Aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance

Abstract

Disclosed is an Al—Mg—Si aluminum alloy extrudate which contains, in terms of mass %, 0.60-1.20% Mg, 0.30-0.95% Si, 0.01-0.40% Fe, 0.30-0.52% Mn, 0.001-0.65% Cu, and 0.001-0.10% Ti and in which the contents of Mg and Si satisfy Mg(%)−(1.73×Si(%)−0.25)≧0 and the remainder comprises Al. The extrudate has an equi-axed recrystallized grain texture in which the areal proportion of recrystallized grains is 65% or higher. In examination with a TEM having a magnification of 5,000, intergranular precipitate grains having a size of 1 μm or more in terms of center-of-gravity diameter are apart from one another at an average spacing exceeding 25 μm. The average areal proportion of Goss-orientation grains is less than 8% throughout the whole thickness of this extrudate.

1 . An aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance, being an extrudate of an Al—Mg—Si aluminum alloy comprising, in terms of percent by mass, Mg in a content of from 0.60% to 1.20%, Si in a content of from 0.30% to 0.95%, Fe in a content of from 0.01% to 0.40%, Mn in a content of from 0.30% to 0.52%, Cu in a content of from 0.001% to 0.65%, and Ti in a content of from 0.001% to 0.10%, the contents of Mg and Si satisfying condition: [Mg (%)]−(1.73×[Si (%)]−0.25)≧0, with the remainder including Al and inevitable impurities, wherein the extrudate has an equiaxed recrystallized structure with an area ratio of recrystallized grains of 65% or more in a cross section in a thickness direction, wherein the aluminum alloy extrudate has an average spacing of more than 25 μm between intergranular precipitates each having a size of 1 μm or more in terms of centroid diameter in observation of the structure under a transmission electron microscope (TEM) of 5000 magnifications, and wherein the aluminum alloy extrudate has an average area ratio of Goss orientation grains of less than 8%, throughout the entire thickness region including an outermost grain growth layer in the cross section in the thickness direction of the extrudate. 
     
     
         2 . The aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance according to  claim 1 , further comprising at least one of Cr in a content of from 0.001% to 0.18% and Zr in a content of from 0.001% to 0.18% as replacing part of Mn, and having a total content of Mn, Cr, and Zr of from 0.30% to 0.52%. 
     
     
         3 . The aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance according to  claim 1 , wherein the aluminum alloy extrudate has such bending crush resistance as to have a critical bending radius (R) of 3.0 mm or less without cracking in a 180-degree bending test according to a press-bending method prescribed in Japanese Industrial Standards (JIS) Z2248 in which a plate-shaped specimen is bent in an extrusion direction. 
     
     
         4 . A method for manufacturing an aluminum alloy extrudate excellent in bending crush resistance and corrosion resistance, the method comprising the steps of soaking a cast billet of an Al—Mg—Si aluminum alloy at a temperature of 560° C. or higher, the aluminum alloy having the chemical composition as defined in  claim 1 ; forcedly cooling the soaked cast billet to a temperature of 400° C. or lower at an average cooling rate of 100° C./hr or more; reheating the cooled cast billet to a temperature of 500° C. or higher and subjecting the reheated billet to hot extrusion so that an extrudate reaches a solid solution temperature of 575° C. or higher at an extruder exit; immediately forcedly cooling the extrudate from the extruder exit at an average cooling rate of 5° C./second or more; and subjecting the cooled extrudate to aging so as to have a 0.2% yield strength of 280 MPa or more.