COLD-ROLLED STEEL SHEET AND METHOD FOR MANUFACTURING SAME, AND HOT-STAMP FORMED BODY

摘要

A cold-rolled steel sheet has a predetermined chemical composition, in which a structure before and after a hot-stamping includes ferrite: 30 area % to 90 area %, martensite: 0 area % or more and less than 20 area %, pearlite: 0 area % to 10 area %, retained austenite: 5 volume % to 20 volume %, and rest structure: bainite, a hardness of the retained austenite measured with a nano indenter before and after the hot-stamping satisfies relations of H2/H1<1.1 and σHM<20, and a relation of TS×El>20000 MPa.% is satisfied.

主权项

1. A cold-rolled steel sheet having a chemical composition comprising, in terms of mass %:
C: 0.1% to 0.3%; Si: 0.01% to 2.0%; Mn: 1.5% to 2.5%; P: 0.001% to 0.06%; S: 0.001% to 0.01%; N: 0.0005% to 0.01%; Al: 0.01% to 0.05%; B: 0% to 0.002%; Mo: 0% to 0.5%; Cr: 0% to 0.5%; V: 0% to 0.1%; Ti: 0% to 0.1%; Nb: 0% to 0.05%; Ni: 0% to 1.0%; Cu: 0% to 1.0%; Ca: 0% to 0.005%; REM: 0% to 0.005%; and rest including Fe and impurities, wherein a structure before and after a hot-stamping includes a ferrite: 30 area % to 90 area %, a martensite: 0 area % or more and less than 20 area %, a pearlite: 0 area % to 10 area %, a retained austenite: 5 volume % to 20 volume %, and a rest structure: a bainite, a hardness of the retained austenite measured with a nano indenter before and after the hot-stamping satisfies Expression 1 and Expression 2, a tensile strength and a ductility satisfy a relation of TS×El>20000 MPa.%,
H2/H1<1.1:  Expression 1,σHM<20:  Expression 2, and the “H1” represents the hardness of the retained austenite existing within a thickness surface portion before and after the hot-stamping, the thickness surface portion being an area within 200 μm in the thickness direction of a surface of the cold-rolled steel sheet, the “H2” represents the hardness of the retained austenite existing within a thickness center portion before and after the hot-stamping, the thickness center portion being an area within ±100 μm along the thickness direction of a center plane of the cold-rolled steel sheet in the thickness direction, the “σHM” represents a variance of the hardness of the retained austenite within the thickness center portion before or after the hot-stamping, “TS” represents the tensile strength of the cold-rolled steel sheet in terms of MPa, and “El” represents the ductility of the cold-rolled steel sheet in terms of %.