IMPROVED FRACTURE-TOUGHNESS OF ULTRAHIGH STRENGTH STEEL THROUGH CONTROL OF NONMETALLIC INCLUSIONS

In recent years, ultrahigh-strength steels, which can be employed successfully at yield strengths of 1400 MPa or higher, have been used increasingly for critical structural applications in aircraft and aerospace vehicles. Most recently, there has been increased demand, however, for ultrahigh-strength steel with superior plane-strain fracture toughness, K(IC), and for the steels suitable for large-sized structural applications; isotropy regarding the property has especially been required. One potential solution to this problem is to control non-metallic inclusions of the steels. This review concentrates on recent topics concerning improved K(IC) of ultrahigh-strength steels, i.e. low-alloy and highly alloyed secondary hardening steels, through control of non-metallic inclusions. The major factors controlling the property are discussed for each of the techniques.