Method of continuously heat-treating 7000-series aluminium alloy sheet material

A method for continuously solution heat-treating aluminium alloy sheet by continuously moving heat-treatable 7000-series aluminium alloy sheet through a continuous heat-treatment furnace arranged to heat the moving aluminium sheet to a set soaking temperature (TSET) in the temperature range of 370° C. to 560° C., the continuous heat-treatment furnace has an entry section and an exit section, the moving aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace, and the moving aluminium sheet is rapidly cooled on leaving the exit section, and before or near the entry section of the continuous heat-treatment furnace the moving aluminium sheet is pre-heated to a temperature of 5° C. to 100° C. below the TSET using an average heat-up rate as function of the sheet thickness of at least Y=−31·ln(X)+50, wherein Y is the heat-up rate in ° C./sec and X is the sheet thickness in mm.