Patent ID: 6902632
Filing Date: 2005-06-07
Classification: C21D,C22C

Abstract:
1. 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: 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 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%, when B≧0.0003%,