STEEL SHEET, HOT-DIP GALVANIZED STEEL SHEET, ALLOYED HOT-DIP GALVANIZED STEEL SHEET, AND PRODUCTION METHODS THEREFOR

摘要

A steel sheet according to one embodiment of the present invention has a prescribed chemical composition. The structure of a portion comprising ¼ of the sheet thickness includes, expressed as volume fractions, at least 70% of tempered martensite, and a total of less than 20% of ferrite and/or bainite. The structure of the portion comprising ¼ of the sheet thickness includes, expressed as volume fractions, less than 10% of retained austenite, not more than 10% of fresh martensite, and not more than 10% of pearlite. The total volume fraction of retained austenite, fresh martensite, and pearlite in said structure is not more than 15%. The number density of iron-based carbides having a longitudinal diameter of at least 5 nm in the tempered martensite in the portion comprising ¼ of the sheet thickness is at least 5×107 per mm2. The ratio of the number of ε carbides to the number of the iron-based carbides having a longitudinal diameter of at least 5 nm in the portion comprising ¼ of the sheet thickness is at least 20%. The tensile strength is at least 780 MPa.