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 having the chemical components recited in the claims. The steel sheet contains composite inclusions that have: a first inclusion phase which includes Ca, at least one of Ce, La, Nd and Pr, and at least one of O and S; and a second inclusion phase which has different components from the first inclusion phase and includes at least one of Mn, Si and Al. The composite inclusions form spherical composite inclusions that have an equivalent circle diameter of 0.5-5 µm, and the number of the spherical composite inclusions is at least 30% or more of the total number of inclusions having an equivalent circle diameter of 0.5-5 µm.