HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET WITH EXCELLENT PROCESSABILITY AND PROCESS FOR PRODUCING THE SAME

摘要

A high-strength hot-dip galvanized steel sheet is provided which has a TS of 590 MPa or higher and is excellent in ductility and stretch flangeability. Also provided is a process for producing the steel sheet. The steel sheet has a composition comprising, in terms of mass%, 0.05-0.3% carbon, 0.01-2.5% silicon, 0.5-3.5% manganese, 0.003-0.100% phosphorus, up to 0.02% sulfur, and 0.010-1.5% aluminum, provided that the sum of the silicon and the aluminum is 0.5-2.5%, with the remainder being iron and incidental impurities. The structure thereof comprises, in terms of areal proportion, at least 20% ferrite phase, up to 10% (including 0%) martensite phase, and 10-60% tempered martensite and has, in terms of volumetric proportion, 3-10% retained austenite phase, the retained austenite having an average crystal-grain diameter of 2.0 µm or smaller. The retained austenite preferably has an average concentration of carbon in a solid solution state of 1% or higher.