TECHNICAL FIELD
[0001] 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
[0002] 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. To be useful in automobile
applications, 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. At the
same time, 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.
[0003] Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000 series are
usually considered for automotive panel applications. The 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.
[0004] To facilitate understanding, a brief explanation of the terminology used to describe
alloy tempers may be in order at this stage. 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. For example, 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. Alternatively, 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.
[0005] 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).
[0006] An alloy that has only been solution heat-treated and artificially aged to peak strength
is said to be in the T6 temper.
[0007] It has been observed that conventionally-produced 6000 series alloy sheet articles
exhibit a good paint-bake response immediately after quenching, but this response
declines somewhat upon natural aging. It would therefore be advantageous to produce
alloy sheet materials of the 6000 series that avoid this decline in the paint bake
response and maintain a high yield strength in the T8X temper.
[0008] In our US patent no. 5,616,189 issued April 1, 1997, we have described processes
for producing aluminum alloy sheet having T4 and potential T8X tempers. These processes
involve heat treatments and controlled cooling. However, there is a need for an alternative
process, since the disclosed process is not always highly convenient.
DISCLOSURE OF THE INVENTION
[0009] 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.
[0010] 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.
[0011] According to one aspect of the invention, there is provided 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 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. about 20°C) to 100°C at a rate of 20 to 30°C (preferably
about 25°C) per minute; and (3) further
[0012] cooling the alloy sheet article having a temperature of 55°C or more after step (2)
to ambient temperature at a rate of less than 2°C per hour.
[0013] For step (3) the sheet article would normally be coiled at the indicated temperature
following step (2) and allowed to cool at the rate indicated for step (3). This final
step (3) brings about an artificial pre-aging of the alloy.
[0014] 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.
[0015] 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 multistep cooling.
[0016] The alloy may be any AA6000 series aluminum alloy and is most preferably an alloy
having the following composition by weight:
Cu |
0 to 1.0% |
Mg |
0.4 to 1.1% |
Si |
0.3 to 1.4% |
Fe |
0.1 to 0.4% |
Mn |
0 to 0.45% |
Al |
balance. |
[0017] Optionally, the preferred alloy may also contain small amounts of Zr, Cr and/or Ti
not exceeding 0.15% in total.
[0018] The most preferred alloy is alloy AA 6111 which has the following composition by
weight (Al forms the balance):
|
Cu |
Fe |
Mg |
Mn |
Si |
Ti |
Cr |
Max |
0.9 |
0.40 |
1.0 |
0.45 |
1.1 |
0.10 |
0.10 |
Min |
0.5 |
0.1 |
0.5 |
0 |
0.6 |
- |
- |
[0019] 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.
[0020] The exact mechanism explaining why the required controlled cooling works is not yet
clear. However, without wishing to be bound to any particular theory, it is currently
believed that 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. In
conventional material, the nuclei formed during natural aging become unstable and
dissolve during subsequent aging at high temperature. As a result, 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
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; and
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.
BEST MODES FOR CARRYING OUT THE INVENTION
[0022] According to a preferred embodiment of the present invention, 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. 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] According to the present invention, 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.
[0027] As noted above, 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. During primary cooling, the material is cooled to an intermediate temperature
between 150 and 250°C at rates typically used in a commercial continuous heat treatment
line. In the secondary stage, the material is then naturally cooled to below 85°C,
and optionally coiled and then coil-cooled to room temperature.
[0028] It should be noted that the use of 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.
[0029] The invention is illustrated in more detail by the following Examples, which are
not intended to limit the scope of the present invention.
EXAMPLE 1 - Laboratory Study
[0030] Commercially-produced AA6111 alloy sheets containing (by weight): 0.74% Cu, 0.24%
Fe, 0.79% Mg, 0.13% Mn, 0.60% Si, 0.06% Ti, 0.05% Cr and balance Al were used in the
following laboratory study.
[0031] 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.
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.
