Process for Production of Cold-Rolled Steel Sheet Having Excellent Press Moldability, and Cold-Rolled Steel Sheet

Abstract

Provided are a cold-rolled steel sheet which undergoes a small load at the time of cold-rolling, has excellent press formability and high strength, and a method of manufacturing the cold-rolled steel sheet. A hot-rolled steel sheet having the composition comprising by mass %, 0.10 to 0.30 C, 0.2 or more Mn, 0.01 or more Ni, 0.5 to 2.5 Mn+Ni, 1.2 to 9.0 Cr, and Fe and unavoidable impurities as a balance, and has a tensile strength of 1000 MPa or less is subjected to pickling and, is subjected to cold rolling at a total rolling reduction of 60% or more thus forming a cold-rolled steel sheet. A final continuous annealing treatment is performed at a soaking temperature of 750° C. or above and at a cooling rate of 3° C./s to 100° C./s so that the cold-rolled steel sheet which has a tensile strength of 1280 MPa or more, breaking elongation of 3% or more, and a thickness of 0.05 to 0.60 mm is manufactured.

1 . A method of manufacturing a cold-rolled steel sheet in which a hot-rolled steel sheet which has a composition by mass % comprising:
 0.10 to 0.30 C,   0.2 or more Mn,   0.01 or more Ni,   0.5 to 2.5 Mn+Ni,   1.2 to 9.0 Cr, and   Fe and unavoidable impurities as a balance, and   has a tensile strength of 1000 MPa or less,   
       is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60% or more, thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750° C. or above and at a cooling rate of 3° C./s to 100° C./s, thus manufacturing a cold-rolled steel sheet having a tensile strength of 1280 MPa or more, breaking elongation of 3% or more and a thickness of 0.05 to 0.60 mm. 
     
     
         2 . The method of manufacturing a cold-rolled steel sheet according to  claim 1 , wherein a thickness of the hot-rolled steel sheet is 1.2 to 3.0 mm. 
     
     
         3 . A cold-rolled steel sheet which is manufactured by a method where a hot-rolled steel sheet which has a composition by mass % comprising:
 0.10 to 0.30 C,   0.2 or more Mn,   0.01 or more Ni,   0.5 to 2.5 Mn+Ni,   1.2 to 9.0 Cr, and   Fe and unavoidable impurities as a balance, and   has a tensile strength of 1000 MPa or less, is subjected to pickling and, thereafter, is subjected to cold rolling at a total rolling reduction of 60% or more thus forming a cold-rolled steel sheet, and final continuous annealing treatment is performed at a soaking temperature of 750° C. or above and at a cooling rate of 3° C./s to 100° C./s, wherein the cold-rolled steel sheet has a tensile strength of 1280 MPa or more, breaking elongation of 3% or more and a thickness of 0.05 to 0.60 mm.   
     
     
         4 . The cold-rolled steel sheet according to  claim 3 , wherein a thickness of the hot-rolled steel sheet is 1.2 to 3.0 mm.