Ultra-high-strength steel sheet having excellent yield ratio and workability

Abstract

An ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Si, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 1 area % or more of a region in which martensite constitutes 90 area % or more, residual austentite constitutes 0.5 area % or more, and the local Mn concentration is at least 1.2 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

1 . An ultra-high-strength steel sheet excellent in yield ratio and workability, having a composition comprising, by mass %,
 C: 0.15% to 0.35%,   Si: 0.5% to 3.0%   Mn: 0.5% to 1.5%,   Al: 0.001% to 0.10% and   the balance being iron and inevitable impurities,   wherein each of P, S and N of the inevitable impurities is limited to   P: 0.1% or less   S: 0.01% or less and   N: 0.01% or less,   the ultra-high-strength steel sheet having a structure comprising, by area ratio based on a whole structure,   martensite: 90% or more and   residual austenite: 0.5% or more,   the ultra-high-strength steel sheet having 1% or more by area ratio of a region where a local Mn concentration is at least 1.2 times a Mn content in a whole steel sheet, and   the ultra-high-strength steel sheet having a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more and an elongation of 10% or more.   
     
     
         2 . The ultra-high-strength steel sheet excellent in yield ratio and workability according to  claim 1 , wherein the composition further comprises, by mass %, at least one of the following (a) to (c):
 (a) one or two or more of Cu: 0.05% to 1.0%, Ni: 0.05% to 1.0% and B: 0.0002% to 0.0050%, (b) one or two or more of Mo: 0.01% to 1.0%, Cr: 0.01% to 1.0%, Nb: 0.01% to 0.3%, Ti: 0.01% to 0.3% and V: 0.01% to 0.3%, and   (c) one or two of Ca: 0.0005% to 0.01% and Mg: 0.0005% to 0.01%.