ULTRAHIGH-STRENGTH STEEL SHEET SUPERIOR IN HYDROGEN EMBRITTLEMENT RESISTANCE AND MANUFACTURING METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide an ultrahigh-strength steel sheet having improved hydrogen-embrittlement resistance.;SOLUTION: The ultrahigh-strength steel sheet includes, by mass%, 0.06-0.6% C, 0.5-3% Si+Al, 0.5-3% Mn, 0.15% or less P and 0.02% or less S, includes 3% or more of a retained austenitic structure and 30% or more of a bainitic structure, by an area rate to the whole structure, and may further include 50% or less of a ferritic structure. In the bainitic structure, an average particle diameter of a bainite block in bainitic ferrite, which has been determined by comparing the same visual fields that have been observed with both an EBSP (Electron Back Scatter Diffraction Pattern) and an SEM, shall be smaller than 20 μm.;COPYRIGHT: (C)2005,JPO&NCIPI