[0032] The primary cooling conditions, to obtain different intermediate cooling temperatures
(ICTs), were determined by performing several calibration runs. A tensile sample having
a thickness of 1.0 mm, with an embedded thermocouple, was heated in a fluidized bed
(sand bed) to 558°C, held for 30 seconds and cooled in forced air to room temperature
(RT). 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.
[0033] To study the effect of the primary cooling conditions on tensile properties, a number
of samples were cooled to a variety of ICTs ranging from 100 to 250°C and then naturally
cooled to room temperature (secondary cooling). The samples were solutionized at 560°C
and cooled in forced air, for a predetermined period of time, to obtain the desired
ICT and further cooled to room temperature in still air. The samples took a maximum
of 8 minutes to cool down to room temperature.
[0034] Additional heat treatments were also performed to determine the combined effect of
controlled cooling and warm coiling temperatures. Samples were solutionized, cooled
to the intermediate cooling temperature between 150 and 250°C and then cooled in still
air. The samples were then subjected to a simulated pre-aging treatment by heat treating
at 60, 80 or 100°C for 5 hours.
[0035] One week later, duplicate tensile tests were performed in the T4 and T8X (2% stretch
plus 30 minutes at 177°C) tempers. Similar results were also obtained without prior
natural aging to examine the stability of the tensile properties. The properties of
the material of this invention were then compared with those of the conventionally
produced material.
Results
Effects of Primary cooling to Intermediate Cooling Temperature (ICT)
[0036] Table 1 below summarizes the average tensile properties of AA6111 in different tempers.
[0037] In the Table:
U.T.S. means "ultimate tensile strength";
Y.S. means "yield strength";
%El. means percentage elongation;
ksi means kilopounds per square inch; and
MPa means megapascals.
[0038] Figure 3 shows the variation in yield strength (YS) as a function of intermediate
cooling temperature. In the Figure:
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.
[0039] From the Figure, the following observations can be made:
(i) The yield strength of the alloy immediately after the heat treatment is ∼82.7
to 89.6 MPa (∼12 to 13 ksi). These values are close to that of as-quenched conventional
material.
(ii) The yield strength in the T4 temper does not vary with the changes in ICT, Figure
3, Curve (a).
(iii) The Yield strength in the T8X temper, Figure 3, Curve (d), immediately after
cooling shows a significantly different aging behavior. The conventionally cooled
material shows -275.8 MPa (∼40 ksi) yield strength at ICT's below 150°C. Beyond this
temperature, the yield strength increases and reaches a value close to 299.9 MPa (43.5
ksi) at 250°C.
(iv) After one week of natural aging, the yield strength of the material is ∼132.4
MPa (∼19.2 ksi), Curve (b) in Figure 3. Like Curve (a), the yield strength in the
T4 temper is independent of the ICT.
(v) As expected, natural aging causes a loss in yield strength in the T8X temper.
The loss of strength is related to the ICT. For example, the material shows 206.8
MPa (30 ksi) yield strength when primary cooling is carried out to <150°C. Higher
values are achieved provided the 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).
Relationship Between ICT and Pre-aging Conditions:
[0040] The Yield strength of the material is increased significantly when controlled cooling
is followed by a pre-aging step (Figure 3). Generally, 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). These data clearly suggest
that the higher pre-aging temperatures are generally better to achieve high strength,
especially for the ICT of 250°C. It should be noted that the choice of higher pre-aging
temperature between 60 and 100°C has only a minor influence on the T8X properties,
Curve (d) in Figure 3 and Table 1.
Summary
[0041] Clearly, there is considerable gain in the paint-bake response if the material is
cooled between 150 and 250°C in primary cooling and then very slowly cooled to a temperature
less than 100°C.
1. A process of heat treating a sheet article made of a 6000 series aluminum alloy, in
which the alloy sheet article is heated at a solutionizing temperature, and then cooled;
characterized in that the 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 4°C per second;
(2) further cooling the alloy to a temperature in the range of ambient to 100°C at
a rate in the range of 20 to 30°C per minute; and
(3) further cooling the alloy sheet article having a temperature of 55°C or more after
step (2) to ambient temperature at a rate of less than 2°C per hour.
