COLD-ROLLED STEEL SHEET

摘要

A high-strength cold-rolled steel sheet excellent in ductility, work hardenability, and stretch flangeability, and having tensile strength of 780 MPa or more includes: a chemical composition containing, in mass percent, C: more than 0.020% to less than 0.30%, Si: more than 0.10% to 3.00% or less, Mn: more than 1.00% to 3.50% or less; and metallurgical structure whose main phase is a low-temperature transformation product, and whose secondary phase contains retained austenite. The retained austenite has a volume fraction relative to overall structure of more than 4.0% to less than 25.0% and an average grain size of less than 0.80 μm, and of the retained austenite, the number density of retained austenite grains whose grain size is 1.2 μm or more is 3.0×10−2 grains/μm2 or less.

主权项

1. A cold-rolled steel sheet characterized by having a chemical composition consisting of, in mass percent, C: more than 0.020% to less than 0.30%; Si: more than 0.10% to at most 3.00%; Mn: more than 1.00% to at most 3.50%; P: at most 0.10%; S: at most 0.010%; sol.Al: at least 0% and at most 2.00%; N: at most 0.010%; Ti: at least 0% and less than 0.050%; Nb: at least 0% and less than 0.050%; V: at least 0% and at most 0.50%; Cr: at least 0% and at most 1.0%; Mo: at least 0% and at most 0.50%; 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 metallurgical structure whose main phase is a low-temperature transformation product, and whose secondary phase contains retained austenite, the retained austenite having a volume fraction of more than 4.0% to less than 25.0% relative to overall structure, and an average grain size of less than 0.80 μm, Wherein of the retained austenite, a number density of retained austenite grains whose grain size is 1.2 μm or more is 3.0×10−2 grains/μm2 or less.