Classifications

• • 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
  • C22F1/05—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 of the Al-Si-Mg type, i.e. containing silicon and magnesium in approximately equal proportions
• • 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
  • C22F1/043—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 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 relates to sheet articles made of heat-treatable aluminum alloys suitable for the fabrication, for example, of automotive skin panels. More particularly, the invention relates to a method of producing sheet articles of this kind in such a way as to minimize disadvantageous effects caused by natural aging of the articles.
• BACKGROUND ART The automotive industry, in order to reduce the weight of automobiles, has increasingly substituted aluminum alloy panels for steel panels. Lighter weight panels, of course, help to reduce automobile weight, which reduces fuel consumption, but the introduction of aluminum alloy panels creates its own set of needs.
• an aluminum alloy sheet product must possess good forming characteristics in the "as-received" (by the automobile manufacturer) T4 temper condition, so that it may be bent or shaped as desired without cracking, tearing or wrinkling.
• the alloy panel after the painting and baking (paint- bake) carried out by the automobile parts manufacturer, must have sufficient strength to resist dents and withstand other impacts.
• AA6000 series alloys contain magnesium and silicon, both with and without copper but, depending upon the Cu content, may be classified as AA2000 series alloys. These alloys are formable in the T4 temper condition and become stronger after painting and baking in the so-called T8X temper (i.e., they exhibit a "paint-bake response" or increase in yield strength) .
• a particularly preferred alloy of this kind is alloy AA6111.
• the temper referred to as T4 is well known (see, for example, Aluminum Standards and Data (1984), page 11, published by The Aluminum Association) and refers to alloy produced in the conventional manner (solutionizing followed by quenching and natural aging for 48 hours or more) .
• This is the temper in which automotive sheet panels are normally delivered to parts manufacturers for forming into skin panels and the like.
• the commercial fabrication of conventional AA6111 sheets in the T4 temper involves solutionizing (subjected to a solution heat treatment) the cold-rolled material between 530 and 560°C in a continuous annealing furnace, rapidly cooling the alloy to a temperature between 35 and 45°C and then naturally aging the alloy for two days or more before subjecting the product to the usual finishing operations.
• the material can be solutionized and coiled between 55 and 85°C and then coil-cooled to room temperature before being subjected to the finishing operations .
• the material produced in this manner performs similarly to the conventional T4 temper sheet in forming and tensile tests and shows a significant improvement in the paint bake response .
• Such a material produced by the alternative heat treatment step is internally referred as T4P temper product.
• T8 temper designates an alloy that has been solution heat-treated, cold worked and then artificially aged. Artificial aging involves holding the alloy at elevated temperature (s) over a period of time.
• T8X temper refers to a condition where T4 material has been stretched by 2% and given an artificial aging at 170°C for 20 minutes or 177°C for 30 minutes (simulating commercial forming and paint-bake) .
• An object of the present invention is to produce alloy sheet articles of 6000 series aluminum alloy exhibit a desirable paint bake response and a high yield strength in the T8X temper.
• Another object of the invention is to provide a heat treatment process that reduces or avoids the reduction of paint bake response of 6000 series aluminum alloys.
• a process of heat treating a sheet article made of a 6000 series aluminum alloy comprising heating the alloy sheet article at a solutionizing temperature followed by cooling the alloy sheet article; wherein said cooling of the article includes the following steps: (1) cooling from said solutionizing temperature to a temperature in the range of 150 - 250 °C at a rate greater than or equal to about 4°C per second (preferably greater than or equal to 225°C per second); (2) further cooling the alloy to a temperature in the range of ambient (room temperature - e.g.
• step (3) brings about an artificial pre-aging of the alloy. If the alloy is solutionized in a continuous annealing line (CAL) involving a heating section and a quenching section, cooling steps (1) and (2) would normally require controlled cooling within a furnace to ensure the required slower cooling rate. Step (3) may be carried out in a conventional storage area of a production facility provided the ambient temperature guarantees the desired slow cooling rate.
• CAL continuous annealing line
• the process of the invention may form part of a process for the continuous production of alloy sheet article involving casting, homogenizing and hot and cold rolling prior to the indicated solutionizing and multi- step cooling.
• the alloy may be any AA6000 series aluminum alloy and is most preferably an alloy having the following composition by weight:
• the preferred alloy may also contain small amounts of Zr, Cr and/or Ti not exceeding 0.15% in total.
• alloy AA 6111 which has the following composition by weight (Al forms the balance) :
• the process of the present invention creates a sheet article that exhibits an improved paint-bake response (increase in yield strength from the T4 temper following painting and baking) compared to an identical alloy produced by the conventional solutionizing, rapid quenching and natural aging procedure, by reducing the tendency of natural aging to reduce this response.
• the exact mechanism explaining why the required controlled cooling works is not yet clear.
• the controlled cooling of steps (1) and (2) allows the formation of stable nuclei which promote the precipitation of fine coherent particles, homogeneously distributed in the alloy matrix, during the artificial aging (pre-aging) step.
• the nuclei formed during natural aging become unstable and dissolve during subsequent aging at high temperature.
• particle distribution in the matrix is coarse and this causes reduced strengthening in the T8X temper compared with that expected from the material produced according to the method of the present invention.
• FIG. 1 is schematic illustration of steps carried out according to a preferred embodiment of the process of the present invention
• Fig. 2 is a chart showing heating and cooling curves of samples as explained in the following Example 1;
