WO1998014626A1

WO1998014626A1 - Aluminium alloy for rolled product process

Info

Publication number: WO1998014626A1
Application number: PCT/GB1997/002600
Authority: WO
Inventors: Ricky Arthur Ricks; Kevin Michael Gatenby; Alan Robert Carr
Original assignee: Alcan International Limited
Priority date: 1996-09-30
Filing date: 1997-09-24
Publication date: 1998-04-09
Also published as: EP0931170A1; AU4314697A

Abstract

Rolled sheet of composition in wt.%: Si 0.8 - 1.5; Mg 0.2 - 0.7; Fe 0.2 - 0.5; Mn 0.01 - 0.1; Cu up to 0.25; Cr up to 0.1; Zn up to 0.4; V up to 0.2; Al balance, can be produced at high line speeds in a form having fine equiaxed grain size and good formability. The rolled sheet is suitable for press forming into automotive exterior panels and closure sheet.

Description

ALUMINIUM ALLOY FOR ROLLED PRODUCT PROCESS

This invention is concerned with aluminium alloys of the 6000 series (of the Aluminum Association Inc. register), and the use of such alloys in the production of rolled sheet for making components for use in vehicles. Rolled aluminium alloy products are used as components in automotive bodies in a variety of situations, both as part of the load- bearing structure of the body and also as facia cladding and skin components which may or may not be visible in the final vehicle. Different components may need to have different properties. The 6000 series alloys used are a compromise between various competing factors including: ability to be press-formed into complex shapes; good age-hardening characteristics and good strength e.g. to reduce denting; surface capable of receiving paint; low quench sensitivity; suitable for production at high line speeds. This invention concentrates on the last of these factors, production at high line speeds, and addresses the alloy modifications that need to be made to cater for or exploit these speeds. Heat treatable alloys of the Al-Mg-Si (6XXX) type are well suited for the application of automotive exterior panels and "closure sheet" (e.g. doors, boot-lids and bonnets). In the solution heat treated condition (T4) the alloys can have high formability to allow complex, shaped panels to be manufactured. Subsequent heat treatment in the process of car manufacture (e.g. paint-bake ovens) develops the precipitation of Mg₂Si from solid solution which generates the strength in the alloy to give the desired in service properties (e.g. denting resistance).

To date, all 6XXX alloys for automotive sheet applications have been developed with a view to controlling the microstructure during processing to generate the required formability in the solution heat-treated condition and the desired strength in the age-hardened condition. As such, they frequently contain alloying additions which can compromise the ability to manufacture the alloy with high productivity for maximum plant throughput and cost efficiency. In particular the use of elements such as Mn (and Cr), which generate a highly stable precipitate ("dispersoid") phase during ingot preheating prior to rolling, are detrimental during quenching from solution heat treatment due to the propensity for the precipitation of the age-hardening phase (Mg₂Si) on the dispersoids during the quench. Thus alloys containing high levels of Mn (i.e. >0.1 wt %) tend to require either slow processing speeds to maximise the effectiveness of a forced air quench, with issues of productivity, or are required to be water quenched, with issues of sheet distortion.

In any continuous heat treatment process for any given line length, power input or heat transfer rate and for equivalent sheet thickness, then as the line speed increases, the heating rate and cooling rate of the sheet will decrease. Heating rate is important in controlling the grain size produced by recrystallisation, which impacts directly on the formability, and resultant surface appearance, of the alloy. The time taken to reach temperature also affects the time available for solution treatment. Cooling rates are important in controlling the degree of precipitation of Mg₂Si on microstructural features which assist nucleation, such as grain boundaries and the dispersoid particles.

To maintain a satisfactory product, this decrease in both heating rate and cooling rate associated with high productivity heat treatment lines must be compensated for by a modification to the alloy microstructure. To compensate for a reduction in heating rate more recrystallisation nucleation sites must be provided, for example: in the form of hard, second phase constituent particles having the right size and dispersion. To compensate for a reduction in cooling rate, assuming the grain size (and hence grain boundary area) is unchanged (for equivalent formability), the volume fraction and number density of dispersoid particles must be controlled to a suitable low number. Both these microstructural features can be developed by the correct combination of composition and homogenisation (ingot preheating) in the sheet ingot prior to hot and cold rolling.

