Patent ID: 8328963

Claim:
An Al—Mg—Si aluminum alloy sheet: comprising an Al—Mg—Si alloy having the following composition: 0.4-1.0 mass % of Mg, 1.0-1.5 mass % of Si, 0.01-0.15 mass % of Mn, and 0.001-1.0 mass % of Cu, with the remainder being aluminum and inevitable impurities, and having a metallographic structure at the center of its thickness which is observed under a transmission electron microscope of 1,000,000 magnifications, wherein the bright field image contains clusters that appear as dark contrast images and each of the clusters is an aggregate of atoms, and wherein the clusters having an equivalent circle diameter from 1 to 5 nm account for 4000-30000/μm 2 in terms of an average number density, wherein the Al—Mg—Si aluminum alloy sheet is produced by the following method: preparing an ingot of the Al—Mg—Si aluminum alloy, subjecting the ingot to solution heat treatment and subsequent hot rolling, subjecting the resulting hot-rolled sheet to cold rolling, subjecting the cold-rolled sheet to solid solution treatment and subsequent quenching down to room temperature, subjecting the cooled sheet to preliminary aging treatment, wherein the preliminary aging treatment is reheating at a reheating temperature of 90-130° C. within 10 minutes after cooling, and subjecting the reheated sheet to heat treatment during which the reheated sheet is cooled from the reheating temperature at an average cooling rate of 0.5-5° C./hr over a period of 3 hours or longer.