HOT-DIP GALVANIZED COLD-ROLLED STEEL SHEET AND PROCESS FOR PRODUCING SAME

摘要

A hot-dip galvanized cold-rolled steel sheet has a tensile strength of 750 MPa or higher, a composition consisting, in mass percent, of C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and 3.0% or less, and optionally containing one or more types of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM, and Bi, P: less than 0.050%, S: 0.010% or less, sol. Al: 0.50% or less, and N: 0.010% or less, and a main phase as a low-temperature transformation product and a second phase as retained austenite. The retained austenite volume fraction is more than 4.0% and less than 25.0% of the whole structure, and has an average grain size of less than 0.80 □m. A number density of retained austenite grains having a grain size of 1.2 □m or more is 3.0□10−2/□m2 or less.

主权项

1. A hot-dip galvanized cold-rolled steel sheet having a hot-dip galvanized layer on a surface of a cold-rolled steel sheet, characterized by having a chemical composition comprising, in mass percent, C: more than 0.10% and less than 0.25%, Si: more than 0.50% and less than 2.0%, Mn: more than 1.50% and at most 3.0%, P: less than 0.050%, S: at most 0.010%, sol. Al: at least 0% and at most 0.50%, N: at least 0.010%, Ti: at least 0% and less than 0.040%, Nb: at least 0% and less than 0.030%, V: at least 0% and at most 0.50%, Cr: at least 0% and at most 1.0%, Mo: at least 0% and less than 0.20%, B: at least 0% and at most 0.010%, Ca: at least 0% and at most 0.010%, Mg: at least 0% and at most 0.010%, REM: at least 0% and at most 0.050%, Bi: at least 0% and at most 0.050%, and the remainder being Fe and impurities, and
by having a metallurgical structure in which a main phase is a low-temperature transformation product and a second phase contains retained austenite, wherein the retained austenite has a volume fraction of more than 4.0% to less than 25.0% with respect to a whole structure, and an average grain size of less than 0.80 μm, and in the retained austenite, a number density of retained austenite grains having a grain size of 1.2 μm or more is 3.0×10−2/μm2 or less.