| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength cold-rolled steel sheet having excellent formability for an extension flange while retaining characteristics equivalent to those of a conventional DP steel; also to provide a hot-dip galvanized steel sheet having the same formability; and a method for manufacturing those. SOLUTION: The high-strength cold-rolled steel sheet comprises 0.03-0.15% C, 1.4-3.5% Mn, 0.05% P or less, 0.01% S or less, 0.15% Al or less, 0.01% N or less, 0.005-0.05% Ti, 0.005-0.04% Nb, 0.0003-0.0020% B, and the balance Fe with unavoidable impurities; and has a structure that contains a ferrite phase and a low-temperature transforming austenite phase in which the area ratio A<SB>ML</SB>/A<SB>M</SB>is 0.30 or higher, where the A<SB>ML</SB>is an area of the low-temperature transforming austenite phase having a grain size 1.5 times larger than the average crystal grain size d<SB>α</SB>of the ferrite phase, and the A<SB>M</SB>is the total area of the low-temperature transforming austenite phase. The hot-dip galvanized steel sheet has the same composition and structure. COPYRIGHT: (C)2007,JPO&INPIT
|
