High strength high ductility steel plate

Abstract

In the present invention, a steel plate contains specific alloy components, has a steel structure that contains specific proportions of retained austenite and ferrite, with the remainder comprising one or more types selected from among bainite, martensite, tempered bainite and tempered martensite, and has a specific average carbon concentration and carbon concentration distribution in the retained austenite.

1 . A high-strength high-ductility steel sheet, having a component composition comprising, in mass %,
 C: from 0.10 to 0.30%,   Si+Al: from 0.5 to 2.0%,   Mn: from 1.0 to 4.0%,   P: from 0 to 0.05%, and   S: from 0 to 0.01%,   
       with the remainder being iron and an unavoidable impurity,
 wherein a steel microstructure comprises, 
 in terms of an area ratio relative to the entire microstructure, 
 8% or more of a retained austenite and 
 more than 5% and 50% or less of ferrite, 
 
       with the remainder being one member or two or more members 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 the carbon concentration of 1.3 mass % or more, relative to the entire microstructure, is 1.0% or more. 
 
     
     
         2 . The high-strength high-ductility steel sheet according to  claim 1 , wherein the component composition further comprises, in mass %, any one of the following (a) to (c):
 (a) a total of 1.0% or less of one member or two or more members of Cu, Ni, Mo, Cr, and B;   (b) a total of 0.2% or less of one member or two or more members of V, Nb, Ti, Zr, and Hf; and   (c) a total of 0.01% or less of one member or two or more members of Ca, Mg and REM.