Patent ID: 6899771

Claim:
A high yield ratio high tensile strength cold rolled steel sheet having excellent strain age hardening characteristics with tensile strength of 440 MPa or higher and a yield ratio of 0.7 or above, characterized in that the sheet has a composition containing, by mass %: 0.15% or less of C; 2.0% or less of Si; 3.0% or less of Mn; 0.08% or less of P; 0.02% or less of S; 0.02% or less of Al; 0.0050 to 0.0250% of N; and 0.007 to 0.04% of Nb; having 0.3 or more of N/Al and 0.0010% or more of N in a solid solution state, and furthermore containing deposited Nb at 0.005% or more, and having the balance of Fe and inevitable impurities; and that the steel sheet has a structure containing a ferritic phase having an average crystal grain size of 10 μm or less at an area ratio of 50% or more, and mainly pearlite as a residual portion.