HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH GALVANIZED STEEL SHEET AND HIGH-STRENGTH ALLOYED GALVANIZED STEEL SHEET HAVING MAXIMUM TENSILE STRENGTH OF 980 MPa AND EXCELLENT DELAYED FRACTURE RESISTANCE CHARACTERISTIC AND THEIR PRODUCTION METHOD

摘要

PROBLEM TO BE SOLVED: To provide a high-strength steel sheet, a high-strength galvanized steel sheet and a high-strength alloyed galvanized steel sheet which have a maximum tensile strength(TS) of 980 MPa or higher and are applicable to impact absorption members on collision.SOLUTION: A high-strength steel sheet has a micro-structure which contains, by volume fraction, 20% or more of ferrite as the main phase and a total of 5-80% of martensite and/or bainite as the second phase and is restricted in volume fraction of retained austenite to lower than 10%. Martensite and bainite have a shape meeting equation (1), where V is the volume of martensite particles or bainite particles; and S is the surface area of martensite particles or bainite particles. The high-strength steel sheet has a maximum tensile strength of 980 MPa or higher and is excellent in delayed fracture resistance characteristics.