An Al alloy currently marketed for automotive body sheet applications AA6016A has the composition range (in wt %): 0.9 - 1.3% Si; Max 0.4% Fe; Max 0.2% Cu; Max 0.25% Mn; 0.3 - 0.5% Mg; Max each 0.10% Cr, 0.15% Zn, 0.15% Ti. This alloy suffers reduced formability and in-service strength due to excessive quench sensitivity of the alloy (i.e. the degree of precipitation of Mg₂Si during the quench). Very high cold reductions are applied during cold rolling, i.e. 90% cold reduction to achieve the required formability.

In WO 95/31580, the authors have proposed the use of an alloy of the above type but containing 0.05 to 0.2 wt % Fe, in order to confer improved formability on the alloy sheet. This is Alcoa AA 6022 alloy. A disadvantage of this proposal is that such alloys are relatively expensive and create scrap reclamation problems, due to the need to keep the Fe content down. In WO 95/14113, the authors have proposed the use of an alloy of the above type but containing 0.1 to 0.8 wt % Mn, in order to improve mechanical strength and formability and to achieve good strain distribution. A disadvantage of such alloys is that they are excessively quench sensitive and so need to be cooled quickly after solution heat treatment e.g. by water quench with the possibility of sheet deformation. The teaching in WO 95/14113 is directed towards solution treatments of 30 minutes or more. This is not possible on continuous annealing lines and can only be achieved in a batch or coil annealing process.

In one aspect the invention provides an alloy product of composition (in wt %) Si 0.8 - 1.5

Mg 0.2 - 0.7

Fe 0.2 - 0.5, preferably 0.3 - 0.5

Mn 0.01 - 0.1 , preferably 0.01 - 0.09

Cu up to 0.25

Cr up to 0.1 , preferably up to 0.05,

Zn up to 0.4

V up to 0.2

Others 0.05 each, 0.15 total

Al Balance

said product being in the form of rolled sheet having fine equiaxed grain size and good formability having an r/t hem flange performance of not more than 0.5 in the T4 condition.

In another aspect the invention provides a method of making an alloy product of the defined composition, which method comprises the steps of. casting; homogenising; hot rolling; optional anneal; final cold rolling; solution heat treating; and quenching at a rate of 2 - 40°C/s from solution heat treat temperature down to 200°C.

The alloy is a variant of AA6016. The Si in combination with Mg strengthens the alloy due to precipitation of Mg₂Si. When the Si content is less than about 0.8 wt %, the strength after painting is unsatisfactory. On the other hand, when the Si content exceeds about 1.5 wt %, the soluble particles cannot all be put into solid solution during heat treatment without melting the alloy.

Mg is an alloy-strengthening element that precipitates as Mg₂Si. At least about 0.2% is needed to provide sufficient strength. At levels above about 0.7%, Mg reduces the formability of the alloy.

Cu enhances the strength and formability of the Al alloys. At high concentrations above 0.3%, however, Cu reduces the corrosion resistance of the alloys. Cu is preferably present at a concentration of up to 0.1 wt %.

Fe is used in the alloys of this invention to generate, along with other alloying elements, the required volume fraction of Al-Fe(-Si-Mn) as-cast constituent phases which become broken up and dispersed during rolling, and which then provide recrystallisation nucleation sites during heating for solution heat treatment. This encourages the formation of a fine recrystallised grain size in continuously annealed sheet even after only a small cold rolling reduction before annealing. Although the prior art teaches that the formability is reduced at an Fe content exceeding 0.2 wt %, the inventors have determined that any reduction in this property is not serious and not progressive. That is to say, alloys containing 0.25 or 0.35 or 0.45 wt % Fe all have roughly equivalent formability properties. Above about 0.5% Fe, the formability problems do become more serious. The Fe content is therefore set at 0.2 to 0.5%, preferably 0.3 to 0.5%. Mn is used in these alloys to refine recrystallised grains and to improve formability, particularly plane strain formability. The Mn concentration is adjusted to develop the desired dispersoid population (volume fraction and number density of dispersoid particles) during homogenisation. If the Mn content is too low, various disadvantages result: a coarse grain size which may give rise to surface roughness when the rolled sheet is stamped ("orange peel" effect). If the Mn level is too high, resulting in a grain size that is too small, the alloy becomes more quench sensitive and formability decreases. The Mn content is set at 0.01 - 0.1 wt % preferably 0.01 - 0.09 wt % or 0.01 - 0.05 wt %. Cr has similar effects to Mn, except that it tends to be a more powerful dispersion-forming component. Its concentration is set at up to 0.1 wt % preferably up to 0.05 wt %. Preferably the (Mn + Cr) content is 0.01 - 0.1 wt %. Alloys containing no Cr are more readily recyclable.