• Fig. 3 is a chart showing variations of yield strength as a function of intermediate cooling temperature of samples as explained in the following Example 1.
• the alloy is direct chill cast, scalped, homogenized between 480 and 580 °C for less than 48 hours, hot/cold rolled to an intermediate gauge, cold rolled to the final gauge, solution heat treated between 480 and 580°C in a continuous heat treatment (CASH) furnace, rapidly cooled in the required controlled manner, coiled at a temperature of less than about 85°C, and is then cooled to room temperature.
• CASH continuous heat treatment
• the material is then normally subjected to various finishing operations including leveling to obtain a flat sheet for forming into parts. Panels formed from the material of this invention will acquire higher strength during the paint cure than conventional AA6111-T4 alloy sheet material. Alloys of the resulting temper are referred to internally as T4CC.
• the processing may also include an inter annealing operation between the hot rolling and the final cold rolling operation to produce roping-free high strength sheet products (for example, as described in co-pending PCT Application Serial No. PCT/CA98/00109 filed February 17, 1998, published on August 27, 1998 as WO 98/37251; the disclosure of which is incorporated herein by reference) .
• FIG. 1 of the accompanying drawings An alternative preferred process according to the present invention, involving twin-belt casting, is shown in simplified schematic form in Fig. 1 of the accompanying drawings.
• Continuous metal strip 10 of series 6000 alloy (preferably AA6111) is cast in a twin belt caster 11 and subjected to hot rolling at rolling station 12. During this rolling step, some precipitates form.
• the hot rolled product is coiled to form coil 14.
• the hot rolled strip 10 is then unwound from coil 14, subjected to cold rolling in cold roll mill 15 and coiled to form coil 16.
• the cold rolled strip 10 is then unwound from coil 16 and subjected to a continuous solution heat treatment and controlled quenching at station 17 to re-solutionize and precipitate constituent particles, and is then coiled at to form coil 18.
• the solution heat treatment by means of which precipitated alloying ingredients are re-dissolved in the alloy, generally involves heating the alloy sheet material to a temperature of between about 500°C and about 570°C (preferably about 560°C) .
• the improved quenching or cooling process of the invention is then carried out.
• the coiled strip 18 is in T4 temper and may be sold to an automobile manufacturer or parts manufacturer for fabrication by forming panels 20 from the strip by deformation followed by painting and baking the panels to form painted panels 22 in T8X temper.
• materials having the properties of the known T4P product can also be produced by controlling the cooling conditions immediately after solutionizing in the indicated way. In fact, the aging response is significantly improved when controlled cooling is combined with warm coiling between 55 and 80°C.
• the controlled cooling from the solutionizing temperature is performed in two stages, which are referred to as steps (1) and (2) or as the primary and secondary cooling steps.
• steps (1) and (2) the material is cooled to an intermediate temperature between 150 and 250 °C at rates typically used in a commercial continuous heat treatment line.
• the secondary stage the material is then naturally cooled to below 85°C, and optionally coiled and then coil-cooled to room temperature .
• heat treatment process of this invention can be readily carried out in long continuous annealing furnaces so that the material can be solutionized, cooled to an intermediate temperature between 150 and 250°C and further cooled slowly to allow formation of stable nuclei.
• the alloy had previously been solutionized at 560°C in a continuous annealing and solution heat treatment (CASH) line, quenched in cold water and stored at room temperature.
• CASH continuous annealing and solution heat treatment
• Several samples prepared from this material were re-solutionized by heating to 558°C in a fluidized bed, and then cooled in forced air in two stages to simulate the primary and secondary cooling operations (first and second cooling steps) of the present invention.
• the primary cooling conditions to obtain different intermediate cooling temperatures (ICTs) , were determined by performing several calibration runs.
• Figure 2 shows the heating and cooling characteristics of the sample. Such experiments were repeated several times and the heating and cooling curves of the sample were found to be highly reproducible.
• the cooling curve in Figure 2 was used to determine the time to reach various ICTs.
• the secondary cooling conditions were simulated by cooling from the ICT to room temperature or a pre-aging temperature in still air.
• U.T.S. means "ultimate tensile strength"
• Y.S. means “yield strength”
• %E1. means percentage elongation; ksi means kilopounds per square inch; and
• MPa means megapascals.
• FIG. 3 shows the variation in yield strength (YS) as a function of intermediate cooling temperature.
• Curve (a) shows the yield strength values after cooling
• Curve (b) shows the yield strength values after one week at room temperature
• Curve (c) shows the yield strength values after one week at room temperature in the T8X temper; and Curve (d) shows the yield strength values after cooling in the T8X temper.
• ICT is increased to a maximum of 250°C. At this temperature, the material shows 256.5 MPa (37.2 ksi) yield strength, which is close to 25% higher than the conventionally produced material (Table 1) .
• the Yield strength of the material is increased significantly when controlled cooling is followed by a pre-aging step (Figure 3).
• a lower ICT gives lower strength in both tempers.
• the strength is increased slightly with an increase in the pre-aging temperature and the absolute T8X Yield strength is increased by up to 20.7 MPa (3 ksi) for the material which was cooled to 250°C (Table 1) .

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Citations (2)

• 1999
  • 1999-07-07 WO PCT/CA1999/000618 patent/WO2000003052A1/en not_active Application Discontinuation
  • 1999-07-07 DE DE69921146T patent/DE69921146T2/de not_active Expired - Fee Related
  • 1999-07-07 CA CA002336687A patent/CA2336687C/en not_active Expired - Fee Related
  • 1999-07-07 EP EP99928955A patent/EP1100977B1/de not_active Revoked
  • 1999-07-07 JP JP2000559267A patent/JP2002520486A/ja active Pending
  • 1999-07-07 AT AT99928955T patent/ATE279547T1/de not_active IP Right Cessation
  • 1999-07-07 BR BR9912522-6A patent/BR9912522A/pt active Search and Examination
• 2001
  • 2001-01-05 NO NO20010079A patent/NO20010079L/no not_active Application Discontinuation

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