Charpy impact toughness prediction using an 'effective' grain size in thermomechanically control rolled (TMCR) microalloyed steels

Thermomechanically controlled rolling of steel plate can involve substantial straining in the intercritical temperature region, which may result in the final ferrite grains not fully recrystallizing, and, hence, the presence of low-angle grain boundaries. It is shown in this article that a Nb-microalloyed thermomechanically controlled rolled (TMCR) steel can contain a high proportion of low-angle grain boundaries (the extent depending on the thermomechanically controlled rolling schedule) and that during toughness testing, the crack front ignores boundaries with less than a 12 deg misorientation. Thus, the average microstructural unit experienced by the crack front (i.e., the cleavage facet) is significantly larger than the average metallographic, two-dimensional grain size. Consequently, use of the metallographic grain size gives a poor prediction of the impact transition temperature (ITT) and fracture stress for these steels. It is also shown that the micromechanism of crack initiation and propagation involves grain-boundary carbides and groups of closely aligned grains that act as single "effective" grains.