A further increase in strength together with a slight increase in formability can be achieved by the addition of up to 0.4%, e.g. 0.1 % to 0.3%, by weight of Zn. An additional V content of up to 0.2% by weight may lead to a further improvement in formability.

These alloy compositions compensate for reduced heating rate to solution heat treatment, by enhancing the number of recrystallisation nucleation sites, ensuring formability, and tolerate a reduced cooling rate by limiting the precipitation nucleation sites, ensuring in-service strength. Another advantage of these alloys is the ease of recyclability which results from the high level of Fe. A further advantage is the greater control of crystallographic texture. The presence of Fe containing intermetallics causes the development of more random textures, as opposed to recrystallisation textures, which develop a more isotropic product which is desired in most forming operations.

In the process of the invention, an aluminium alloy ingot having the above composition is cast by a conventional continuous casting or semi-continuous DC casting method.

The ingot is subjected to homogenisation to improve the homogeneity of solute and to refine the recrystallised grains of the final product. The homogenisation temperature is generally 450°C to 580°C. The ingot time at homogenising temperature is preferably 1 - 24 hours. The optimum temperature is related to the Mg/Si content of the alloy. For an alloy containing 1.0% Si and 0.4% Mg, a preferred homogenising temperature is 480 - 550°C e.g. 520°C. For an alloy containing 1.2% Si and 0.6% Mg, a preferred homogenising temperature is 540 - 580°C, e.g. 560°C. The homogenising temperature is preferably kept down to a level at which a precipitate of dispersed elemental Si is present. Preferably the precipitate is in the sub-micron range or at most a few microns in diameter. This Si precipitate improves formability and helps with control of grain size, without causing corrosion problems. Excess Si is known to improve formability of these alloys - see Anil K Sachder, Metallurgical Transactions Vol 21 A January 1990 pages 165-175 and S J Murtha SAE Technical Papers Series 950718 (International Congress Detroit 27 February - 2 March 1995). It may be assumed that a Si precipitate plays a role in this improved formability. The homogenised ingot is then subjected to hot rolling preferably to 2 - 8 mm, e.g. 2.5 - 6 mm, generally followed by cold rolling. Hot rolling conditions may be conventional. The homogenised ingot may be immediately hot rolled without an intermediate cool to ambient temperature. Or the ingot may be cooled to room temperature following homogenisation and then reheated to the desired rolling temperature. One or more annealing steps may be incorporated in the rolling procedure, either between hot and cold rolling or between cold rolling steps. An optional interanneal is performed preferably at above the recrystallisation temperature of the alloy, preferably at a temperature of 250 - 500°C e.g. 400 - 460°C. After an optional interanneal, final cold rolling is preferably effected under conditions that may be conventional; and in which preferably the sheet thickness is reduced by at least 40%. Cold rolling may be interrupted by a recovery anneal. The amount of final cold rolling is preferably enough to control grain size but not so much as to cause "roping". A sheet thickness reduction of at least 20% e.g. 20 - 40% during final cold rolling is preferred.

