Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
1998-06-11
2001-02-27
King, Roy V. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S552000
Reexamination Certificate
active
06193818
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for manufacturing strips less than 5 mm thick in aluminium alloys whose alloying elements are silicon and possibly magnesium, manganese and/or copper, by continuous casting between cooled twin rolls and, if required, cold rolling, these strips offering high mechanical resistance and good formability, intended for mechanical applications, in particular automotive body panel work.
DESCRIPTION OF RELATED ART
Strips in aluminium alloys intended for mechanical applications such as automotive body panel work are customarily produced by semi-continuous casting of plates, hot rolling and cold rolling, with a certain number of intermediate or final heat treatments.
Continuous casting processes may also be used, in particular continuous casting between cooled twin rolls, which offer the advantage of limiting and often avoiding the hot rolling operation, but their operation raises problems for alloys containing large quantities of alloying elements.
U.S. Pat. No. 4,126,486 by ALCAN therefore describes a process for the manufacture of strips in AlSi alloy (Si being between 4 and 15% by weight) with possible additions of Mg, Cu, Zn, Fe and/or Mn, obtained for example by continuous twin roll casting at a speed of >0.25 m/mn, of a strip whose thickness gauge is between 5 and 8 mm, followed by cold rolling at a reduction rate of more than 60% and annealing.
A cast structure is obtained having intermetallic compounds in rod form, which are converted into fine particles by cold rolling, which improves formability.
Japanese patent application JP 62-207851 by SKY ALUMINIUM relates to strips in AlSiMg alloy with Si lying between 0.4 and 2.5%, and Mg between 0.1 and 1.2% (by weight), having a fine intermetallic structure, obtained by continuous casting of a strip with a thickness of between 3 and 15 mm, followed by cold rolling, solution treatment and quenching. These strips may be used for automotive body panel work and other mechanical applications such as air conditioners or gasoline tanks.
This range of alloys comes under the conventional compositions of aluminium alloys in the 6000 series which can be obtained by conventional casting, and it does not make use of the hardening potential of copper and silicon, as their formability is limited by the presence of rough silicon phases which restricts applications thereof.
In general, the production of alloys having a high content of alloying elements by continuous roll casting raises problems, since the presence of intermetallic phases may, at the time of casting, lead to a microstructure that does is unfit for subsequent working. If it is required to obtain strips in aluminium alloys, even with a low alloy element content, that provide both high mechanical resistance and good formability, publications on the subject recommend that substantial force is applied between the rolls in order to obtain a segregate-free microstructure.
P. M. THOMAS and P. G. GROCOK from the DAVY INTERNATIONAL company in their report on “High speed thin strip casting comes of age” presented at the ALUMITECH Congress in Atlanta (USA) on Oct. 26-28, 1994, indicated in this respect that considerable forces of between 0.5 to 1 t per mm width of strip need to be applied to the rolls in order to obtain pure or low-alloy aluminium, which implies the use of barrel-shaped rolls.
According to these authors, the applied forces need to be higher, the thinner the thickness of the cast strip. The effect of applied force on the formation of central segregation is summarized in the article in the form of a diagram reproduced in
FIG. 1
, showing the limits of segregation onset in relation to the force applied and the thickness of the strip. According to the authors, this diagram shows that it is possible to obtain a microstructure free of centre line microstructural defects under all conditions except under relatively low applied forces. With narrow thicknesses, greater specific forces need to be applied so that the structure remains free of segregates.
The paper presented by B. TARAGLIO and C. ROMANOWSKI from the HUNTER ENGINEERING company on “Thin-gauge/High-speed roll casting technology for foil production” at the AIME/TMS Light Metals 95 Congress, indicated that the power of the rolling mill used for continuous roll casting is 3000 t. This value underlines the necessity of using high forces during continuous roll casting. It is evident that a reduction in this force would be of great interest, as it would allow lighter and therefore cheaper equipment to be built.
It is known to men of the art, as shown by the above article, that the operating point of a continuous roll casting machine is determined by three variables: the force exerted by the rolls on the strip (expressed in tonnes per metre of strip width), the thickness of the strip on exiting the roll mill (in mm) and casting speed (in m/mn). Any two of these variables may be adjusted independently and, for each operating point thus defined, it is the quality of the product obtained and the efficiency of the machine which determine the industrial advantage of the process.
To summarize, according to the state of the art, an operating point must be sought in the area of strong force, all the more so when the alloy has a high alloying content. Also, it is ascertained that up until now, no such high content alloys have been manufactured by continuous roll casting. This is shown for example by the list of alloys given in table 1 of the previously mentioned article by B. TARAGLIO et al., which lists those which can be cast using the casting machine he describes.
SUMMARY OF THE INVENTION
It came to the notice of the inventors that, unlike the teaching of the prior art, the use of an operating point corresponding to a low level of force between the rolls, led in surprising manner to an improvement in the microstructural quality of the cast strips compared with strips that were cast using higher forces, and allowed thin strips to be obtained in alloys containing silicon, magnesium, manganese and/or copper, in particular AlSiMg and AlSiMgCu alloys which up until now could be not be obtained by continuous casting, and which moreover offered strong mechanical properties and good formability
The object of the invention is therefore a process for the manufacture of strips in aluminium alloy having high mechanical resistance and good formability, entailing:
the preparation of an aluminium alloy containing (by weight) from 0.5 to 13% of Si, from 0 to 2% Mg, and/or from 0 to 1% manganese, and/or from 0 to 2% Cu, and/or from 0 to 2% Fe, the other elements being less than 0.5% each and 2% overall.
continuous twin roll casting of this alloy between 2 cooled rolls to obtain a strip whose thickness lies between 1.5 and 5 mm,
possible cold rolling of this strip to a thickness of less than 2 mm,
process in which the operating point, in a diagram whose X axis is the thickness of the strip (in mm) and whose Y-axis is the specific force applied to the rolls (in t per metre of cast strip width), is located below the straight line AB, preferably below the straight line A
1
B
1
, A, B, A
1
and B
1
having the co-ordinates:
A
1.5 mm
750 t/m
A
1
1.5 mm
700 t/m
B
5 mm
500 t/m
B
1
5 mm
300 t/m
The process may possibly also comprise annealing of the cast strip, before rolling, at a temperature of between 420 and 600° C. depending upon the alloy composition, and also heat treatment of the rolled strip by solution treatment at between 420 and 600° C., quenching and artificial ageing at a temperature of <300° C.
The invention preferably applies to alloys having a (weight %) composition of:
Si: 2.6-13; Mg: 1.4-2; Cu <2; Fe <0.4 (and, preferably, <0.25); Mn<0.5.
REFERENCES:
patent: 4126486 (1978-11-01), Morris et al.
patent: 5286315 (1994-02-01), Iwayama et al.
patent: 5503689 (1996-04-01), Ward et al.
patent: 5571347 (1996-11-01), Bergsma
patent: 5655593 (1997-08-01), Wyah-Mair et al.
patent: 2291284 (1976-06-01), None
patent: 62-207851 (1987-09-01), None
patent: 9
Gehanno Herve
Jarry Philippe
Legresy Jean-Marc
Menet Pierre Yves
Schmidt Martin Peter
Dennison, Scheiner Schultz & Wakeman
King Roy V.
McGuthry-Banks Tima
Pechiney Rhenalu
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