Metallurgical Design of High Strength/High Toughness Steels

The proper balance between yield strength, YS, and ductile to brittle transition temperature, DBTT, has been the main concern during development of high strength engineering steels and the effect of microstructure on impact toughness has attracted a great attention during the last decades. In this paper a review concerning the relationship between strength and toughness in steels will be presented and the effect of different microstructural parameters will be discussed, aiming to improve such properties in designing new high strength steels. Complex microstructures, obtained by quenching and tempering (Q&T) and thermo-mechanical (TM) processing are considered. The steels are low/medium carbon steels (C=0.04%-0.40%) with yield strength in the range YS=500-1000 MPa. Results show that the strength and the impact toughness behaviour are controlled by different microstructural parameters and not, as in the case of polygonal ferritic steels, by the same structural unit (the grain size) and that a “fine” microstructure is required in order to achieve high levels of both strength and toughness. The metallurgical design of high strength steels with toughness requirements is discussed using the same approach for both Q&T and TMCP processes.