2. A process according to claim 1, characterized in that said cooling step (1) is carried out at a rate greater than or equal to 225°C per
second.
3. A process according to claim 1, characterized in that said cooling step (2) is carried out at a temperature of about 25°C per minute.
4. A process according to claim 1, characterized in that said alloy sheet article is coiled at said temperature following step (2) prior to
said cooling of step (3).
5. A process according to claim 1,
characterized in that the process is carried out on an alloy having the following composition by weight:
Cu |
0 to 1.0% |
Mg |
0.4 to 1.1% |
Si |
0.3 to 1.4% |
Fe |
0.1 to 0.4% |
Mn |
0 to 0.4% |
Zr and/or Cr and/or Ti |
0 to 0.15% |
Al |
balance. |
6. A process according to claim 1,
characterized in that the process is carried out on an alloy having the following composition by weight:
|
Cu |
Fe |
Mg |
Mn |
Si |
Ti |
Cr |
Max |
0.9 |
0.40 |
1.0 |
0.45 |
1.1 |
0.10 |
0.10 |
Min |
0.5 |
0.1 |
0.5 |
0 |
0.6 |
- |
- |
7. A process according to claim 1, characterized in that the process is carried out on alloy AA6111.
8. A process according to claim 1, characterized in that said alloy sheet article subjected to said solutionizing is produced by casting an
ingot, scalping said ingot, homogenizing said ingot at a temperature in the range
of 480 to 580°C, hot/cold rolling the ingot to form a sheet article of intermediate
gauge, and cold rolling the sheet article of intermediate gauge to final gauge.
9. A process according to claim 8, characterized in that said intermediate gauge sheet article is subjected to an inter annealing operation.
1. Verfahren zur Wärmebehandlung eines Blechartikels aus einer Aluminiumlegierung der
6.000er Serie, worin der Legierungsblechartikel auf eine Auflösungstemperatur erwärmt
und anschliessend abgekühlt wird;
dadurch gekennzeichnet, dass die Kühlung des Artikels die folgenden Schritte einschliesst:
(1) Abkühlen von der Lösungstemperatur auf eine Temperatur im Bereich von 150-250°C
mit einer Geschwindigkeit von ≥ 4°C/sek;
(2) weiteres Abkühlen der Legierung auf eine Temperatur im Bereich von Umgebungstemperatur
bis 100°C mit einer Geschwindigkeit im Bereich von 20-30°C/min; und
(3) weiteres Abkühlen des Legierungsblechartikels, der nach dem Schritt (2) eine Temperatur
von 55°C oder mehr aufweist, auf Umgebungstemperatur in einer Geschwindigkeit von
weniger als 2°C/std.
2. Verfahren gemäss Anspruch 1, dadurch gekennzeichnet, dass der Kühlschritt (1) mit einer Geschwindigkeit von ≥ 225°C/sek durchgeführt wird.
3. Verfahren gemäss Anspruch 1, dadurch gekennzeichnet, dass der Kühlschritt (2) bei einer Temperatur von etwa 25°C/min durchgeführt wird.
4. Verfahren gemäss Anspruch 1, dadurch gekennzeichnet, dass der Legierungsblechartikel bei der Temperatur nach Schritt (2) vor dem Abkühlen gemäss
Schritt (3) aufgewickelt wird.
5. Verfahren gemäss Anspruch 1,
dadurch gekennzeichnet, dass das Verfahren mit einer Legierung durchgeführt wird, die die folgende Gewichtszusammensetzung
aufweist:
Cu |
0 bis 1,0 % |
Mg |
0,4 bis 1,1 % |
Si |
0,3 bis 1,4 % |
Fe |
0,1 bis 0,4 % |
Mn |
0 bis 0,4 % |
Zr und/oder Cr und/oder Ti |
0 bis 0,15 % |
Al |
Rest |
6. Verfahren gemäss Anspruch 1,
dadurch gekennzeichnet, dass das Verfahren mit einer Legierung mit der folgenden Gewichtszusammensetzung durchgeführt
wird:
