Aluminum alloy sheet for structural components

Abstract

Disclosed is a 7xxx-series aluminum alloy sheet that has a specific chemical composition and is produced by a common procedure. The sheet maintains high strength via a good balance between Zn and Mg, while the Zn content is controlled. The average compositional ratio of Zn to Mg in grain-boundary precipitates is controlled in the sheet that is naturally aged after production; and the average compositional ratio of Zn to Mg in intragranular precipitates is controlled in the sheet that is further artificially aged after the natural aging. Thus, the aluminum alloy sheet is allowed to have formability, corrosion resistance, and such high strength as to be demanded of structural components.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength aluminum alloy sheet for structural components, where the aluminum alloy sheet offers better workability and still has excellent corrosion resistance. As used herein the term “aluminum alloy sheet” refers to a rolled sheet that is obtained after subjecting a sheet produced via rolling to solution treatment and quenching and subsequently to natural aging at room temperature for 2 weeks or longer and is one before forming into a structural component and before artificial aging. Also as used herein the term “microstructure” of the sheet after natural aging at room temperature refers to the microstructure of the sheet after subjected to natural aging at room temperature for 2 weeks or longer subsequent to solution treatment and quenching.BACKGROUND ART[0002]Social requirements for reduction in automobile body weight have increased more and more in consideration typically of global environment. To meet these requiremen...

AI Technical Summary

Benefits of technology

[0016]The inventors of the present invention selected a 7xxx-series aluminum alloy sheet having a chemical composition in which the Zn content is decreased for better corrosion resistance, but the Mg content is increased for ensuring strength and formability at certain levels; focused attention on grain-boundary precipitates and intragranular precipitates in the microstructure of the sheet after natural aging at room temperature; and analyzed how the chemical compositions of these precipitates affect the properties.

[0017]As a result, the inventors found that the reduction in Zn content (Zn compositional ratio) in the grain-boundary precipitates reduces the amount of Zn to be (uselessly) consumed in the grain-boundary precipitates and allows Zn solute in the matrix to be present in a certain amount, where the gain-boundary precipitates are formed via natural aging at room temperature of the sheet after production. When the Zn solute is allowed to be present in the matrix in a certain amount as above, the amount of Zn, which is necessary to form artificial aging precipitates (intragranular precipitates) upon subsequent artificial aging, can be maximized, and this leads to maximization of BH response (bake hardening response) and balance between strength and formability, even when the Zn content in the alloy chemical composition is lowered.

[0018]The inventors also found that reduction of the Zn content (Zn compositional ratio) in intragranular precipitates, which are aging precipitates formed upon artificial aging, reduces the Zn amount necessary for the formation of the intragranular precipitates (precipitated amount) formed via artificial aging. Thus, the amount of precipitated aging precipitates can be maximized, and this leads to maximization of BH response and balance between strength and formability, even when the Zn content in the alloy chemical composition is lowered, where the aging precipitates contribute to strengthening by artificial aging.

[0019]By controlling the chemical compositions of the precipitates as above, the present invention allows even a 7xxx-series aluminum alloy sheet having a lowered Zn content to have better BH response and better balance between strength and ductility (formability). The resulting 7xxx-series aluminum alloy sheet can be provided as a 7xxx-series aluminum alloy sheet for structural components, which is a rolled sheet produced according to the common procedure, has strength and formability both at satisfactory levels, and still has corrosion resistance such as SCC resistance at excellent levels. The “formability” is hereinafter also referred to as “forming workability” or “workability”.

[0020]The present invention specifies the chemical composition of grain-boundary precipitates formed via natural aging at room temperature. The microstructure measurement with the TEM is therefore performed not on the sheet immediately after the temper (heat treatment) and before natural aging at room temperature, but on the sheet after natural aging at room temperature (standing at room temperature) for 2 weeks or longer as a rough reference and before forming into a structural component and before artificial aging.

Problems solved by technology

The 6xxx-series aluminum alloy sheets, however, are far from the high strength as mentioned above, even when the chemical composition and temper (heat treatments) are controlled.

However, the 7xxx-series aluminum alloys, which are Al—Zn—Mg alloys, have poor general corrosion resistance.

These alloys may therefore possibly cause stress corrosion cracking (the stress corrosion cracking is hereinafter also referred to as “SCC”).

This reduces the features of the alloys as high-strength alloys.

However, the repeated warm rolling is performed because hot rolling and / or cold rolling according to a common procedure fails to give such a metallic texture including small angle grain boundaries in an amount of 25% or more.

Accordingly, the technique is significantly different in process (steps) from the common procedure and is not considered as a practical technique for producing a target sheet.