HIGH-STRENGTH STEEL SHEET EXHIBITING EXCELLENT STRETCH-FLANGE FORMABILITY AND BENDING WORKABILITY, AND METHOD OF PRODUCING MOLTEN STEEL FOR THE HIGH-STRENGTH STEEL SHEET

摘要

The present invention provides a high-strength steel sheet including: C: 0.03 to 0.25 mass %, Si: 0.1 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not more than 0.05 mass %, T.O: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %, N: 0.0005 to 0.01 mass %, acid-soluble Al: more than 0.01 mass %, Ca: 0.0005 to 0.0050 mass %, and a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, with a balance including iron and inevitable impurities, in which the steel sheet contains a chemical component on a basis of mass that satisfies 0.7100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70 and 0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10, the steel sheet contains compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, and Al, the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in the range of 0.5 μm to 5 μm, and a ratio of the number of the spherical compound inclusion relative to the number of all inclusions having the equivalent circle diameter in the range of 0.5 μm to 5 μm is 30% or more.