Strength and toughness tradeoffs for an ultrafine-grain size ferrite/cementite steel produced by warm-rolling and annealing

For an ultrafine grain ferrite/cementite (UGF/C) steel, the Charpy impact energy was measured at temperatures from 373 K to 4.2 K, and tensile tests were carried out at temperatures between 323 K and 77 K. For the steel with annealed microstructure, the ductile-to-brittle transition appearance temperature (DBTT) was lower than the Charpy transition temperature (CTT). With increasing annealing time at 873 K, the DBTT and the CTT increased, and the DBTT approached the CTT. The DBTT decreased with decreasing effective grain size. The effective grain size correlated to the grain size of the larger grain size peak in the distribution of grains with {1 0 0} planes. The annealed microstructures had higher yield strength for equivalent toughness (including upper shelf energy, DBTT and CTT) compared to the conventional ferrite/pearlite steel.