A novel dual-strengthening technology combining external second phase nanoparticle method and hot compression deformation process in micro-alloyed medium carbon steel

Herein, a novel dual-strengthening technology that combines the external second-phase nanoparticle addition method with the hot compression deformation tests was carried out to investigate the effects of surface-modified nanoparticles (NPs) and deformation conditions on the characteristics of inclusions and the evolution of microstructure in the micro-alloyed (MA) medium carbon steels. The results revealed that the inclusions in the steels were divided into single phase inclusions and composite phase inclusions. With the addition of NPs, Al2O3 inclusions were replaced by MgAl2O4 spinel, and the composite inclusions mainly consisted of TiN–VN, MgAl2O4–TiN and MgAl2O4–TiN–MnS inclusions, and the same types of inclusions became significantly smaller in size. According to the EDS results, some of the inclusions acted to induce ferrite nucleation, and the MgAl2O4–TiN–MnS composite inclusion was more favorable for the nucleation of acicular ferrite (AF). The deformation induced ferrite (DIF) started to form at the deformation temperature of 750 °C and the maximum stress peak in the NPs-bearing steel was 516 MPa, which was 28.4% higher than that in the original steel. The proportion of ferrite increased as the deformation temperature decreased. The dispersed NPs contributed to the ferrite nucleation and the grain refinement as the inclusion-modifying agent, which improved the material properties, and the relevant strengthening mechanisms were explained.