The rolled sheet is subjected to solution heat treatment, generally at a temperature of 450°C - 580°C for long enough to solutionise Mg-containing soluble precipitates, preferably 500 - 560°C for less than 60s e.g. less than 30s. Preferably the annealing treatment is carried out at a temperature and for a time that does not dissolve all of the precipitated silicon. The rate of heating for this step may be as high as possible. But high production line speeds, which are particularly desired according to this invention, place a limit on heating rates which are accordingly likely to be in the range of 2 - 40°C per second. There is some correlation between this heating rate and the Fe content of the alloy, with higher Fe contents being appropriate at lower heating rates.

The rolled alloy sheet should be cooled from solution heat treating temperature as fast as possible, consistent with avoiding distortion of the sheet. Although the sheet can in principle be quenched by immersing it in water or spraying water on to a surface, this does tend to cause distortion and is not preferred. Cooling is preferably effected by forced air cooling. At the high production line speeds desired according to this invention, this is likely to result in cooling the sheet at a rate of 2 - 40°/degrees per second, particularly 5 - 30°C per second, down to a temperature not exceeding 200°C.

Immediately after cooling, the sheet may be stretched e.g. by 0.1 - 2.0% for tension levelling.

The resulting solution heat treated sheet may then be subjected to one or a series of subsequent heat treatments in which the sheet is heated to a temperature in the range of 100 - 300°C (preferably 130 - 270°C) and then cooled. In the (or in each) such heat treatment, the metal is preferably heated directly to a peak temperature, held at peak temperature for a period of time less than about 1 minute and then cooled to 85°C or less. This procedure has the effect of maintaining good ductility of the metal in the T4 temper, while maximising the improvement in mechanical properties achieved by age hardening (the paint-bake response). This effect is described in more detail in WO 96/07768. The treatment may conveniently be effected in the course of cleaning, pretreating and pre-priming the rolled sheet. The resulting product has a microstructure with a fine equiaxed grain size. The grain size, which is to some extent dependent on the Mn content of the alloy, is preferably below 50 μm e.g. 30 - 40 μm. The aspect ratio (ratio of grain size in the longitudinal and short transverse directions) is preferably below 2.0, desirably below 1.5.

The Mn dispersoid precipitates typically have a median size of 0.04 - 0.09 μm with a maximum size of 0.20 μm. The relatively low Mn concentration of the alloy results in a rather large dispersoid spacing, of about 0.1 - 1.0 μm typically about 0.5 μm. The resulting rolled sheet is in the solution heat treated T4 temper and has good formability. Formability of 6000 series alloys may be assessed by means of their r/t hem flange performance. Preferred alloy products according to this invention have an r/t ratio below 0.5, particularly below 0.3. The resulting rolled sheet may be formed by conventional means into the shape of components for vehicles. These components are envisaged for use mainly in motor vehicles such as cars, vans, lorries and buses. But other vehicular use of such components is also contemplated. The shaped components are then age hardened, by being heated for a time and to a temperature to achieve dispersion strengthening which is characteristic of 6000 series alloys. For automotive use, the components are generally provided with a protective surface paint coating, which also helps to avoid any corrosion problems that might otherwise result from the presence of dispersed elemental Si. Preferably a thermosetting protective organic paint coating is applied and cured on the surface of the component. The heat treatment required to effect age hardening may be the same heat treatment used to cure the protective organic coating.

Reference is directed to the accompanying drawings in which:- Figure 1 is a diagram of temperature vs time showing a simulation of a continuous heat treatment and anneal (CASH) line incorporating reheat stabilisation steps.

Figure 2 is a bar chart showing grain size and aspect ratio of certain AA6016 alloys.

Figure 3 is a bar chart showing r/t hem flange performance of the same alloys.

