| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for working an ultrahigh-strength steel sheet showing superplasticity, without oxidizing the surface, causing the unevenness of the strength, and lowering the strength of the steel sheet after having been worked. SOLUTION: The ultrahigh-strength steel sheet includes, by mass%, 0.02-0.15% C, 0.05-0.7% Mo and 0.03-0.35% Ti; has substantially a single phase structure of ferrite in which precipitates containing Ti and Mo with an average particle diameter of less than 10 nm are dispersed; and has a tensile strength of 780 MPa or higher at room temperature. The working method comprises the steps of: superplastically forming the ultrahigh-strength steel sheet while heating it at 500 to 750°C; and cooling it to room temperature. COPYRIGHT: (C)2007,JPO&INPIT
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