| 摘要 |
PROBLEM TO BE SOLVED: To provide an ultrahigh-strength steel sheet which has remarkably enhanced hydrogen embrittlement resistance in an ultrahigh-strength area in which a tensile strength is 1,180 MPa or more, without impairing the superior ductility which is a characteristic of a TRIP steel sheet. SOLUTION: The ultrahigh-strength steel sheet includes 0.10-0.25% C, 1.0-3.0% Si, 1.0-3.5% Mn, 0.010% or less P, 0.002% or less or 0.004-0.01% S, 1.5% or less Al, 0.003-2.0% Cr and the balance iron with unavoidable impurities, while satisfying [Mn]×1,000[S] of 2.2 or less or 12.5-25, when [Mn] represents an amount of Mn and [S] represents an amount of S; and also contains 1% or more retained austenite by an area rate with respect to the whole structure. Crystal grains of the retained austenite have an average axial ratio (major axis/minor axis) of 5 or more and an average length of the minor axis of 1μm or shorter, and satisfy that the shortest distance between the crystal grains of the retained austenite is 1μm or shorter. The ultrahigh-strength steel sheet also has a tensile strength of 1,180 MPa or more. COPYRIGHT: (C)2011,JPO&INPIT
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