Strength and impact toughness of low-carbon steel with fibrous ultrafine-grained structure

The effect of severe warm rolling on the structure and mechanical characteristics of the 12GBA low-carbon steel has been studied. A fibrous ultrafine-grained structure has been formed; the average transverse size of structural elements was 0.5 μm and the length of fibers in the longitudinal section was 20–25 μm. An analysis of this ultrafine-grained structure in transverse and longitudinal sections was carried out using the method of electron backscatter diffraction. It has been shown that the formation of the fibrous structure results in a twofold increase in the strength of the steel in comparison with the original coarse-grained state, with retaining satisfactory ductility. The additional annealing of the steel leads to a slight decrease in its strength characteristics, but results in the complete restoration of its ductile characteristics to the values typical of the coarse-grained state. Impact tests at negative temperatures have shown that, after rolling, the cold-brittleness threshold shifts toward a lower temperature range (from −30 to −60°C) in comparison with the coarse-grained state. It has been found that the formation of the ultrafine-grained state leads to a decrease in the cold-brittleness threshold of the 12GBA steel, as well as to an increase in its impact toughness in the low-temperature range and in the contribution of the crack-propagation work to the total work of fracture of a specimen.