| 摘要 |
PROBLEM TO BE SOLVED: To provide an ultrahigh-strength steel sheet superior in hydrogen-embrittlement resistance. SOLUTION: The ultrahigh-strength steel sheet superior in hydrogen-embrittlement resistance comprises, by mass, 0.10-0.25% C, 1.0-3.0% Si, 1.0-3.5% Mn, 0.15% or less P, 0.02% or less S, 1.5% or less (excluding 0%) Al, further 0.003-0.5% Cu and/or 0.003-1.0% Ni, and the balance iron with unavoidable impurities; has a metallurgical structure in which retained austenite occupies 1% or more, bainitic ferrite and martensite occupy 80% or more in total, and ferrite and pearlite occupy 9% or less (including 0%) in total by an area rate with respect to the total structure, and in which crystal grains of the retained austenite have an average axial ratio (major axis/minor axis) of 5 or higher; and has further a tensile strength of 1,180 MPa or higher. COPYRIGHT: (C)2006,JPO&NCIPI
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