Ultrahigh strength hot rolled microalloyed steel: microstructure and properties

Abstract

A low carbon steel alloyed with Ni, Mn, Mo, Cu and microalloyed with Nb and Ti was prepared. Continuous cooling transformation behaviour of the steel was evaluated. Formation of polygonal or Widmanstätten ferrite is suppressed at high temperature and the 'C' curve is shifted to an extreme right. At lower temperatures a flat top 'C' curve with a mixed structure of bainite and martensite was obtained and the transformation temperatures do not vary much with a wide range of cooling rates. The steel was thermomechanically processed at different finishing temperatures and ultrahigh strength values were obtained as a result of austenite grain refinement, highly dislocated fine lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride at all finish rolling temperatures. The stable and large TiN/TiCN particles formed during casting have impaired the impact toughness values at ambient and at ?40°C temperatures.