Superhigh-strength dual-phase steel sheet of excellent fatigue characteristic in a spot welded joint

摘要

A superhigh-strength dual-phase steel sheet containing ferritic microstructure and a martensitic microstructure—containing composite-phase steel sheet containing:;C: 0.08-0.20% (mass% here and hereinafter),;Si: 0.5% or less (inclusive of 0%);Mn: 3.0% or less (exclusive of 0%);P: 0.02% or less (inclusive of 0%);S: 0.02% or less (inclusive of 0%), and;Al: 0.001-0.15%, and further containing;Mo: 0.05-1.5%, and;Cr: 0.05-1.5%, and which satisfying that:;the average Vickers hardness of the ferritic microstructure is 150 Hv or more and the average Vickers hardness of the martensitic microstructure is 500 Hv or more, the superhigh-strength dual-phase steel sheets being of excellent fatigue characteristic in a spot welded joint.