Metal treatment – Process of modifying or maintaining internal physical... – With casting or solidifying from melt
Reexamination Certificate
2001-10-31
2003-11-25
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
With casting or solidifying from melt
C148S691000, C148S695000
Reexamination Certificate
active
06652678
ABSTRACT:
External closure sheet panels for automotive applications require a high degree of surface finish including the absence of surface roughening due to forming operations. AA6000 sheet is prone to a phenomenon called roping, which is the effect seen from macroscopic surface undulations caused by stretching during pressing. Conventional routes to prevent this phenomenon, i.e. to provide roping-free sheet, involve a recrystallisation anneal either before or between cold rolling passes and can be performed either by a batch or a continuous process. These processes are costly in terms of both time and energy. Additionally, the introduction of an annealing step can adversely influence the ability to solution heat-treat at final gauge, thus lowering the attainable strength before and after paint bake.
It is known that certain aluminium alloys (not including 6000 series alloys) can be subjected to hot rolling under conditions which cause them to be self-annealing, that is to say, to recrystallise without the need of a specific recrystallisation annealing step. This invention concerns the treatment of 6000 series alloys in such a way as to make hot-rolled sheet self-annealing.
In one aspect the invention provides a method of converting an ingot of a 6000 series aluminium alloy to self-annealing sheet, which method comprises subjecting the ingot to a two-stage homogenisation treatment, the first stage being at a temperature of at least 560° C. and the second stage at a temperature of 450° C. to 480° C., and then hot-rolling the homogenised ingot at a starting hot roll temperature of 450° C. to 480° C. and a finishing hot roll temperature of 320° C. to 360° C. The hot-rolled sheet is caused to be self-annealing by a careful control of treatment conditions, as discussed in more detail below, and also by control over the alloy composition. Preferred alloy composition is (in wt %)
Si
0.3-1.8 preferably 0.9-1.3
Fe
up to 0.5 preferably 0.15-0.4
Mg
0.30-1.5 preferably 0.35-0.50
Cu
up to 0.3 preferably up to 0.2
Mn
0.03-0.2 preferably 0.04-0.10
Cr
up to 0.35 preferably 0.01-0.15
Others
up to 0.05 each and 0.15 total
Al
balance.
Alloys containing a high copper content would not show satisfactory self-annealing properties. Hence Cu is preferably kept at a low level. During homogenisation of the ingot, Mn-containing dispersoids coarsen and these coarsened dispersoids later contribute to the self-annealing properties of the hot-rolled sheet. For this effect to be notable, the Mn content of the alloy needs to be at least 0.03 or 0.04% by weight. At Mn contents above 0.1 or 0.2 weight % the recrystallisation temperature increases to a level impractical to attain in hot rolling. Cr is preferably included in the alloy in order to keep Mn in a finely dispersed form. Other alloy components, e.g. Si, Fe and Mg, may be present at concentrations usual for AA6000 alloys for they do not have any major effect on the self-annealing properties described herein.
Alloy of the required composition is cast into ingots, typically by d.c. casting although the casting technique is not material to the invention. Ingots are subjected to a two-stage homogenisation, the first stage being at a temperature of at least 560° C., preferably at least 570° C. for at least one hour. A maximum homogenisation temperature is set by the need to avoid re-melting the ingot, and is for practical purposes 590° C. Mn is present as dispersoids and a major purpose of this high-temperature homogenisation is to coarsen the dispersoids, e.g. to a mean D
C
(equivalent diameter) of at least 0.25 &mgr;m, to an extent that they enhance recrystallisation at a later stage. Homogenisation time and temperature should be chosen with this in mind.
In the second stage of homogenisation, the ingots are brought to a temperature of 450° C. to 480° C., preferably 460° to 480° C. Ingots may be cooled from first stage homogenisation to ambient temperature and then re-heated, or more preferably may simply be cooled from first stage to second stage homogenisation temperature. Ingots cooled from first stage homogenisation to below hot rolling temperature should preferably be reheated to at least 500° C., in order to re-solutionise Mn dispersoids, prior to cooling to the second homogenisation temperature of 450° C. to 480° C. The ingots should be brought into thermal equilibrium at the second stage homogenisation temperature, which is not otherwise metallurgically significant.
The homogenisation ingots are then hot rolled at a starting hot roll ingot temperature of 450° C. to 48
0
0C, preferably 460° C. to 480° C., and a finishing hot roll ingot temperature of 320° C. to 360° C., preferably 330° C. to 350° C. Preferably hot rolling is performed in two stages. In a first stage, an ingot is passed repeatedly forwards and backwards through a breakdown mill to reduce the thickness to 30 to 50 mm. This first stage is typically performed under substantially isothermal conditions, and the resulting slab preferably has a temperature of 430° C. to 470° C. If the slab is too cold, it may be unrollable in the next stage. If the slab is too hot, it may be difficult to roll fast enough to achieve the desired final hot rolled sheet microstructure.
A second hot rolling stage typically involves passage through a three or four or five stand Tandem mill. Typically passage through each stand cools the slab by 40° C. to 50° C., but in the current invention this is reduced by high speed rolling of a relatively cold slab. Preferably there is at least a 90% thickness reduction during this second hot-rolling stage with preferably (to encourage recrystallisation) a larger than average reduction in the last stand. Preferably the thickness reduction in the last stand is greater than in the immediately preceding stand e.g. is at least 45%.
Energy imparted during this Tandem mill rolling stage should be enough to cause recrystallisation, but not so much that significant recovery takes place between rolling passes.
The hot rolled sheet exits the last stand at a temperature of 320° C. to 360° C. preferably 330° C. to 350° C. If the exit temperature is either too high or too low, then recrystallisation may not take place due to a lack of either stored energy or thermal energy, respectively. The hot rolled sheet is coiled and allowed to cool to ambient temperature.
Recrystallisation typically takes place during the early stages of cooling, while the sheet is still above 270° C. to 290° C. The hot rolled sheet typically has a thickness of 2 to 4 mm. It is then cold-rolled down to a desired final thickness, under conditions which may be conventional except that no recrystallisation anneal is required either before or during cold rolling (although a recovery anneal or recrystallisation anneal. is not excluded). The cold rolled sheet is subjected to solution heat treatment under conditions which may be conventional, is optionally lubricated or coated, and may then be coiled or cut to length.
The as hot rolled sheet constitutes another aspect of this invention. It is in a recrystallised state and has a texture characterised by a Cube recrystallisation component lower than that found in an alloy of the same composition that has been given a recrystallisation anneal after hot rolling. Preferably the Cube recrystallisation component of the invention product is at least 3 volume % less than that of a comparable product produced by a conventional process. For example, in the alloy used in the experimental section below, the invention product had a Cube component of 29.0 volume %, where the conventional product had a Cube component of 35.9 to 37.4 volume % (see Table 2).
The sheet which has been hot rolled, cold rolled and then solution heat treated, constitutes another aspect of the invention which may be defined in different ways. Preferably the sheet has a texture in which the combined volume % of the Brass (Bs) and Cu and S recrystallisation components is at least 1.5 times the combined volume % of the Cube and Goss recrystallisation components. Products according to the invention are substantially more
Hamerton Richard
Heinze Martin
Herbst Karl Albert
Marshall Graeme John
Alcan International Limited
Combs Morillo Janelle
Cooper & Dunham LLP
Wyszomierski George
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