System for preparing aluminum alloy strip having improved formability and bake hardenability

A system for preparing an aluminum alloy strip having improved formability and bake hardenability and a minimized secular change at room temperature, the aluminum alloy consisting essentially of in % by weight, 0.3 to 1.5% of Mg, 0.5 to 2.5% of Si, and the balance of aluminum, said system comprising

• means for continuously unraveling a rolled strip of the aluminum alloy from its coil,
• a continuous heat treatment section for continuously receiving the rolled strip from said unraveling means, heat treating it, and delivering it outward,
• means for continuously winding up the rolled strip exiting from the continuous heat treatment section into a coil form, and
• means for holding the coil of rolled strip at a temperature in the range of 50.degree. to 140.degree. C. for at least 3 hours,
• said continuous heat treatment section including
• a first accumulator for continuously receiving the strip from said unraveling means and feeding it downstream,
• a primary heating zone disposed downstream of said first accumulator for continuously receiving the strip therefrom and heating it to a temperature of at least 480.degree. C.,
• a first cooling zone disposed downstream of said primary heating zone for continuously receiving the strip therefrom and cooling it at a rate of at least 100.degree. C./min. to a temperature of not greater than 100.degree. C., and
• a secondary heating zone disposed downstream of said first cooling zone for continuously receiving the strip therefrom and heating it to a temperature in the range of 120.degree. to 250.degree. C. within 10 minutes from the end of cooling.