An ultra-high strength cold rolled steel sheet having a tensile strength of
880 to 1170 MPa, that consists essentially of 0.01 to 0.07% C, 0.3% or
less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or
less N, 1.6 to 2.5% of the sum of at least one element selected from the
group consisting of Mn, Cr, and Mo, by mass, and a balance of Fe, and that
has an inner zone deeper than 10 .mu.m from the surface of the steel sheet
being substantially a martensitic single phase structure. Since the steel
sheet has a hole expansion ratio of 75% or more, specified by the Standard
of Japan Iron and Steel Federation, JFST1001-1996, the steel sheet has a
tensile strength in a range of from 880 to 1170 MPa, and an excellent
mechanically joining property, and is suitable for fabricating automobile
seat frames.
