° C / s, after which 1006511 - 5 - the aluminum sheet in a temperature range of 450 - 570 ° C is annealed for 5-300 sec. and preferably 5-30 sec. With such a high heating rate, it is achieved that the aluminum plate has a fine grain structure, whereby the occurrence of orange 5 peel after deformation of the aluminum plate is avoided. The aluminum sheet according to the invention is preferably heated homogeneously in the continuous annealing furnace by means of inductive heating.

Very good results with regard to a well-seamable aluminum plate which is furthermore free from roping are obtained with the method according to the invention, wherein the aluminum plate comprises the alloying elements in weight percentages in the range: 0.4 - 1.7% Si, 0.2 - 0.9% Mg, 0.25% Mn (maximum), 0.2% Cu (maximum), 0.5%

Fe (maximum), the balance mainly aluminum and unavoidable impurities. Furthermore, the aluminum sheet according to the invention comprises elements such as Ti, TiB2, TiC, etc. as grain-refining elements of the casting structure.

Preferably, the Si content is in a range from 1.0 to 1.3 weight percent. This achieves that the mechanical properties and the deformability are optimized for the use of the aluminum sheet in automotive exterior parts.

The Mg content is preferably in the range of 0.2 - 0.6 weight percent. Mg-Si clusters or precipitates are formed by cooperation with the Si, which contribute to the mechanical properties of the aluminum sheet. More preferably, the Mg content is in the range of 0.3 - 0.5 weight percent, in that optimum deformability is achieved when the Si / Mg ratio equals about 3.

The Mn content is preferably in the range of 0.05 - 0.20 weight percent. At a relatively high Μη content, for example 30 more than 0.3%, ductility decreases to an unacceptable level with increasing Mn content. At too low a Μη content, for example less than 0.05%, the Mn does not contribute sufficiently to an effective grain refinement during the solution annealing as a result of the formation of so-called dispersoids. The optimum combination for the desired contribution to the grain refining effect after the solution annealing and the decreasing elongation at an increasing Mn content is obtained in a range of 0.05 - 0.20% by weight Mn.

Dispersoid alloying elements other than Mn 40 can also be used for this purpose, such as, for example, Cr, Zr, V

1006511-6 or Ti. If Cr or Zr is added in a range of 0.05 - 0.25% by weight, almost equal results are obtained with respect to the grain size after solution annealing as with Mn in this range. This is because the dispersoids formed are of the same order-5 size in size as when Mn is added, which is due to the interaction of Zr or Cr with the Si and Mg. Addition of Zr or Cr to an AA6xxx type alloy does not significantly improve grain size, while the costs for Zr or Cr containing alloys will be higher than if Mn 10 is added in the same range. In addition, the value and applicability of Zr or Cr-containing scrap material decreases.

The Fe content has a major influence on the deformability of the aluminum sheet. At a high Fe content, for example more than 0.5%, relatively large intermetallic compounds are formed, which greatly decrease the deformability. A low Fe content is therefore desirable. The Fe present also contributes to the control of the grain size in the sheet during solution annealing. The ferrous intermetallic compounds can favorably influence the nucleation process during solution annealing, resulting in a small grain size. Good results are obtained in the method according to the invention at an Fe content of less than 0.3% by weight.

More preferably, the Fe content is in the range of 0.10-0.30 weight percent. In this range, the best compromise is achieved between good formability and favorable contribution to the grain size during solution annealing.

The alloying element Cu can make a significant contribution to increasing the mechanical properties of the aluminum sheet. It may be generally known that with an increasing Cu content in the aluminum plate, the corrosion resistance decreases sharply. In view of the applications of the aluminum sheet in, for example, automotive exterior parts, good corrosion resistance is particularly important. The Cu content in the aluminum sheet must therefore be low. For optimum corrosion resistance, the Cu-35 content should preferably be less than 0.1% by weight.

The invention is also embodied in a curved car-series plate part suitable for use for automotive outer parts made of the aluminum plate obtained from the method according to the invention.

The invention will now be elucidated on the basis of a few examples, which are not limitative of the 1006511-7 invention.

