The current research studied the microstructure and mechanical properties of low carbon steels reinforced by a trace amount of TiC-TiB 2 nanoparticles. An innovative method was proposed to introduce the nanoparticles into molten iron through the use of an aluminum master alloy. It was found that the addition of nanoparticles refined the grains and lamellar structures in pearlite of low carbon steels. During the liquid-solid phase transformation, the amount of widmanstatten structures containing acicular ferrite decreased after adding the nanoparticles. For mechanical properties, the simultaneous enhancement of hardness, tensile properties and impact toughness of low carbon steels was achieved. According to the edge-to-edge matching model, it was found that the nanoparticles could be utilized as heterogeneous cores to facilitate the nucleation of austenite and ferrite. A higher nucleation rate resulted in finer microstructure under as-cast and annealed conditions. Moreover, the enhanced yield strength and tensile strength were attributed to the inhibition of migration of dislocations due to more grain boundaries and the nanoparticles. The enhanced ductility and toughness were associated with the fine microstructure and scattering behavior of cracks by the nanoparticles. These results can be used as references in the development of high-performance steels by using the nano-sized ceramic particles. • Nanoparticles-reinforced low carbon steels were prepared by using a TiC-TiB 2 /Al master alloy. • The as-cast and annealed microstructures were refined by adding TiC-TiB 2 nanoparticles. • The mechanical properties were simultaneously enhanced by adding TiC-TiB 2 nanoparticles. • An E2EM model indicated that the nanoparticles could be utilized as heterogenous cores.