HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

摘要

A high-strength cold-rolled steel sheet includes, as a component composition, by mass %: C: 0.075% to 0.300%; Si: 0.30% to 2.50%; Mn: 1.30% to 3.50%; P: 0.001% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 1.500%; and N: 0.0001% to 0.0100%, in which a surface microstructure contains residual austenite of 3% to 10% and ferrite of 90% or less by volume fraction, an inner microstructure at a depth of t/4 from the surface assuming that a sheet thickness is t contains residual austenite of 3% to 20% by volume fraction, a ratio Hvs/Hvb between a surface hardness Hvs of the steel sheet surface and a hardness Hvb at a depth of ¼ of the thickness is more than 0.75 to 0.90, and a maximum tensile strength is 700 MPa or more.

主权项

1. A high-strength cold-rolled steel sheet comprising, as a component composition, by mass %:
C: 0.075% to 0.300%; Si: 0.30% to 2.50%; Mn: 1.30% to 3.50%; P: 0.001% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 1.500%; N: 0.0001% to 0.0100%; Ti: 0.150% or less; Nb: 0.150% or less; V: 0.150% or less; Cr: 2.00% or less; Ni: 2.00% or less; Cu: 2.00% or less; Mo: 1.00% or less; W: 1.00% or less; and 0.5000% or less in total of at least one of Ca, Ce, Mg, Zr, Hf, and REM; and a balance being Fe and unavoidable impurities, wherein a surface microstructure of the steel sheet surface at a depth of 20 μm from the surface contains residual austenite of 3% to 10%, ferrite of 90% or less, fresh martensite limited to 15% or less, and pearlite of 5% or less by volume fraction, an inner microstructure at a depth of t/4 from the surface assuming that a sheet thickness is t contains residual austenite of 3% to 30%, fresh martensite limited to 15% or less, and pearlite of 5% or less by volume fraction, a ratio Hvs/Hvb between a surface hardness Hvs of the steel sheet surface and a hardness Hvb at a depth of t/4 is more than 0.75 to 0.90, a maximum tensile strength TS is 700 MPa or more, and a relationship between the maximum tensile strength TS and a total elongation EL satisfies TS×EL≧18000 (MPa·%).