Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel

The current study concerns the development of a low-carbon, microalloyed ultra-high-strength steel on a pilot scale. The continuous cooling transformation has been evaluated, and a flat top “C” curve with a mixed microstructure of bainite and martensite has been obtained at a lower transformation temperature. The steel has been processed thermomechanically, followed by air cooling and water quenching. In addition, a variation in microstructure and mechanical properties at different finish rolling temperatures has been studied. Although a mixture of granular bainite and bainitic ferrite with interlath and intralath precipitation of NbC/NbC(N)/TiC(N) particles are the characteristic microstructural feature of air-cooled steel, the lath martensitic structure along the fine NbC/NbC(N)/TiC(N) precipitate is obtained in case of a water-quenched steel. The high-strength value obtained in the current steel is caused by the accumulated contribution of fine-grained, pancaked austenite, highly dislocated fine lath martensitic structure along with the presence of tiny precipitates of microalloy carbide/carbonitride.