Abstract

Recent advances show that tough as-quenched ultra-high-strength steels in fully and partially martensitic conditions demand proper control of the effective coarse grain size, which is the key microstructural parameter controlling the toughness in the ductile-brittle transition region. The most effective way to reduce this grain size and texture components detrimental to toughness with thermomechanically rolled steels is to apply a high level of austenite pancaking. The effective coarse grain size (d80%) can be used to reliably estimate impact toughness transition temperatures. Adding the fraction of {100} cleavage planes close to the specimen notch/crack plane further improves these estimates. A recent straightforward semi-physical model consists of just two parameters, the first term describes the temperature-dependency of a local brittle fracture, and the second term relates to the size of these locally cleaved areas. Here, we present the concept reference toughness and study its applicability to the estimation of both impact toughness and fracture toughness transition temperatures. Fractographic evidence demonstrates that failure initiation is a complicated interaction between large grains and large brittle inclusions. With locally varying, inhomogeneous microstructural properties, the failure is likely to initiate and propagate when a large particle locates in a coarse-grained matrix, whose dimensions are in line with the effective coarse grain size. Application of these microstructure-based estimates of the impact toughness and fracture toughness transition temperatures can further assist the design and production of steels with lath-like microstructures.