Aluminum alloy sheet excellent in formability and hardenability during baking of coating and method for production thereof

Abstract

A sheet of a 6000 type aluminum alloy containing Si and Mg as main alloy components and having excellent formability sufficient to allow flat hemming, excellent resistance to denting, and good hardenability during baking a coating, which exhibits an anisotropy of Lankford values of more than 0.4 or the strength ratio for cube orientations of the texture thereof of 20 or more, and exhibits a minimum bend radius of 0.5 mm or less at 180° bending even when the offset yield strength thereof exceeds 140 MPa through natural aging; and a method for producing the sheet of the aluminum alloy, which comprises subjecting an ingot to a homogenization treatment, cooling to a temperature lower than 350° C. at a cooling rate of 100° C./hr or more, optionally to room temperature, heating again to a temperature of 300 to 500° C. and subjecting it to hot rolling, cold rolling the hot rolled product, and subjecting the cold rolled sheet to a solution treatment at a temperature of 400° C. or higher, followed by quenching.

1 . An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, the aluminum alloy sheet having a minimum inner bending radius of 0.5 mm or less by 180° bending after 10% tensile stretching, even if yield strength of the aluminum alloy sheet exceeds 140 MPa through natural aging after a solution heat treatment and quenching.  
     
     
         2 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 1 , which comprises 0.5-1.5% (mass %, hereinafter the same) of Si and 0.2-1.0% of Mg, with the balance consisting of Al and impurities.  
     
     
         3 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 1 , which comprises 0.5-1.5% of Si and 0.2-1.0% of Mg, with the balance consisting of Al and impurities, in which the maximum diameter of Mg—Si compounds is 10 μm or less and the number of Mg—Si compounds having a diameter of 2-10 μm is 1000 per mm 2  or less.  
     
     
         4 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 2  or  3 , which further comprises 0.1-0.3% of Zn.  
     
     
         5 . An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises 0.8-1.2% of Si, 0.4-0.7% of Mg, and 0.1-0.3% of Zn, with the balance consisting of Al and impurities, in which the maximum diameter of Mg—Si compounds is 10 μm or less and the number of Mg—Si compounds having a diameter of 2-10 μm is 1000 per mm 2  or less, the aluminum alloy sheet having a minimum inner bending radius of 0.2 mm or less by 180° bending after 10% tensile stretching, even if yield strength of the aluminum alloy sheet exceeds 140 MPa through natural aging after a solution heat treatment and quenching.  
     
     
         6 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  1 - 5 , which further comprises at least one of 0.3% or less (excluding 0%, hereinafter the same) of Mn, 0.3% or less of Cr, 0.2% or less of V, and 0.15% or less of Zr.  
     
     
         7 . An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises 0.4-1.5% of Si, 0.2-1.2% of Mg, and 0.05-0.3% of Mn, with the balance consisting of Al and impurities, in which the percentage of crystal grain boundaries in which misorientation of adjacent crystal grains is 15° or less is 20% or more.  
     
     
         8 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 7 , which further comprises 0.5% or less of Zn.  
     
     
         9 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 7  or  8 , which further comprises at least one of 0.3% or less of Cr, 0.2% or less of V, and 0.15% or less of Zr.  
     
     
         10 . An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, in which an anisotropy of Lankford values is more than 0.4.  
     
     
         11 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 10 , which comprises 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2%, and Si %−0.58Mg %≧0.1% being satisfied and the balance consisting of Al and impurities.  
     
     
         12 . An aluminum alloy sheet with excellent formability and paint bake hardenability, which comprises Si and Mg as major alloy components, in which an intensity ratio of cube orientation of crystal graphic texture is 20 or more.  
     
     
         13 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 12 , which comprises 0.5-2.0% of Si and 0.2-1.5% of Mg, with 0.7Si %+Mg %≦2.2% being satisfied and the balance consisting of Al and impurities.  
     
     
         14 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 11  or  13 , which further comprises 0.5% or less of Zn.  
     
     
         15 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  11 ,  13 , and  14 , which further comprises at least one of 1.0% or less of Mn, 0.3% or less of Cr, 0.2% or less of V, and 0.2% or less of Zr.  
     
     
         16 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  2 - 9 ,  11 , and  13 - 15 , which further comprises 1.0% or less of Cu.  
     
     
         17 . The aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  2 - 9 ,  11 , and  13 - 16 , which further comprises at least one of 0.1% or less of Ti and 50 ppm or less of B.  
     
     
         18 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  2 - 6 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  2 - 6 ,  16 , and  17  at a temperature of 450° C. or more, cooling the ingot to a temperature of 350-500° C. at a cooling rate of 100° C./h or more, starting hot rolling of the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         19 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  2 - 6 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  2 - 6 ,  16 , and  17  at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         20 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  2 - 6 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  2 - 6 ,  16 , and  17  at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 350-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         21 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  7 - 9 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  7 - 9 ,  16 , and  17  at a temperature of 480° C. or more, cooling the ingot to a temperature of 300-450° C. at a cooling rate of 150° C./h or more, starting hot rolling of the ingot at the temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         22 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  7 - 9 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  7 - 9 ,  16 , and  17  at a temperature of 480° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 150° C./h or more, heating the ingot to a temperature of 300-450° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         23 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  7 - 9 ,  16 , and  17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  7 - 9 ,  16 , and  17  at a temperature of 480° C. or more, cooling the ingot to a temperature of less than 300° C. at a cooling rate of 150° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 300-450° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 500° C. or more, and quenching.  
     
     
         24 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  11  and  13 - 17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  11  and  13 - 17  at a temperature of 450° C. or more, cooling the ingot to a specific temperature of less than 350° C. at a cooling rate of 100° C./h or more, hot rolling the ingot at the specific temperature, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.  
     
     
         25 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  11  and  13 - 17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  11  and  13 - 17  at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.  
     
     
         26 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  11  and  13 - 17 , the method comprising homogenizing an ingot of an aluminum alloy having the composition defined in any one of claims  11  and  13 - 17  at a temperature of 450° C. or more, cooling the ingot to a temperature of less than 350° C. at a cooling rate of 100° C./h or more, cooling the ingot to room temperature, heating the ingot to a temperature of 300-500° C. and starting hot rolling of the ingot, cold rolling the hot-rolled product, and subjecting the cold-rolled product to a solution heat treatment at a temperature of 450° C. or more, and quenching.  
     
     
         27 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  18 - 26 , wherein the hot rolling is finished at a temperature of 300° C. or less.  
     
     
         28 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to any one of claims  18 - 27 , comprising quenching the solution-treated product to 120° C. at a cooling rate of 5° C./s or more, and subjecting the quenched product to a heat treatment at a temperature of 40-120° C. for 50 hours or less within 60 minutes after the quenching.  
     
     
         29 . A method for producing the aluminum alloy sheet with excellent formability and paint bake hardenability according to  claim 28 , comprising subjecting the heat-treated product to a reversion treatment at a temperature of 170-230° C. for 60 seconds or less within seven days after the heat treatment.