The characteristics and formation mechanisms of intragranular acicular ferrite (IAF) in steel with MgO nanoparticle additions were systematically investigated for different isothermal heat-treatment temperatures, and its influence on mechanical properties was also clarified. The results indicate that the inclusions were finely dispersed and refined after adding MgO nanoparticles. In addition, with decreasing heat-treatment temperature, the microstructure changed from grain boundary ferrite (GBF) and polygonal ferrite (PF) to intragranular acicular ferrite. Moreover, the steel with MgO additions had excellent mechanical properties in the temperature range of 973 to 823 K and an average Charpy absorbed energies value of around 174 J at 873 K due to the significant refinement of the microstructure and nucleation of intragranular acicular ferrite.
