Patent ID: 6902632

Claim:
A production of a high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics, formability and impact resistance with tensile strength of 440 MPa or more, characterized in that sequentially carried out are: a hot rolling step wherein a steel slab has a composition containing, by mass %: 0.15% or less of C; 3.0% or less of Mn; 0.02% or less of S; 0.02% or less of Al; and 0.0050 to 0.0250% of N; and furthermore, containing one or two elements of 0.05 to 1.0% of Mo and 0.05 to 1.0% of Cr, and having N/Al of 0.3 or higher, or furthermore, containing one group, or two or more groups of the following e to h: Group e: one, or two or more elements of Si at 0.05 to 1.5%, P at 0.03 to 0.15%, and B at 0.0003 to 0.01%; Group f: one, or two or more elements of Nb at 0.01 to 0.1%, Ti at 0.01 to 0.2%, and V at 0.01 to 0.2%; Group g: one or two elements of Cu at 0.05 to 1.5% and Ni at 0.05 to 1.5%; and Group h: one or two elements of Ca and REM at a total of 0.0010 to 0.010% is heated at a slab heating temperature of 1,000° C. or above and is roughly rolled to form a sheet bar, and the sheet bar is finish rolled at finish rolling delivery-side temperature of 800° C. or above and is coiled at coiling temperature of 750° C. or below to form a hot rolled sheet; a cold rolling step in which the hot rolled sheet is pickled and cold rolled to form cold rolled sheet; and a cold rolled sheet annealing step in which the cold rolled sheet is annealed at a temperature between (Ac 1 transformation point) and (Ac 3 transformation point) for a holding time of 10 to 120 seconds, and is cooled at an average cooling rate that is a critical cooling rate CR defined by the following formula (1) or (2), or above from 600 to 300° C.: when B<0.0003%, log CR=−1.73[Mn+2.67Mo+1.3Cr+0.26Si+3.5P+0.05Cu+0.05Ni]+3.95 (1); and when B≧0.0003%, log CR=−1.73[Mn+2.67Mo+1.3Cr+0.26Si+3.5P+0.05Cu+0.05Ni]+3.40 (2) wherein CR is a cooling rate (° C./s); and Mn, Mo, Cr, Si, P, Cu and Ni are contents of each element (mass %).