Ductile-to-Brittle Transition and Brittle Fracture Stress of Ultrafine-Grained Low-Carbon Steel

Ductile-to-brittle transition (DBT) temperature and brittle fracture stress, sigma(F), are important toughness criteria for structural materials. In this paper, low-carbon steels with an ultrafine elongated grain (UFEG) structure (transverse grain size 1.2 mu m) and with two ferrite (alpha)-pearlite structure with grain sizes 10 mu m and 18 mu m were prepared. The UFEG steel was fabricated using multipass warm biaxial rolling. The tensile tests with a cylindrical specimen and three-point bending tests with a single-edge-notched specimen were performed at -196 degrees C. The local stress near the notch was quantitatively calculated via finite element analysis (FEA). The sigma(F) for each sample was quantified based on the experimental results and FEA. The relationship between sigma(F) and d(alpha) in the wide range of 1.0 mu m to 138 mu m was plotted, including data from past literature. Finally, the conditions of grain size and temperature that cause DBT fracture in low-carbon steel were shown via the stress-d(-1/2) map. The results quantitatively showed the superiority of alpha grain size for brittle fracture.