The aluminum alloy according to the invention with the chemical composition in percent by weight: 1.2% Si, 0.33% Mg, 0.07% Mn, 0.07% Cu, 0.20% Fe, balance in 5 mainly aluminum and unavoidable impurities, has been processed in three different ways into a thin sheet of 1.15 mm thickness and subsequently assessed on both flaring and roping behavior.

The three aluminum alloy process routes followed are shown in Table 1; wherein route 3 is the method according to the invention, and route 1 the method is in accordance with that known from international patent application WO 95/22634. In route 1, the homogenization cycle was heated at a heating rate of 50 ° C / hour to a homogenization temperature of 540 ° C and held at this temperature for 5 hours, then cooled to 400 ° C, which temperature was the entry temperature also called Break-down mill temperature. In routes 2 and 3 it was warmed to 570 ° C at a heating rate of 50 ° C / hour, kept at 570 ° C for 5 hours, then cooled in the oven to 510 ° C and held at this temperature for 1 hour, and then cool to an entry temperature of 480 ° C.

The exit temperature after hot rolling, tandem exit temperature, was 260 ° C for all routes, with the plate thickness being 6 mm.

Only during route 3, the aluminum sheet for cold rolling is heat-treated at 400 ° C for 1 hour, after which the sheet cools to room temperature at a cooling rate of about 10 ° C / hour.

For all process routes, solution annealing includes a heat treatment heating at a heating rate of 100 ° C / sec to a temperature of 570 ° C for 10 sec. kept at this temperature and then cooled to room temperature at a cooling rate of about 100 ° C / sec.

For assessment of the aluminum sheet's seamability, a standard test method was used in which a 100 x 50 mm test panel was bent into a flat over a 1.0 mm thickness panel, which panel acts as the inner panel. In order to make a good assessment, it is customary that the test panel is stretched beforehand, for example 2 - 10% plane strain. The provision of such a pre-stretch causes cracks to form on the seam more quickly. The outside of the folded seam is then visually assessed and classified, with special attention being paid to the prevention of cracks in 1006511-8. For classification, use is made of the classification according to Table 2. A performance higher than six indicates good flaring behavior, while a performance of six or lower indicates moderate or even poor flaring behavior.

Roping is mainly due to the presence of meta-stable texture components created after recrystallization of the aluminum sheet. In principle, this makes it possible to describe roping using microstructure parameters such as grain size distribution and location-dependent texture measurements. However, this is a very cumbersome procedure. A quantitative way to describe roping is by means of a simple test in which sheet material of 100 x 300 mm is stretched 15% in a tensile testing machine, then lightly polished and classified by means of Table 3. 15 The assessment considers both height and thickness of the lines (roughening), if the degree of presence of ridging lines plays a role. A performance of six or higher indicates fair to good roping behavior, while performance below six indicates moderate to poor roping behavior.

20 The results of the assessment of the flaring and roping behavior for the different process routes are shown in Table 4. From these results it can be concluded that route 1 is not good for obtaining both good roping and good flaring behavior. By adjusting the homogenization cycle and increasing the entry temperature (route 2), the flaring behavior is improved, but the roping behavior deteriorates. In Table 5 the seaming behavior as a function of the pre-stretch is given for aluminum sheet manufactured according to the method according to the invention (route 3).

With the method according to the invention, both good roping behavior and particularly good flaring behavior are obtained, which is important when using the aluminum sheet to form automotive outer parts.

1 00 68 1 1 - 9 -

Table 1 Followed process routes

Route

Process step I

1_2_3

Homogenization temperature 540 570/510 570/510 5 Break-down mill temperature 400 480 480

Tandem exit temperature 260 260 260

Heat treatment - - yes

Cold rolling deformation 80% 80% 80 X

Solution annealing standard standard standard 10

Table 2 Classification of flaring behavior

Performance Rating 15 10 no visible roughening 8 surface roughened 6 groove-shaped roughening 4 cracks, not continuous 2 crack, continuous 20 0 fracture

