HOT-DIP ZINC-COATED STEEL SHEETS AND PROCESS FOR PRODUCTION THEREOF

摘要

The invention provides hot-dip zinc-coated steel sheets excellent in the balance between strength and ductility and in hardenability and a process for the production of the same. The sheets have a chemical composition which contains C: 0.005 to 0.04%, Si: 1.5% or below, Mn: 1.0 to 2.0%, P: 0.10% or below, S: 0.03% or below, Al: 0.01 to 0.1%, N: less than 0.008%, and Cr: 0.2 to 1.0% and satisfies the relationship: 2.1 < Mn(mass%) + 1.29Cr(mass%) < 2.8 with the balance being iron and unavoidable impurities, and a structure which consists of ferrite and 3.0 to less than 10% by volume of martensite and in which the mean grain diameter of the ferrite exceeds 6µm and is up to 15µm with at least 90% of the martensite existing in ferrite grain boundaries. In producing the hot-dip zinc-coated steel sheets, cold rolled steel sheets are annealed in the temperature range of Ac1 to Ac3.