High-strength high-ductility steel sheet

Abstract

A high-strength high-ductility steel sheet has a specific component composition and a steel microstructure containing, in terms of an area ratio relative to the entire microstructure, 8% or more of a retained austenite, with the remainder being at least one of bainite, martensite, tempered bainite, and tempered martensite. With respect to the carbon concentration in the retained austenite, an average carbon concentration is from 0.9 to 1.2 mass %, a standard deviation of the carbon concentration distribution is 0.35 mass % or more, and an area ratio of a region having the carbon concentration of 1.5 mass % or more, relative to the entire microstructure, is 1.0% or more.

1 . A high-strength high-ductility steel sheet, comprising, in mass %,
 C: 0.10% or more and less than 0.35%,   Si+Al: from 0.5 to 2.0%,   Mn: from 1.0 to 4.0%,   P: from 0 to 0.05%,   S: from 0 to 0.01%, and   iron,   wherein   the steel sheet has a steel microstructure comprising, in terms of an area ratio relative to entire microstructure,
 8% or more of a retained austenite, 
 more than 5% and 50% or less of ferrite, and 
 at least one of bainite, martensite, tempered bainite, and tempered martensite, and 
   with respect to a carbon concentration in the retained austenite, an average carbon concentration is from 0.8 to 1.1 mass %, a standard deviation of a carbon concentration distribution is 0.25 mass % or more, and an area ratio of a region having a carbon concentration of 1.3 mass % or more, relative to the entire microstructure, is 1.0% or more.   
     
     
         2 . The steel sheet according to  claim 1 , further comprising, in mass %, at least one of (a) to (c):
 (a) a total of 1.0% or less of at least one of Cu, Ni, Mo, Cr, and B;   (b) a total of 0.2% or less of at least one of V, Nb, Ti, Zr, and Hf; and   (c) a total of 0.01% or less of at least one of Ca, Mg and REM.