Aluminum alloy plate having excellent moldability and bake finish hardening properties

Abstract

An aluminum alloy sheet excellent in terms of formability and bake hardenability is provided. The aluminum alloy sheet contains, in terms of mass %, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities. A differential scanning calorimetry curve of the aluminum alloy sheet has an endothermic peak in a temperature range of 150 to 230° C. and an exothermic peak in a temperature range of 240 to 255° C. The endothermic peak corresponds to a dissolution of a Mg—Si cluster and has a peak height of 8 μW / mg or less, including 0 μW / mg. The exothermic peak corresponds to a formation of a Mg—Si cluster and has a peak height of 20 μW / mg or larger.

Description

TECHNICAL FIELD[0001]The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention means an aluminum alloy sheet that is a rolled sheet such as a hot rolled sheet or a cold rolled sheet and has been subjected to refining such as a solution heat treatment and a quenching treatment, but is not yet subjected to a press forming and a bake hardening treatment. Further, aluminum is hereinafter also referred to as Al.BACKGROUND ART[0002]In recent years, because of environmental awareness and the like, the society's requirement for weight reduction in a vehicle such as an automobile has been steadily increasing. In order to respond to such requirement, as a material for a large body panel structure (an outer panel or an inner panel) of an automobile instead of a steel material such as a steel sheet, application of an aluminum alloy material excellent in formability and bake hardenability and lighter in weight has been increasing.[000...

AI Technical Summary

Benefits of technology

[0031]With regard to the first aspect, Sn exerts such effects in the structure of the Al—Mg—Si alloy sheet that, at room temperature, it captures (traps) atomic holes and thereby inhibits diffusion of Mg and Si at room temperature, inhibits the strength from increasing at room temperature and, during the forming of the sheet into panels, improves the press formability including hem workability, drawability and punch stretch formability (hereinafter, this press formability is referred to also as hem workability as a representative). During an artificial aging treatment of the panels, such as a baking treatment, it releases the captured holes and hence in turn enhances the diffusion of Mg and Si. Consequently, the BH response can be enhanced.

[0032]However, the present inventors have found that the addition of such Sn poses a new problem due to peculiar properties of Sn. Specifically, in the cases when Sn is added and a sheet is produced by an ordinary method, the addition of Sn rather leads, depending on the production conditions, to a decrease in the amount of Mg—Si clusters which contribute to strength. There are hence cases where the addition of Sn results in an insufficient amount of precipitates which precipitate after a bake hardening treatment, making it impossible to obtain the strength required as automotive panels as described above.

[0033]The reason for this is presumed to be because the Sn's effect of capturing and releasing atomic holes is related with the fact that the solid-solution amount of Sn in the matrix is exceedingly small (in an ordinary means, even when the added amount of Sn is controlled to equal to or less than a theoretical solid-solution amount, a large proportion thereof crystallizes out or precipitates as compounds without coming into a solid-solution state). However, this presumption is uncertain.

[0034]In any case, the addition of Sn itself may become meaningless unless problems such as the decrease in the amount of Mg—Si clusters which contribute to strength and the insufficient amount of precipitates which precipitate after a bake hardening treatment are overcome, such problems being regarded as side effects of the addition of Sn.

[0035]Because of this, in the present aspect, the inventors have ventured to reconsider sheet production processes and contrived production conditions concerning, for example, a preliminary aging treatment (reheating treatment) after a solution quenching treatment as will be described later, so that addition of Sn does not result in a decrease in the amount of Mg—Si clusters which contribute to strength or in an insufficient amount of precipitates which precipitate after a bake hardening treatment.

[0036]The inventors have further discovered that a DSC (differential scanning calorimetry curve) of this sheet can be applied as a standard of the structure which can, even when Sn has been added thereto, prevent the Mg—Si clusters that contribute to strength from being diminished and increase or ensure the amount of precipitates that precipitate after a bake hardening treatment. Specifically, in the present aspect, based on the DSC, an endothermic peak corresponding to the dissolution of relatively small Mg—Si clusters, which do not contribute to strength, is controlled and meanwhile an exothermic peak corresponding to the formation of relatively large Mg—Si clusters, which contribute to strength, is enhanced. Thus, Mg—Si clusters that do not contribute to strength are suppressed and the Mg—Si clusters that contribute to strength are increased, thereby obtaining desired BH response.

Problems solved by technology

Here, the 6000-series aluminum alloy had an advantage of having excellent BH response, but had a problem of having aging properties at room temperature, that is, of age hardening during retention at room temperature after solution heat treatment and quenching treatment to increase the strength, thereby deteriorating formability into a panel, particularly the bendability.

Particularly, in the outer panel subjected to severe bending, there was such a problem that, although forming was possible without any problem immediately after manufacturing, cracking occurred in hem working after the lapse of 1 month.

Moreover, in the case where such room-temperature aging is great, a problem also occurs in that the BH response deteriorate and the proof stress is not improved to the strength required as a panel by heating during an artificial aging (hardening) treatment at a comparatively low temperature, such as a bake treatment and the like of the panel after forming described above.

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