| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tension cold-rolled steel sheet having an ultrafine grain structure and excellent strength-elongation balance and toughness among mechanical characteristics. SOLUTION: The structure consists of a composition containing, in steel components, particularly C, Si, Mn, Ni, Ti, and Nb within a range satisfying the following formulas (1) (2), (3), respectively, and the balance Fe and inevitable impurities, in which the fraction of the ferrite phase is≥65 vol%, and the average crystal grain size of the ferrite is≤3.5μm. Further, the structure has a martensite phase at≥3 vol% in the fraction with respect to the entire part of the structure, where 637.5+4,930äTi*+(43/93)×[%Nb]}≥A<SB>1</SB>, (1), A<SB>3</SB>≤860 (2), [%Mn]+[%Ni]≥1.3 (3), where Ti<SP>*</SP>=[%Ti]-(48/32)×[%S]-[48/14]×[%N], A<SB>1</SB>: a predicted value(°C) of A<SB>1</SB>traformation point caliculated by a caliculating formula, A<SB>3</SB>: a predicted value (°C) of Ag transformation point caliculated by a caliculating formula. COPYRIGHT: (C)2009,JPO&INPIT
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