Acquiring high density of ultrafine second-phase nanoparticles quickly in low-carbon steel

Abstract

In the general smelting process, nonmetallic compounds manifest as inclusions. These inclusions, typically larger than 5 μm, can significantly reduce the strength, toughness, and processability of steel. More critically, they pose a threat to the service life of steel. To mitigate these adverse effects, the practice of inclusion modification has been developed. Typically, calcium and magnesium elements are employed for this purpose. However, despite these efforts, the size of modified inclusions often remains at the micron level. Although there is some improvement in mechanical properties, the presence of these inclusions still compromises the steel matrix. Therefore, controlling the size, quantity, and distribution of nonmetallic inclusions during steel manufacturing becomes imperative. When inclusions are reduced to nanometer size, the formation of a high density of nanosized second-phase particles can substantially improve the strength and toughness of the steel. In this study, we prepared Fe–0.04C–1.5Mn–0.5Ti–0.5Al2O3 (Fe–TAMO) steel by applying a dynamic magnetic field during the smelting process. This was followed by grain size optimization and equiaxed optimization through rolling and annealing processes. Transmission electron microscopy revealed that the second-phase particles were uniformly dispersed within the as-cast Fe–TAMO steel matrix. The density of these particles reached 3.3 × 1015 m−2, with an average diameter of 2.75 ± 0.803 nm. Energy-dispersive x-ray spectroscopy analysis identified these particles as oxides of Ti–Al–Mn. The compressive mechanical properties of Fe–TAMO steel, in its as-cast, as-rolled, and as-annealed states, were evaluated using a universal testing machine. The grain size of the as-cast Fe–TAMO steel is 143 μm, with a compressive yield strength of 150 MPa, approximately double that of as-cast pure ferritic steel. After rolling, the grain size decreased to 119 μm, and the compressive yield strength increased to 484 MPa. Following annealing, the grain size was further reduced to 64 μm, with a compressive yield strength of 334 MPa. These results demonstrate that the applied method effectively minimizes the size of the second-phase particles, with most controlled within 5 nm. The densely distributed second-phase nanoparticles significantly improve the steel strength, while subsequent heat treatments allow for adjustments in grain size to further enhance mechanical properties. This method streamlines the process flow to a single step, achieving uniform dispersion of ultrafine nanosized second-phase particles in the steel matrix in about a mere 3 minutes. Moreover, it holds great potential for industrial production, offering a new avenue for the mass production of high-performance steel.

Keywords

• induction melting
• dynamic magnetic field
• ferrite steel
• second-phase nanoparticles
• mechanical properties