Table 3 Classification of the roping behavior 25 Performance Roughing Ridging lines 10 no visual surface no ridging lines roughing 8 light surface roughening short, discontinuous lines 30 6 moderate surface roughening discontinuous lines 4 light surface roughening continuous lines 2 moderate surface roughening continuous lines 0 serious surface roughening continuous lines 35

Table 4 Result flaring and roping behavior for the different process route

Route Flaring behavior Roping behavior 40 1 4 8 2 10 4 I_3__10_ 8 1 1006511 - 10 -

Table 5 The seam behavior as a function of the pre-stretch (%) for sheet manufactured via route 3

Pre-stretch (%) Flaring behavior 5 0 10 2 10 5 8 10 8 10 1006 511

Claims (18)

A method of manufacturing an AA6000 type AlMgSi alloy aluminum sheet comprising Si in a range from 0.4 to 1.7 weight percent and Mg in a range from 0.2 to 0.9 weight percent, suitable for conversion into automotive outer parts, wherein such a deformation process of hot and / or cold rolling and heat treating, such as homogenization and / or solution annealing, is chosen that substantial coarse Si and 10 Mg2Si particles and / or precipitates in the aluminum sheet are substantially prevented. Method according to claim 1, characterized in that the method successively comprises the steps of: (i) homogenizing; (ii) cooling the aluminum sheet to a temperature range of 450-525 ° C; (iii) hot rolling, the aluminum sheet having a temperature in a range of 230 - 300 ° C immediately after hot rolling; (iv) heat treating by keeping the aluminum sheet in a temperature range of 300 - 450 ° C for a residence time of at least 30 min; (v) cold rolling with a total cold rolling reduction of at least 70%; (vi) solution annealing; (vii) cooling to below 100 ° C at a cooling rate of at least 300 ° C / min. Method according to claim 2, characterized in that in (iv) the aluminum sheet is maintained in a temperature range of 350-400 ° C. Method according to claim 2 or 3, characterized in that in 35 (v) the total cold rolling reduction is at least 80 X. Method according to any one of the preceding claims 2-4, characterized in that in (ii) the aluminum sheet is maintained in a temperature range of 450-500 ° C. A method according to any one of the preceding claims 2-5, characterized in that the 1006511-12 is characterized in that immediately after the hot rolling the aluminum plate has a temperature in a range of 250-280 ° C. 7. A method according to any one of the preceding claims 2-6, characterized in that (i) comprises a two-step homogenization cycle in which in the second step the temperature of the aluminum plate is in a range of 470-530 ° C. 8. Method according to any one of the preceding claims 2-7, characterized in that the aluminum sheet in (vi) at final thickness is solution-annealed in a continuous annealing furnace with a heating rate in the heating section of the continuous annealing furnace of at least 50 ° C / s . Method according to any one of the preceding claims 2-8, characterized in that the cooling rate in (vii) is equal to or greater than 50 ° C / s. 10. Method according to any one of claims 1-9, characterized in that the aluminum plate comprises the alloying elements in weight percent in the range 51 0.4 to 1.7% Mg 0.2 to 0.9% Mn 0.25% (max.) 25 Cu 0.2% (max.) Fe 0.5% (max.) Balance mainly aluminum and unavoidable impurities. Process according to any one of claims 1-10, characterized in that the Si content is in the range from 1.0 to 1.3% by weight. 12. A method according to any one of claims 1 to 11, characterized in that the Mg content is in a range from 0.2 to 0.6% by weight. Process according to claim 12, characterized in that the Mg content is in a range from 0.3 to 0.5% by weight. 40 1006511 - 13 - IA. A method according to any one of claims 1-13, characterized in that the Mn content is in a range from 0.05 to 0.20% by weight. A method according to any one of claims 1 to 14, characterized in that the Fe content is in a range less than 0.3% by weight. 16. A method according to claim 15, characterized in that the Fe content is in a range from 0.10 to 0.30% by weight. A method according to any one of claims 1 to 16, characterized in that the Cu content is in a range less than 0.1% by weight. 15 Body panel part made from the aluminum sheet obtained from the method according to any one of claims 1-17. 1006511