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.

1 . An Al—Zn—Mg alloy sheet, comprising: in mass percent,
 Zn in a content of 3.0% to 6.0%; 
 Mg in a content of 2.5% to 4.5%; 
 Cu in a content of 0.05% to 0.5%; and 
 wherein, 
 the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.3[Mg]+4.5; 
 grain-boundary precipitates observed with a transmission electron microscope at 60000-total magnification have an average compositional ratio of Zn to Mg of from 0.5 to 3.0 in a microstructure of the sheet subjected to a natural aging at room temperature subsequent to a solution treatment and quenching; 
 intragranular precipitates observed with a transmission electron microscope at 60000-total magnification have an average compositional ratio of Zn to Mg of from 1.5 to 3.5 in a microstructure of the sheet further subjected to one of a two-stage artificial aging and a single-stage artificial aging after the natural aging at room temperature 
 where the two-stage artificial aging comprises:
 a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer; and 
 a second-stage heat treatment at a temperature of from 100° C. to 170° C. for 5 hours or longer, and 
 
 the single-stage artificial aging comprises
 a heat treatment at a temperature of from 100° C. to 150° C. for 12 to 36 hours. 
 
 
     
     
         2 . The Al—Zn—Mg alloy sheet according to  claim 1 ,
 further comprising: in mass percent, at least one of (a) and (b) as follows: 
 (a) at least one element selected from the group consisting of:
 Zr in a content of 0.05% to 0.3%; 
 Mn in a content of 0.1 to 1.5%; 
 Cr in a content of 0.05% to 0.3%; and 
 Sc in a content of 0.05% to 0.3%, and 
 
 (b) at least one element selected from the group consisting of:
 Ag in a content of 0.01% to 0.2%; and 
 Sn in a content of 0.001% to 0.1%. 
 
 
     
     
         3 . The Al—Zn—Mg alloy sheet according to  claim 1 ,
 wherein the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.5[Mg]+5.75, and 
 the sheet after the artificial aging has a 0.2% yield strength of 400 MPa or more. 
 
     
     
         4 . The Al—Zn—Mg alloy sheet according to  claim 2 ,
 wherein the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.5[Mg]+5.75, and 
 the sheet after the artificial aging has a 0.2% yield strength of 400 MPa or more.