| 摘要 |
PROBLEM TO BE SOLVED: To provide a manufacturing method with a low cost and high productivity by securing excellent internal quality and HIC resistance performance even with a super thin steel sheet having sheet thickness of 50 mm or more.SOLUTION: There is provided an ultra thin steel sheet having sheet thickness of 50 mm or more and excellent in HIC resistance performance, consisting of a specific component composition, where a micro structure from a surface layer to a 1/8 t to 7/8 t position, where t is a sheet thickness, contains a bainite generated from old austenite grain having aspect ratio of 1.5 or less of 90% or more of the total micro structure, Vickers hardness of a center segregation part is 300 or less, an inclusion consisting of a compound containing a gap existing in the center segregation part, a MnS-based inclusion and Nb or Ti or both and an inclusion cluster consisting of a compound containing Al or Ca or both has both major axis of less than 200 μm and 30% or more of the number of oxide containing Al and Ca in the steel is oxide having AlO/CaO=0.7 to 1.3 by molar ratio.SELECTED DRAWING: None |
