MORPHOLOGY CONTROL OF DUCTILE 2ND PHASE AND IMPROVED MECHANICAL-PROPERTIES IN HIGH-STRENGTH LOW-ALLOY STEELS WITH MIXED STRUCTURE

Mixed structures of martensite and non-martensitic decomposition product (ductile second phase) can often be produced in commercial heat treatments of low-alloy steels. Such mixed structures can sometimes produce a detrimental effect on the mechanical properties of steels, but they can significantly improve strength, ductility and notch toughness if the ductile second phase appears in a suitable morphology (size, shape and distribution) in association with tempered martensite. Therefore, microstructures have recently been produced in a deliberate attempt to improve the mechanical properties of ultrahigh-strength low-alloy steels. In this review, an attempt has been made to present the effect of the morphology (shape, size and distribution) of the ductile second phase on improved mechanical properties of ultrahigh-strength low-alloy steels having mixed structures. This review first discusses the effect of the morphology of the second-phase bainite on mechanical properties of high-strength low-alloy steels with mixed structures of martensite and bainite, in which great improvement in the mechanical properties of the steel has been associated with the morphology of the bainite. Then, knowledge of the recent development of a steel having the mixed structures for low-temperature ultrahigh-strength applications is also reported.