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

Abstract

A 0.15%C-0.3%Si-1.5%Mn steel bar with ultrafine elongated grain structures of transverse grain size 1.2 μm 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 °C to 200 °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 °C and the specimen did not separate into two pieces even at a low temperature of −196 °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.