Cold-rolled steel sheet with excellent shape fixability and method of manufacturing the same

Abstract

A steel material having a chemical composition contains 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, whereby a resulting cold-rolled steel sheet has a microstructure dominated by ferrite with an average grain size of 10 μm to 30 μm, a proportional limit of 100 MPa or less, and excellent shape fixability.

1 - 20 . (canceled) 
     
     
         21 . A cold-rolled steel sheet with excellent shape fixability, having a chemical composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; a microstructure dominated by ferrite with an average grain size of 10 μm to 30 μm and a proportional limit of 100 MPa or less. 
     
     
         22 . The cold-rolled steel sheet according to  claim 21 , further containing 0.009% or less Nb on a mass basis in addition to the chemical composition. 
     
     
         23 . The cold-rolled steel sheet according to  claim 21 , further containing 0.06% or less Cr on a mass basis in addition to the chemical composition. 
     
     
         24 . The cold-rolled steel sheet according to  claim 21 , further containing 0.009% or less Nb and 0.06% or less Cr on a mass basis in addition to the chemical composition. 
     
     
         25 . The cold-rolled steel sheet according to  claim 22 , wherein the content of Nb is 0.001% to 0.009% on a mass basis. 
     
     
         26 . The cold-rolled steel sheet according to  claim 23 , wherein the content of Cr is 0.001% to 0.06% on a mass basis. 
     
     
         27 . The cold-rolled steel sheet according to  claim 21 , wherein the B/C is greater than or equal to 0.5 and less than or equal to 5. 
     
     
         28 . The cold-rolled steel sheet according to  claim 27 , wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3. 
     
     
         29 . The cold-rolled steel sheet according to  claim 28 , wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3. 
     
     
         30 . The cold-rolled steel sheet according to  claim 21 , wherein the proportional limit is greater than or equal to 40 MPa and less than or equal to 100 MPa. 
     
     
         31 . The cold-rolled steel sheet according to  claim 21 , wherein the microstructure dominated by ferrite contains 95% or more ferrite in terms of area fraction. 
     
     
         32 . A method of manufacturing a cold-rolled steel sheet with excellent shape fixability, comprising subjecting a steel material to a hot-rolling step, a pickling step, a cold-rolling step, and an annealing step in that order, wherein the steel material has a composition containing 0.0010% to 0.0030% C, 0.05% or less Si, 0.1% to 0.5% Mn, 0.05% or less P, 0.02% or less S, 0.10% or less Al, 0.0050% or less N, 0.021% to 0.060% Ti, and 0.0005% to 0.0050% B on a mass basis such that B/C satisfies 0.5 or more, the remainder being Fe and incidental impurities; the hot rolling step is a step in which the steel material is heated, roughly rolled, finish-rolled at a finishing delivery temperature of 870° C. to 950° C., and coiled at a coiling temperature of 450° C. to 630° C.; the cold-rolling step is a step in which cold rolling is performed at a rolling reduction of 90% or less; and the annealing step is a step in which heating is performed up to a holding temperature in the range of 700° C. to 850° C. at an average heating rate of 1° C./s to 30° C./s in a temperature region not lower than 600° C., retention is performed at the holding temperature for 30 s to 200 s, and cooling is performed at a cooling rate of 3° C./s or more in a temperature region down to 600° C. 
     
     
         33 . The method according to  claim 32 , wherein the chemical composition further contains 0.009% or less Nb on a mass basis. 
     
     
         34 . The method according to  claim 32 , wherein the chemical composition further contains 0.06% or less Cr on a mass basis. 
     
     
         35 . The method according to  claim 32 , wherein the chemical composition further contains 0.009% or less Nb and 0.06% or less Cr on a mass basis. 
     
     
         36 . The method according to  claim 33 , wherein the content of Nb is 0.001% to 0.009% on a mass basis. 
     
     
         37 . The method according to  claim 34 , wherein the content of Cr is 0.001% to 0.06% on a mass basis. 
     
     
         38 . The method according to  claim 32 , wherein the B/C is greater than or equal to 0.5 and less than or equal to 5. 
     
     
         39 . The method according to  claim 38 , wherein the B/C is greater than or equal to 1.0 and less than or equal to 3.3. 
     
     
         40 . The method according to  claim 39 , wherein the B/C is greater than or equal to 1.5 and less than or equal to 3.3.