| 摘要 |
PROBLEM TO BE SOLVED: To provide an ultrahigh strength cold rolled steel sheet excellent in hydrogen embrittlement resistance and having tensile strength of 1300 MPa or more.SOLUTION: There is provided an ultrahigh strength cold rolled steel sheet having a steel structure containing C:0.15 to 0.3%, Si:0.001 to 2%, Mn:2.1 to 4%, P:0.05 or less, S:0.01% or less, N:0.01% or less, Al:0.001 to 1%, Ti:0.001 to 0.1%, B:0.0001 to 0.01%, having values of solid solution B amount solB [mass%] and old austenite grain size Dγ[μm] satisfying a relationship of solB Dγ≥0.0010, polygonal ferrite of 10% or less, bainite of 30% or less, retained austenite of 6% or less, tempered martensite of 60% or more, number density of Fe carbide in the sintered martensite of 1×10/mmor more, average dislocation density of whole steel of 1.0×10to 2.0×10/mand crystal grain particle of 7.0 μm or less.SELECTED DRAWING: Figure 1 |
