Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/047—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with magnesium as the next major constituent

Definitions

• 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.
• 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).
• 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
• 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 Mg 2 Si on microstructural features which assist nucleation, such as grain boundaries and the dispersoid particles.
• 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 Mg 2 Si during the quench).
• Very high cold reductions are applied during cold rolling, i.e. 90% cold reduction to achieve the required formability.
• 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.
• 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.
• the invention provides an alloy product of composition (in wt %) Si 0.8 - 1.5
• Mn 0.01 - 0.1 preferably 0.01 - 0.09
• 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.
• 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 Mg 2 Si.
• the Si content is less than about 0.8 wt %, the strength after painting is unsatisfactory.
• 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 Mg 2 Si. 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.
• 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.
• 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 %.
• 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.
• 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.
• 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.
• a preferred homogenising temperature is 480 - 550°C e.g. 520°C.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m e.g. 30 - 40 ⁇ m.
• 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 ⁇ m with a maximum size of 0.20 ⁇ m.
• the relatively low Mn concentration of the alloy results in a rather large dispersoid spacing, of about 0.1 - 1.0 ⁇ m typically about 0.5 ⁇ m.
• 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.
• 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.
• 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.
• 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.
• FIG. 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.
• 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.
• 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.
• a primer coating is applied and cured by heating the sheet at about 240°C for up to 10 seconds.
• an ASC (anti stone chip) coating is applied and cured by heating the sheet at about 250°C for up to 10 seconds.
• the coated strip is coiled for transport and sale.
• 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
• 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.
• An alloy was commercially cast in an ingot 500 mm wide and 275 mm thick with the following composition.
• 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.
• 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.

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Citations (5)

• 1997
  • 1997-09-24 EP EP97941125A patent/EP0931170A1/de not_active Withdrawn
  • 1997-09-24 AU AU43146/97A patent/AU4314697A/en not_active Abandoned
  • 1997-09-24 WO PCT/GB1997/002600 patent/WO1998014626A1/en not_active Application Discontinuation

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