Microstructure evolution and acicular ferrite nucleation in inclusion-engineered steel with modified MgO@C nanoparticle addition

Abstract To investigate the effect of surface-modified nanoparticles (NPs) on the inclusion refinement and microstructure evolution, deoxidized experiment ingots with different amounts of modified NPs were manufactured under different cooling conditions. Laser scanning confocal microscope (LSCM) was hereby used for in-situ observation of the phase transition and microstructural evolution during heat cycle process. The results revealed that the inclusion size was always smaller under water quenching than under air cooling, and the number of inclusions was greater under water quenching. After NP addition, the nucleant inclusions were identified as MgAl2O4 spinel and irregular TiN inclusion from SEM-EDS measurement and equilibrium calculations using Factsage thermodynamic software. The higher cooling rate under water quenching resulted in less polygonal ferrite decrease and the formation of bainite in the steel. The LSCM experiments showed that ferrite side plates (FSP) always formed on the boundary prior to the formation of acicular ferrite (AF) on the intragranular inclusions, and the start transformation temperatures of FSP and AF phases both lowered after NP addition. The higher cooling rate and NP addition contributed to AF formation and increased the degree of interlocking of the AF phase. Finally, the relationship between the characteristics of inclusions and the kinetics of AF was investigated.