|
Cu |
Fe |
Mg |
Mn |
Si |
Ti |
Cr |
max |
0,9 |
0,40 |
1,0 |
0,45 |
1,1 |
0,10 |
0,10 |
min |
0,5 |
0,1 |
0,5 |
0 |
0,6 |
-- |
-- |
7. Verfahren gemäss Anspruch 1, dadurch gekennzeichnet, dass das Verfahren mit einer AA6111-Legierung durchgeführt wird.
8. Verfahren gemäss Anspruch 1, dadurch gekennzeichnet, dass der Legierungsblechartikel, der der Auflösungsbehandlung unterworfen wird, hergestellt
wird durch Giessen eines Barrens, Abschälen des Barrens, Homogenisieren des Barrens
bei einer Temperatur im Bereich von 480-580°C, Heiss/Kaltwalzen des Barrens unter
Bildung eines Blechartikels mit Zwischendicke, und Kaltwalzen des Blechartikels mit
Zwischendicke auf Enddicke.
9. Verfahren gemäss Anspruch 8, dadurch gekennzeichnet, dass der Blechartikel mit Zwischendicke einer Zwischentemperung unterworfen wird.
1. Procédé de traitement à chaud d'un article en tôle composé d'un alliage d'aluminium
série 6000, dans lequel l'article en tôle d'alliage est chauffé à une température
de solution, et ensuite refroidi ;
caractérisé en ce que le refroidissement de l'article comprend les étapes suivantes consistant à :
(1) refroidir à partir de ladite température de solution dans la plage comprise entre
150 et 250°C à un taux supérieur ou égal à 4°C par seconde ;
(2) refroidir davantage l'alliage à une température dans la plage comprise entre la
température ambiante et 100°C à un taux dans la plage comprise entre 20 et 30°C par
minute ; et
(3) refroidir davantage l'article en tôle d'alliage présentant une température de
55°C ou plus après l'étape (2) à la température ambiante à un taux inférieur à 2°C
par heure.
2. Procédé selon la revendication 1, caractérisé en ce que ladite étape de refroidissement (1) est réalisée à un taux supérieur ou égal à 225°C
par seconde.
3. Procédé selon la revendication 1, caractérisé en ce que ladite étape de refroidissement (2) est réalisée à une température d'environ 25°C
par minute.
4. Procédé selon la revendication 1, caractérisé en ce que ledit article en tôle d'alliage est enroulé à ladite température suite à l'étape
(2) avant ledit refroidissement de l'étape (3).
5. Procédé selon la revendication 1,
caractérisé en ce que le procédé est réalisé sur un alliage présentant la composition en poids suivante
:
Cu |
0 à 1,0 % |
Mg |
0,4 à 1,1 % |
Si |
0,3 à 1,4 % |
Fe |
0,1 à 0,4 % |
Mn |
0 à 0,4 % |
Zr et/ou Cr et/ou Ti |
0 à 0,15 % |
Al |
complément. |
6. Procédé selon la revendication 1,
caractérisé en ce que le procédé est réalisé sur un alliage présentant la composition en poids suivante
:
|
Cu |
Fe |
Mg |
Mn |
Si |
Ti |
Cr |
Max. |
0,9 |
0,40 |
1,0 |
0,45 |
1,1 |
0,10 |
0,10 |
Min. |
0,5 |
0,1 |
0,5 |
0 |
0,6 |
- |
- |
7. Procédé selon la revendication 1, caractérisé en ce que le procédé est réalisé sur un alliage AA6111.
8. Procédé selon la revendication 1, caractérisé en ce que ledit article en tôle d'alliage soumis à ladite solution est produit en coulant un
lingot, en scalpant ledit lingot, en homogénéisant ledit lingot à une température
dans la plage comprise entre 480 et 580°C, en laminant à chaud/à froid le lingot pour
former un article en tôle d'épaisseur intermédiaire, et en laminant à froid l'article
en tôle d'épaisseur intermédiaire en un calibre final.
9. Procédé selon la revendication 8, caractérisé en ce que ledit article en tôle d'épaisseur intermédiaire est soumis à une opération de recuit
intermédiaire.