| 摘要 |
PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet having≥780 MPa tensile strength, which has good bendability, elongation, work-hardenability and high yield ratio at high grade. SOLUTION: The cold-rolled steel sheet has the composition composed of 0.08-0.18% C,≤0.5% Si, 1.8-3.0% Mn,≤0.1% P,≤0.01% S, 0.005-0.5% sol.Al,≤0.01% N, 0.05-0.2% Ti and the balance Fe with impurities and≥0.06 C* in the formula (1): C*=C+(12/14)×N-(12/48)×Ti-(12/93)×Nb and has the metallurgical structure composed of ferrite having≤5μm average grain diameter, bainite having≤5μm average grain diameter and retained austenite having≤3μm the longest diameter; and 0.40-0.70 volume ratio Vf of the ferrite and also, satisfying the relation of the formula (2); 0.50≤äC*/(1-Vf)}+ä(Mn+Ni)/6}+(Cr/5)+(Mo/2)≤0.80 and≥0.05 volume ratio Va of the retained austenite, and further, has mechanical characteristics of≥780 MPa tensile strength,≥0.19 work-hardening index (n) between two points of 5% and 10% elongations and also,≥72% yield ratio. COPYRIGHT: (C)2010,JPO&INPIT
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