Improvement of strength, toughness and ductility in ultrafine-grained low-carbon steel processed by warm bi-axial rolling

A 0.15%C-0.3%Si-1.5%Mn steel bar with ultrafine elongated grain structures of transverse grain size 1.2 mu m was fabricated via multi-pass bi-axial rolling process at a warm working temperature. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and 0.15% low-carbon steel with a ferrite/pearlite structure were also prepared. The full-size Charpy V-notch impact and tensile tests were conducted at a temperature range of 196 degrees C to 200 degrees C, and the relationship between microstructures, yield stress, reduction in area and impact energy was studied. In the developed steel bar, the main orientation in the microstructure changed in the cross-sectional plane, and it was dominated by {001}< 100 > cube orientation at the center, {111}< 110 > at the quarter and rolling direction//< 110 > at the surface. Crack branching started to occur with decreasing temperature in the Charpy test. The fracture surfaces are very complicated at temperatures below 100 degrees C and the specimen did not separate into two pieces even at a low temperature of 196 degrees C. The strength-toughness balance of the developed steel was significantly improved compared with conventional steels. This advantage in the developed steel was also seen in the strength-reduction in area balance. As a result, the steel fabricated by warm bi-axial rolling was best balance in correlation between strength and ductility and between strength and toughness at all temperatures.