Aluminum alloy sheet for motor vehicle and process for producing the same

Abstract

An aluminum alloy sheet for motor vehicles is produced by casting a melt, containing 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, and less than 0.1 mass % Mn, a balance substantially being inevitable impurities and Al, into a slab having a thickness of 5 to 15 mm in a twin-belt caster so that cooling rate at ¼ depth of thickness of the slab is 20 to 200° C./sec; winding the cast thin slab into a coiled thin slab subjected to cold rolling with a roll having a surface roughness of 0.2 to 0.7 μm Ra at a cold rolling reduction of 50 to 98%; subjecting the cold rolled sheet to final annealing either continuously in a CAL at a holding temperature of 400 to 520° C. or in a batch annealing furnace at a holding temperature of 300 to 400° C.; and subjecting the resulting sheet to straightening with a leveler.

1 . An aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability, the sheet comprising:
 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, and further a limited amount of less than 0.1 mass % Mn; and   a balance substantially being inevitable impurities and Al;   the sheet having an intermetallic compound maximum size of 5 μm or less by circle-equivalent diameter in a region at ¼ depth of the sheet thickness, an average recrystallized grain size of 15 μm or less, a surface roughness of 0.2-0.6 μm Ra, a yield strength of 145 MPa or less, and a tensile strength of 225 MPa or more.   
     
     
         2 . The aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability according to  claim 1 , wherein the aluminum alloy sheet has a punch stretch forming height of 29 mm or more. 
     
     
         3 . The aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability according to  claim 1 , wherein the aluminum alloy sheet further comprises 0.001-0.1% Ti. 
     
     
         4 . A process for producing an aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability, the process comprising:
 casting a melt comprising 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15 mass % Si, further a limited amount of less than 0.1 mass % Mn, and a balance substantially being inevitable impurities and Al, into a thin slab having a thickness of 5 to 15 mm in a twin-belt caster so that the cooling rate at ¼ depth of a thickness of the thin slab is 20 to 200° C./sec;   winding the cast thin slab into a coiled thin slab;   subjecting the coiled thin slab to cold rolling with a roll having a surface roughness of 0.2 to 0.7 μm Ra at a cold rolling reduction of 50 to 98% to produce a cold rolled thin sheet;   subjecting the cold rolled thin sheet to final annealing continuously in a CAL at a holding temperature of 400 to 520° C.; and   subjecting the resulting sheet to straightening with a leveler.   
     
     
         5 . The process for producing the aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability according to  claim 4 , wherein the cold rolled thin sheet is subjected to final annealing in a batch annealing furnace at a holding temperature of 300 to 400° C. 
     
     
         6 . The aluminum alloy sheet for motor vehicles excellent in press formability, resistance to surface roughening and shape fixability according to  claim 2 , wherein the aluminum alloy sheet further comprises 0.001-0.1% Ti.