High-strength hot-dip galvanized steel sheet excellent in mechanical cutting properties, high strength galvannealed steel sheet, as well as methods for their preparation

Provided is a high-strength hot-dip galvanized steel sheet and the like, said steel sheet providing high ductility while ensuring high strength with a maximum tensile strength of at least 900 MPa, and having excellent mechanical cutting characteristics. The high-strength hot-dip galvanized steel sheet has a thickness of 0.6 to 5.0 mm, and has a plating layer on a surface of the steel sheet for which component compositions have been set within appropriate ranges. The structure of the steel sheet includes a ferrite phase with a volume fraction of 40 to 90%, and a retained austenite phase with a volume fraction of at least 3%. The retained austenite phase has a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 mum or less, and an average distance between particles of 0.1 to 5.0 mum. The thickness of a decarburized layer in a surface section of the steel sheet is 0.01 to 10.0 mum, and the average particle size and average density of oxides contained in the surface section of the steel sheet are 30 to 120 nm and 1.0 × 1012 oxides/m2 or higher, respectively. In addition, the work hardening coefficient (n value) when plastic deformation of 3 to 7% occurs is, on average, at least 0.080.