Cold-rolled steel sheet with excellent bending workability, method for manufacturing the same, and member using the same

Abstract

A cold-rolled steel sheet with excellent bending workability has a component composition of 0.025% or less C, 0.1% or less Si, 0.05% to 0.5% Mn, 0.03% or less P, 0.02% or less S, and 0.01% to 0.1% sol. Al on a mass basis, the remainder being Fe and unavoidable impurities; a microstructure that is a ferrite rolling texture; a tensile strength TS of 390 MPa or more; a thickness of 0.4 mm or more; and a sheet thickness direction ultimate ductility of 1.3 or more, wherein the sheet thickness direction ultimate ductility is the natural logarithm Ln(t 0 /t 1 ) of the ratio of the thickness t 0 of an untested steel sheet to the thickness t 1 of the fracture surface of the tested steel sheet as determined by a tensile test.

1 . A cold-rolled steel sheet with excellent bending workability, having a component composition of 0.025% or less C, 0.1% or less Si, 0.05% to 0.5% Mn, 0.03% or less P, 0.02% or less S, and 0.01% to 0.1% sol. Al on a mass basis, the remainder being Fe and unavoidable impurities; a microstructure that is a ferrite rolling texture; a tensile strength TS of 390 MPa or more; a thickness of 0.4 mm or more; and a sheet thickness direction ultimate ductility of 1.3 or more, wherein the sheet thickness direction ultimate ductility is the natural logarithm Ln(t 0 /t 1 ) of the ratio of the thickness t 0  of an untested steel sheet to the thickness t 1  of the fracture surface of the tested steel sheet as determined by a tensile test. 
     
     
         2 . A cold-rolled steel sheet with excellent bending workability, having a component composition of 0.025% or less C, 0.1% or less Si, 0.05% to 0.5% Mn, 0.03% or less P, 0.02% or less S, and 0.01% to 0.1% sol. Al on a mass basis, the remainder being Fe and unavoidable impurities; a tensile strength TS of 390 MPa or more, a yield ratio of 80% or more; a thickness of 0.4 mm or more; and a sheet thickness direction ultimate ductility of 1.3 or more, wherein the sheet thickness direction ultimate ductility is the natural logarithm Ln(t 0 /t 1 ) of the ratio of the thickness t 0  of an untested steel sheet to the thickness t 1  of the fracture surface of the tested steel sheet as determined by a tensile test. 
     
     
         3 . A cold-rolled steel sheet with excellent bending workability, having a component composition of 0.025% or less C, 0.1% or less Si, 0.05% to 0.5% Mn, 0.03% or less P, 0.02% or less S, and 0.01% to 0.1% sol. Al on a mass basis, the remainder being Fe and unavoidable impurities; a tensile strength TS of 390 MPa or more; a yield ratio of 80% or more; a thickness of 0.4 mm or more; and a sheet thickness direction ultimate ductility of 1.3 or more, wherein the number of cementite precipitates in the hot-rolled steel sheet is less than 5.0×10 3  per mm 2  and the sheet thickness direction ultimate ductility is the natural logarithm Ln(t 0 /t 1 ) of the ratio of the thickness t 0  of an untested steel sheet to the thickness t 1  of the fracture surface of the tested steel sheet as determined by a tensile test. 
     
     
         4 . The cold-rolled steel sheet with excellent bending workability according to  claim 3 , wherein the hot-rolled steel sheet has an average crystal grain diameter of 25 μm or less. 
     
     
         5 . The cold-rolled steel sheet with excellent bending workability according to  claim 1 , wherein in the component composition, the content of C is 0.0040% or less on a mass basis. 
     
     
         6 . The cold-rolled steel sheet with excellent bending workability according to  claim 1 , wherein the component composition further contains one or two of 0.002% to 0.05% Ti and 0.002% to 0.05% Nb. 
     
     
         7 . The cold-rolled steel sheet with excellent bending workability according to  claim 1 , wherein the component composition further contains 0.0001% to 0.005% B. 
     
     
         8 . The cold-rolled steel sheet with excellent bending workability according to  claim 1 , wherein the tensile strength TS is 490 MPa or more. 
     
     
         9 . A member having a bent portion, using the cold-rolled steel sheet according to  claim 1 . 
     
     
         10 . A method for manufacturing a cold-rolled steel sheet with excellent bending workability, comprising hot-rolling a steel having a component composition of 0.025% or less C, 0.1% or less Si, 0.05% to 0.5% Mn, 0.03% or less P, 0.02% or less S, and 0.01% to 0.1% sol. Al on a mass basis, the remainder being Fe and unavoidable impurities, at a finishing temperature not lower than the Ar 3  transformation point thereof; coiling the hot-rolled steel sheet at a coiling temperature of 500° C. to 650° C.; pickling the hot-rolled steel sheet; and cold-rolling the hot-rolled steel sheet at a rolling reduction of 85% or less such that the cold-rolled steel sheet has a tensile strength TS of 390 MPa or more and a thickness of 0.4 mm or more. 
     
     
         11 . The method for manufacturing the cold-rolled steel sheet with excellent bending workability according to  claim 10 , wherein in the component composition, the content of C is 0.0040% or less on a mass basis. 
     
     
         12 . The method for manufacturing the cold-rolled steel sheet with excellent bending workability according to  claim 10 , wherein the component composition further contains one or two of 0.002% to 0.05% Ti and 0.002% to 0.05% Nb. 
     
     
         13 . The method for manufacturing the cold-rolled steel sheet with excellent bending workability according to  claim 10 , wherein the component composition further contains 0.0001% to 0.005% B. 
     
     
         14 . The method for manufacturing the cold-rolled steel sheet with excellent bending workability according to  claim 10 , further comprising recovery annealing after cold rolling.