| 摘要 |
PROBLEM TO BE SOLVED: To provide a low yield ratio and high strength steel sheet whose elongation in a tensile test and hole expansibility are made compatible with each other, and to provide its production method. SOLUTION: The steel controlled to be (%Ti)/(%S)>=5 (wherein, %S and %Ti respectively denote S and Ti contents) is hot-rolled at the finish outlet side temperature of (900+50×%Si) deg.C or lower and is then annealed at 700 to 900 deg.C being a two phase coexistent temperature region of ferrite and austenite to form a metallic structure in which the total volume ratio of martensite and retained austenite is >=6%, and the hard phase structure of martensite, retained austenite and bainite are contained by fixed ratios in accordance with the amounts of precipitation strengthening elements such as Ti, Nb, Mo and V to be added.
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