Process for producing an aluminum alloy sheet

Details Process for producing an aluminum alloy sheet Process for producing an aluminum alloy sheet

2000-05-12

2001-06-19

Yee, Deborah (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Heating or cooling of solid metal

C148S694000, C148S697000, C148S551000

Reexamination Certificate

active

06248193

ABSTRACT:

TECHNICAL FIELD
This invention concerns a process for the production of an Al—Mg alloy sheet, which affords enhanced resistance to stress corrosion cracking and improved shape fixability after press.
BACKGROUND ART
Aluminum alloy sheets are light in weight as compared to a steel sheet and have good formability, and therefore, they have today taken the place of the steel sheet in sectors of body sheets for automotive vehicles, skeletal structures, ship components and the like. For its great strength and excellent formability, an alloy of an Al—Mg type (JIS Type 5000 series) has been proposed as typically applicable to the aluminum alloy sheets noted above.
The Al—Mg alloy, however, has the problem that upon lapse of a prolonged period of time after deforming, it tends to cause &bgr; phase (Al
2
Mg
3
) to preferentially precipitate as a form of film in its grain boundary, thus bringing about stress corrosion cracking. Various techniques have been found in solving this problem. For instance, Japanese Unexamined Patent Publication No. 4-187748 discloses a method of the production of an aluminum alloy sheet for automotive use having high resistance to stress corrosion cracking. The method comprises homogenizing an aluminum alloy ingot having Mg in a content of 3.5 to 5.5% by weight, hot-rolling and then cold-rolling the ingot, annealing the resultant sheet, without further cold rolling, and subjecting the annealed sheet to hold for 0.5 to 24 hours at a temperature of 150 to 230° C. As a like instance, JP5-179413A or JP63-255346A discloses a method in which process comprises homogenizing aluminum alloy ingot after casting, hot-rolling and then cold-rolling the ingot, and annealing and slowly cooling the resultant sheet.
In order to improve the shape retention after deforming of an Al—Mg type alloy sheet, namely the shape fixability thereof, it is desired that the proof stress (or 0.2% yield strength) of such sheet be rendered to be as low as possible. To this end, a certain method is known as taught in Japanese Examined Patent Publication No. 6-68146. This prior art method contemplates cold-rolling a hot-rolled sheet or a continuously cast slab of an Al—Mg type alloy containing Mg in an amount of 2 to 6% by weight, and recrystallizing, quenching and solution heat treatment the cold-rolled sheet by means of quick heating and quick cooling, followed by annealing and correction treatment of the resultant sheet. In such method, when the heating temperature after correction is preset to range from 60 to 200° C., heating and cooling is carried out at a rate of 4×10
−3
° C./sec or above. In the case of the heating temperature at from 200 to 360° C., heating and cooling are effected at a rate of 1.225×10
−3
T−0.241° C./sec or more where T denotes the heating temperature, this definition applying as such to the following instances. Alternatively, heat treatment is conducted for 10
5
seconds or less in the case of the heating temperature at from 60 to 160° C., for −5.33×10
5
T+9.5×10
5
seconds or less in the case of the heating temperature at from 160 to 175° C., for −1.65×10 T+4.89×10
4
seconds or less in the of the heating temperature at from 175 to 290° C., and for −7.14 T+3.07×10
3
seconds or less in the case of the heating temperature at from 290 to 360° C. In that way, an aluminum alloy sheet is producible which is suitable for automotive use and has high strength and good formability.
However, the Al—Mg type alloy sheet obtained from continuous casting and rolling with use of the above cited method has the drawback that when heat-treated, it fails to attain sufficient resistance to stress corrosion cracking and adequate reduction in proof stress.
DISCLOSURE OF INVENTION
With the aforementioned defects of the prior art in view, the present invention provides a process for the production of an aluminum alloy sheet that is fabricated from continuous casting and rolling and is excellent in respect of stress corrosion cracking resistance under stress and shape fixability.
Through research made to solve those prior problems and leading to the present invention, it has now been found by the present inventors that as sharply contrasted to a conventional production method of an Al—Mg type alloy sheet, an Al—Mg type alloy sheet fabricated from continuous casting and rolling can be stabilized at a by far higher temperature which is then allowed to drop at a by far slower cooling rate so as to effect cooling so that resistance to stress corrosion cracking may be enhanced, proof stress be reduced, and shape fixability after press be improved. Namely, the Al—Mg type alloy sheet continuously cast and rolled does not undergo homogenization treatment and hence causes Mg to be segregated to a marked extent. This means that sensibility to stress corrosion cracking is conversely objectionably increased by treatment at those heating temperatures and cooling rates commonly known in the art. To be more specific, Mg would presumably get continuously precipitated, as &bgr; phase along the associated grain boundary, at a markedly segregated region at which stress corrosion cracking might take place. This problem can be obviated by application of the process concept found above by the present inventors; that is, &bgr; phase is caused to discontinuously precipitated in an Al—Mg alloy sheet having a small content of Mg and fabricated from continuous casting and rolling. Such specific process leads to high resistance to stress corrosion cracking, small proof stress and good shape fixability after press.
According to the present invention, there is provided a process for the production of an aluminum alloy sheet having enhanced resistance to stress corrosion cracking and improved shape fixability. The process comprises: annealing a continuously cast and rolled sheet of an aluminum alloy having Mg in a content of 3 to 6% by weight; strain-correcting the annealed sheet; heating the corrected sheet at a temperature chosen from a preset temperature zone, the preset temperature zone being defined in such a manner that a rectangular ordinate system is drawn with an abscissa axis of heat treatment temperature (° C.) and an ordinate axis of cooling rate (° C./sec), a heating temperature region being surrounded by connecting a straight line between coordinate (240, 5.0×10
−3
) and coordinate (340, 2.5×10
−3
), a straight line between coordinate (240, 1.0×10
−3
) and coordinate (340, 1.0×10
−3
), a straight line between coordinate (240, 5.0×10
−3
) and coordinate (240, 1.0×10
−3
) and a straight line between coordinate (340, 2.5×10
−3
) and coordinate (340, 1.0×10
−3
), respectively; subjecting the resultant sheet to hold for one hour or more; and subsequently cooling the same at a cooling rate corresponding to the preset temperature zone.


REFERENCES:
patent: 3617395 (1971-11-01), Ford
patent: 0 259 700 (1988-03-01), None
patent: 63-255346 (1988-10-01), None
patent: 4-187748 (1992-07-01), None
patent: 4-276049 (1992-10-01), None
patent: 5-179413 (1993-07-01), None
patent: 0 646 655 (1995-04-01), None

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