| 摘要 |
PROBLEM TO BE SOLVED: To provide a strain aging hardening type steel sheet which has excellent cold elongation deterioration resistance, cold delayed aging properties and low temperature bake hardenability, and to provide its production method. SOLUTION: The steel sheet has a composition comprising, by mass,≤0.2% C,≤0.7% Si,≤3.0% Mn,≤0.1% P,≤0.02% S, 0.0030 to 0.0180% N, 0.2 to 1.5% Cr,≤0.020% Al and 0.0020 to 0.02% O, wherein Cr/N is≥25, and further comprising≤0.0040% solid solution C, 0.0020 to 0.0085% solid solution N so as to include 0.0020 to 0.012% solid solution C+12/14 (solid solution N), and the balance Fe with inevitable impurities. In the steel sheet, the area ratio of ferrite is≥80%, the ratio between BH170, which is the bake hardening quantity after 5% tensile deformation+150°C×20 min heat treatment, and BH 150, which is the bake hardening quantity after 5% tensile deformation+150°C×20 min heat treatment, i.e., BH 150/BH170, is≥0.9, the BH150 is≥40 MPa, the reduction of the total elongation after 100°C×1 hr aging is≤3%, and the increase of the yield point elongation is≤0.6%. COPYRIGHT: (C)2005,JPO&NCIPI
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