Figure 4 is a bar chart showing the effect of Fe content on formability (r/t bend performance) in a commercial scale production trial. A typical temperature profile of rolled sheet on a high speed line is shown in Figure 1. The first temperature peak from the left shows a solution heat treatment followed by a forced air quench down to 100°C and a water quench to ambient temperature. The metal sheet is then subjected to an optional stretch of no more than 2% and usually about 0.2% which takes a few seconds as a routine levelling operation. This is carried out by stretching the strip over specially positioned rolls to remove waviness. Then the strip is subjected to electrocleaning in the course of which it is heated to about 80°C and held at that temperature for a few seconds. Then the cleaned surface is subjected to a pretreatment step which involves heating the sheet to about 130°C and holding it at that temperature for 1 to 2 seconds before again cooling to ambient temperature. Then a primer coating is applied and cured by heating the sheet at about 240°C for up to 10 seconds. Then an ASC (anti stone chip) coating is applied and cured by heating the sheet at about 250°C for up to 10 seconds. Then the coated strip is coiled for transport and sale. The following examples illustrate the invention. EXAMPLE 1

Four alloys were tested whose compositions are set out in the following table.

Si Mg Fe Cu Mn Cr

A 1.01 0.42 0.29 0.09 0.054 0.019

B 1.21 0.41 0.31 0.09 0.090 0.033

C 1.12 0.35 0.28 0.07 0.060 0.036

D 0.98 0.40 0.24 0.08 0.029 0.021

Ingots of these compositions were subjected to the following treatment schedule: homogenised for 17 hours at 520°C and immediately hot rolled to 6 mm thick followed by cold rolling to 1.2 mm. Sheets were then solution treated in a salt bath at 530°C for 1 min and water quenched. Grain size data of the resulting rolled sheet is set out in Figure 2. As can be seen, the grain size of each alloy was below about 25 μm in the longitudinal direction; and below about 20 μm in the short transverse direction. The aspect ratio was distinctly below 1.5 in all cases. Thus all alloys showed the desired fine equiaxed grain structure. Figure 3 shows the formability, measured as r/t bend performance in the longitudinal and transverse directions, of these alloys. In all cases the r/t ratio was well below 0.5, often below 0.25. With an r/t ratio of about 0 in both directions, the alloy D showed outstandingly good formability performance. TEM inspection of the sheets showed dispersed elemental Si to be present as sub-micron particles. EXAMPLE 2

A commercial scale production trial was carried out on two alloys of the following compositions E and F.

Si Fe Cu Mn Mg Cr

E 0.94 0.33 0.08 0.06 0.47 0.03

F 0.93 0.46 0.07 0.05 0.45 0.03

These alloys were subjected to the following treatment schedule. Ingots were DC cast on a large scale unit. One batch of ingots of each composition was homogenised at 520°C for 15 hours. Another batch was homogenised at 550°C for 15 hours. Both batches were then immediately hot rolled on a production mill to a thickness of 6 mm and cold rolled to 1.2 mm. The cold rolled sheet was then continuously annealed at 560°C peak metal temperature for 10 seconds followed by a forced air quench. In order to assess formability, the rolled sheet was subjected to bend testing, either in the T4 condition or after a 5% prestrain. The results are set out in Figure 4, in which the left hand bar chart shows sheet in the T4 condition and the right hand bar chart shows sheet after a 5% prestrain. In each group of four bars:

- The left hand bar shows the alloy E after homogenising at 520°C. - The second bar from the left shows the alloy E after homogenising at 550°C.

- The third bar shows the alloy F after homogenising at 520°C.

- The fourth bar shows the alloy F after homogenising at 550°C In all cases, the r/t ratios are well below 0.5, often below 0.25. This demonstrates that the formability of these alloys is not adversely affected by the unusually high Fe concentrations present.

EXAMPLE 3

An alloy was commercially cast in an ingot 500 mm wide and 275 mm thick with the following composition.

Si Mg Fe Cu Mn Cr

1.21 0.58 0.24 0.09 0.09 0.04

The ingot was homogenised for 4 h at 550°C before being hot rolled to 5.0 mm and cold rolled to 2.0 mm. The coil was batch annealed for 2 hours at 400°C and further cold rolled to the final gauge of 1.0 mm. Samples from this coil were solution treated in the laboratory for 2 minutes in a fluidised bed at 560°C and forced air quenched to simulate the commercial continuous annealing practice.

This material had the following properties.

.T4* T8Xf

0.2% Proof 0.2% Proof

Elongation Stress UTS r/t Stress

(%) (MPa) (MPa)

0° 145.6 279.7 30 0.50 230.4

90° 140.6 270.4 31 0.32

* indicates as solution treated properties t strained 2% and aged 30 minutes at 180°C The example demonstrates that an alloy with a composition containing more Si and Mg is capable of producing very high paint baked strengths combined with acceptable formability.

