Evolution of Phases and Mechanical Properties of Thermomechanically Processed Ultra High Strength Steels

The present study concerns the development of two low carbon microalloyed ultra high strength steels on a pilot scale. This recent endeavour has been made towards the reduction of weight by achieving high strength to weight ratio together with improved weldability for the various prospective high performance defence applications such as explosive ammunition, gun barrel, missile skins, light-weight military bridges etc. These steels were thermomechanically processed and finished at different finish rolling temperatures followed by water quenching. Variation in microstructure and mechanically properties at different finished rolling temperatures was studied. The experimentally determined continuous cooling transformation diagrams have revealed that adequate hardenability is achievable in these steels usually at a cooling rate >5 °C/s. Lath martensite along with the microalloy (Ti, Nb) CN precipitate particles are the characteristic microstructural feature of the investigated steels. The high strength value obtained in the present steels is due to the accumulated contribution of fine grained pan-caked austenite, highly dislocated lath martensite along with the presence of tiny precipitates of microalloy carbide/carbonitride and Cu rich precipitates. The good combination of strength (1,364–1,538 MPa) and ductility (11–16 %) has been achieved for the selected range of finish rolling temperature. The Charpy impact toughness values (30–80 J) reveal approximately consistent fall with the lowering of testing temperature.