| 摘要 |
PROBLEM TO BE SOLVED: To produce a high tensile strength cold rolled steel sheet excellent in ductility and strain age hardening characteristics and further excellent in impact resistance and to provide its producing method. SOLUTION: A thin cold rolled steel sheet having a composition containing 0.05 to 0.30% C, 0.4 to 2.0% Si, 0.7 to 3.0% Mn, <=0.08% P, <=0.02% Al and 0.0050 to 0.0250% N and an N/Al ratio satisfying >=0.3 is heated at (Ac1 transformation point) to (Ac3 transformation point + 50 deg.C), is then cooled at least in the range of 600 to 500 deg.C at a cooling rate of 5 to 150 deg.C/s and is held in the temperature range of 350 to 500 deg.C. In this way, the steel sheet is provided with excellent ductility and strain age hardening characteristics attainingΔTs of >=50 MPa, and its impact resistance is improved.
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