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, 35-80% of polygonal ferrite, 5% or more of un-recrystallized ferrite, and 5-25% of martensite, as well as, in terms of volume ratio, 8% or more of residual austenite, the average crystal grain diameter of the polygonal ferrite being 6 µm or smaller, the average crystal grain diameter of the martensite being 3 µm or smaller, the average crystal grain diameter of the residual austenite being 3 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 590 MPa or greater, a high yield ratio of 68% or above, and excellent formability stemming from the high YR.