HIGH-STRENGTH COMPOSITE STEEL SHEET HAVING EXCELLENT MOLDABILITY AND DELAYED FRACTURE RESISTANCE

摘要

Disclosed is a high-strength composite steel sheet having a tensile strength of 980 MPa or greater and excellent in moldability, delayed fracture resistance and spot weldability. The steel sheet comprises a steel having the following chemical composition: C: 0.12-0.25%, Si: 1.0-3.0%, Mn: 1.5-3.0%, P: 0.15% or less, S: 0.02% or less, Al: 0.4% or less, and Cr: 1.0% or less, with the remainder being iron and unavoidable impurities, wherein the contents of Si, Al, Mn and Cr satisfy the relationship expressed by the following formula: (Si+Al)/Mn or (Si+Al)/(Mn+Cr)=0.74-1.26 and the steel sheet has a specified microstructure.