The strain age hardening die steel sheet and its production mannered null mass which are superior in resistance extension degradation Characteristic, normal temperature resistance to aging and low temperature printing hardening property at

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