Estimates of toughness in terms of Charpy impact energy and the critical stress intensity factor, K(ICV) using deeply chevron-notched specimens were made for two casts of niobium-bearing high-strength low-alloy (HSLA) steels. K(ICV) determinations are carried out for the first time using both three- and four-point bend loading configurations for this material. Quantitative analyses of the material microstructures are made with, respect to the amount of the phases, ferrite grain size, and the volume fraction, length, aspect ratio, and mean inter-spacings of the inclusions. A comparative study of impact and fracture toughness with regard to the microstructural parameters, indicates that the latter toughness characterization approach is far superior to the former. The compatibility of the estimated values of K(ICV) using the two different loading configurations is discussed.