HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET WITH EXCELLENT FATIGUE PROPERTY AND HOLE EXPANSIBILITY AND PROCESS FOR PRODUCING SAME

摘要

Provided are a high-strength hot-dip galvanized steel sheet which has a tensile strength TS of 590 MPa or higher and has excellent fatigue properties and hole expansibility, and a process for producing the steel sheet. The galvanized steel sheet contains, in terms of mass%, 0.04-0.13% C, 0.9-2.3% Si, 0.8-1.8% Mn, up to 0.1% P, up to 0.01% S, up to 0.1% Al, and up to 0.008% N, with the remainder comprising iron and incidental impurities, and has a structure which comprises, in terms of areal proportion, at least 80% ferrite phase, at least 1.0% bainitic ferrite phase, and 1.0-10.0% pearlite phase, has a martensite phase content of 1.0-5.0%, excluding 5.0%, in terms of areal proportion, and has an average ferrite crystal grain diameter of 14 µm or less and an average martensite crystal grain diameter of 4 µm or less, the martensite having a mean free path of 3 µm or more and the ferrite having a Vickers hardness of 140 or above, and which satisfies (areal proportion of martensite)/((areal proportion of bainitic ferrite)+(areal proportion of pearlite))≤0.6.