Ferritic lightweight steel sheet having excellent strength and ductility and method for manufacturing the same

Abstract

A ferritic steel sheet according to an exemplary embodiment of the present invention includes C at 0.01 to 0.3 wt %, Mn at 0.5 to 8 wt %, Al at 5 to 12 wt %, and Nb at 0.015 to 0.2 wt % based on an entire composition of 100 wt %, and a remaining part of Fe and an impurity.

What is claimed is: 
     
         1 . A ferritic steel sheet including C at 0.01 to 0.3 wt %, Mn at 0.5 to 8 wt %, Al at 5 to 12 wt %, and Nb at 0.015 to 0.2 wt % based on an entire composition of 100 wt %, and a remaining part of Fe and an impurity,
 wherein an average grain size of a ferrite crystal existing in the steel sheet is 30 μm or less.   
     
     
         2 . The ferritic steel sheet of  claim 1 , wherein:
 the average grain size of the ferrite crystal is 15 μm or less.   
     
     
         3 . The ferritic steel sheet of  claim 1 , wherein:
 the ferritic steel sheet includes Si at 0.04 to 2.0 wt %, Cr at 2.0 wt % or less (0% is not included), Mo at 1.0 wt % or less (0% is not included), Ni at 1.0 wt % or less (0% is not included), Ti at 0.1 wt % or less (0% is not included), V at 0.2 wt % or less (0% is not included), B at 0.01 wt % or less (0% is not included), Zr at 0.2 wt % or less (0% is not included), or a combination thereof based on the entire composition of 100 wt %.   
     
     
         4 . The ferritic steel sheet of  claim 1 , wherein:
 a κ-carbide of a spherical shape, an oval shape, an acicular shape, or a band shape existing inside the ferritic steel sheet is included.   
     
     
         5 . The ferritic steel sheet of  claim 4 , wherein:
 the κ-carbide at 1 to 10 vol % is included based on the entire 100 vol % of the steel sheet.   
     
     
         6 . The ferritic steel sheet of  claim 4 , wherein:
 a particle size of the κ-carbide is in a range of 20 nm to 10 μm, and   the κ-carbide is present in the range of 5×10 3  to 1×10 6  particles per unit area (mm 2 ).   
     
     
         7 . The ferritic steel sheet of  claim 1 , wherein:
 a NbC compound existing inside the ferritic steel sheet is included.   
     
     
         8 . The ferritic steel sheet of  claim 7 , wherein:
 the NbC compound at 0.1 to 1 vol % is included based on the entire 100 vol % of the steel sheet.   
     
     
         9 . The ferritic steel sheet of  claim 7 , wherein:
 a particle size of the NbC compound is in the range of 10 nm to 1 μm, and   the NbC compound is present in the range of 5×10 4  to 3×10 5  particles per unit area (mm 2 ).   
     
     
         10 . The ferritic steel sheet of  claim 1 , wherein:
 a content of Al is in the range of 10 to 12 wt %.   
     
     
         11 . A method for manufacturing a ferritic steel sheet, comprising:
 heating a slab including C at 0.01 to 0.3 wt %, Mn at 0.5 to 8 wt %, Al at 5 to 12 wt %, and Nb at 0.015 to 0.2 wt % based on an entire composition 100 wt %, and a remaining part of Fe and an impurity;   hot rough-rolling the heated slab;   cold-rolling the steel sheet of which the hot rough rolling is completed; and   annealing the cold-rolled steel sheet of which the cold rolling is completed.   
     
     
         12 . The method of  claim 11 , wherein:
 a heating temperature is in a range of 1000 to 1250° C. in the step of heating the slab.   
     
     
         13 . The method of  claim 11 , wherein:
 a temperature is in the range of 700 to 1250° C. in the step of hot rough rolling the slab.   
     
     
         14 . The method of  claim 11 , further comprising:
 warm-rolling the slab at a temperature of 600 to 850° C. after the hot rough rolling.   
     
     
         15 . The method of  claim 14 , further comprising:
 intermediate-annealing the slab at a temperature of 700 to 900° C. after the warm rolling.   
     
     
         16 . The method of  claim 15 , further comprising:
 warm-rolling the slab in a temperature of 600 to 850° C. after the intermediate annealing.   
     
     
         17 . The method of  claim 11 , further comprising:
 hot rolling the slab in a temperature of 1000 to 1250° C. after the hot rough rolling.   
     
     
         18 . The method of  claim 17 , further comprising:
 intermediate annealing the slab at a temperature of 700 to 900° C. after the hot rolling.   
     
     
         19 . The method of  claim 11 , wherein:
 in the step of the cold-rolled sheet annealing, the cold-rolled sheet annealing temperature is in a range of 650 to 900° C.   
     
     
         20 . The method of  claim 11 , wherein:
 the slab includes Si at 0.04 to 2.0 wt %, Cr at 2.0 wt % or less (0% is not included), Mo at 1.0 wt % or less (0% is not included), Ni at 1.0 wt % or less (0% is not included), Ti at 0.1 wt % or less (0% is not included), V at 0.2 wt % or less (0% is not included), B at 0.01 wt % or less (0% is not included), Zr at 0.2 wt % or less (0% is not included), or a combination thereof based on the entire composition of 100 wt %.