POSCO Holdings Inc.
High strength cold-rolled steel sheet and hot-dip galvanized steel sheet having excellent hole expansion, ductility and surface treatment properties, and method for manufacturing same

Abstract:

Provided are a high-strength cold-rolled steel sheet and a hot-dip galvanized steel sheet comprising in % by weight: 0.05 to 0.3% of carbon (C); 0.6 to 2.5% of silicon (Si); 0.01 to 0.5% of aluminum (Al); 1.5 to 3.0% of manganese (Mn); and the remainder being Fe and unavoidable impurities, the steel sheet has a microstructure comprised of, in an area fraction, ferrite in an amount of 60% or less, lath-type bainite of 25% or more, martensite of 5% or more, and lath-type retained austenite in an amount of 5%, wherein the ferrite has an average grain diameter of 2 .mu.m or less and the ferrite satisfies Fn2, defined by relational expression 1, is 89% or more and Fa5, defined by relational expression 2, is 70% or less.