HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET, HIGH-STRENGTH MOLTEN ALUMINUM-PLATED STEEL SHEET, AND HIGH-STRENGTH ELECTROGALVANIZED STEEL SHEET, AND METHODS FOR MANUFACTURING SAME.

摘要

Provided is a high-strength steel sheet comprising a prescribed component composition, the steel structure of which includes, expressed in terms of area ratio, 15-55% of polygonal ferrite, 8% or more of un-recrystallized ferrite, and 15-30% of martensite, as well as, in terms of volume ratio, 12% or more of residual austenite, the average crystal grain diameter of the polygonal ferrite being 4 µm or smaller, the average crystal grain diameter of the martensite being 2 µm or smaller, the average crystal grain diameter of the residual austenite being 2 µm or smaller, and the value calculated by dividing the amount of Mn (mass%) in the residual austenite by the amount of Mn (mass%) in the polygonal ferrite being 2.0 or greater, whereby the material has a TS of 980 MPa or greater, a high yield ratio of 68% or above, and excellent formability stemming from the high YR.