EXAMPLE 4

Samples of sheet with the following composition

Si Mg Fe Cu Mn Cr

0.94 0.47 0.35 0.07 0.05 0.03

were cast and commercially processed to 1.2 mm gauge by pre-heating the ingots for 17 hours at 520°C hot rolling them to 6.0 mm before cold rolling them to 1.2 mm. Samples were then put through a simulation of a commercial continuous annealing practice using heated steel plates to achieve a heating rate close to that achieved in a commercial line.

Each sample was fitted with a thermocouple to record the temperature and samples were removed from the heating apparatus at temperatures of 350, 475, 500, 520, 540 and 560°C and examined in the transmission electron microscope in order to follow the microstructural evolution during dissolution.

From these microstructural studies it was possible to follow the dissolution of Mg₂Si particles present from the previous casting and thermo-mechanical processing which is complete at 560°C. In all cases larger particles with the morphology recognisable as that of elemental silicon were present. Chemical analysis using energy dispersive x-ray analysis confirmed the particles were silicon. In the final solution treated condition the particles are approximately 0.5 to 1.0 μm in size.

Claims

1. An alloy product of composition (in wt %)

Si 0.8 - 1.5

Mg 0.2 - 0.7

Fe 0.2 - 0.5, preferably 0.3 - 0.5

Mn 0.01 - 0.1 , preferably 0.01 - 0.09

Cu up to 0.25

Cr up to 0.1 , preferably up to 0.05,

Zn up to 0.4

V up to 0.2

Others up to 0.05 each, 0.15 total

Al Balance

said product being in the form of rolled sheet having fine equiaxed grain size and good formability having an r/t hem flange performance of not more than 0.5 in the T4 condition..

2. An alloy product as claimed in claim 1 , wherein the grain size is less than 50 μm and the aspect ratio is less than 2.0.

3. An alloy product as claimed in claim 1 or claim 2 having an r/t hem flange performance of not more than 0.3.

4. An alloy product as claimed in any one of claims 1 to 3, wherein a paint primer coating is present on the surface of the rolled sheet.

5. An alloy product as claimed in any one of claims 1 to 4, wherein a precipitate of dispersed elemental Si is present.

6. An age-hardened component for a vehicle formed from the alloy product of any one of claims 1 to 5. 7. A method of making an alloy product of composition (in wt %):

Si 0.8 - 1.5

Mg 0.2 - 0.

7

Fe 0.2 - 0.5, preferably 0.3 - 0.5

Mn 0.01 - 0.1 , preferably 0.01 - 0.09

Cu up to 0.25

Cr up to 0.1 , preferably up to 0.05,

Zn up to 0.4

V up to 0.2

Others up to 0.05 each, 0.15 total

Al Balance

which method comprises the steps of: casting; homogenising; hot rolling; optional anneal; optional cold rolling; optional recovery anneal; optional final cold rolling solution heat treating; and quenching at a rate of 2 - 40°C/s from solution heat treat temperature down to 200°C.

8. A method as claimed in claim 7, wherein: homogenising is performed at 450 - 580°C, preferably

480 - 520°C, for 1 - 24 hrs; hot rolling is performed down to a sheet thickness of 2.0 - 8 mm; interannealing is optionally performed at 250 - 500°C, preferably 400 - 460°C; solution heat treating is performed at a heating rate of 2 - 40°C/s to a hold temperature of 450 - 580°C; and cooling from solution heat treatment is effected by air quenching.

9. A method as claimed in claim 7 or claim 8, wherein final cold rolling is performed to reduce the sheet thickness by 20 - 40%.

10. A method as claimed in any one of claims 7 to 9, wherein before substantial age hardening has taken place, the rolled sheet is subjected to at least one subsequent heat treatment by being heated to a peak temperature in the range of 100 - 300°C, held at peak temperature for a period of time less than about one minute, and cooled to 85°C or less.