HIGH-STRENGTH HOT-DIP GALVANIZED STEEL PLATE HAVING EXCELLENT IMPACT RESISTANCE AND METHOD FOR PRODUCING SAME, AND HIGH-STRENGTH ALLOYED HOT-DIP GALVANIZED STEEL SHEET AND METHOD FOR PRODUCING SAME

摘要

A base steel sheet has a hot-dip galvanized layer formed on a surface thereof, in which, in a steel sheet structure in a range of 1/8 thickness to 3/8 thickness centered around 1/4 thickness of a sheet thickness from a surface, a volume fraction of a retained austenite phase is 5% or less, and a total volume fraction of a bainite phase, a bainitic ferrite phase, a fresh martensite phase and a tempered martensite phase is 40% or more, an average effective crystal grain diameter is 5.0 µ m or less, a maximum effective crystal grain diameter is 20 µ m or less, and a decarburized layer with a thickness of 0.01 µ m to 10.0 µ m is formed on a surface layer portion, in which a density of oxides dispersed in the decarburized layer is 1.0 × 10 12 to 1.0 × 10 16 oxides/m 2 , and an average grain diameter of the oxides is 500 nm or less.