Basic research on the effect of nitrogen content on nitride formation and grain size in titanium microalloyed low carbon steel

Abstract

This study targeted titanium microalloyed low-carbon steel, focusing on the effects of nitrogen content on nitride formation and austenite grain size. A continuous nitrogen-increase experiment was conducted in molten steel, supported by thermodynamic calculations and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), optical microscopy (OM), high-temperature confocal laser scanning microscopy (HT- CLSM) and electron probe microanalysis (EPMA). The results show that the nitrogen content in the steel rises with prolonged nitrogen blowing, reaching saturation after approximately 35 min. SEM-EDS revealed the mechanism of complex nitride inclusions in steel gradually complicated with the increase of nitrogen content. These progress from almost no nitrides (0.0064 % N) to combinations such as TiN, BN, and TiN-BN (0.0128 % N), and further to complex inclusions including TiN–Al₂O₃ and BN-Al₂O₃ (0.0232 % N), eventually reaching Al₂O₃-TiN-BN (0.0386 % N). Thermodynamic calculations reveal a sequential formation of nitrides: TiN→BN→AlN. The maximum quantity of nitrides increases with nitrogen content, and the revelation of the mechanism by which BN inhibits formation of AlN through thermodynamic calculations combined with in situ observations. BN and AlN formation involve nitrogen extraction from TiN, reducing the quantity of TiN. Additionally, the lattice mismatch calculation and EPMA show that the three nitrides can significantly promote austenite grain refinement during heat preservation. This behavior pins austenite grains, refining their size. Nitrogen solubility in this system ranges from 0.0400 % to 0.0451 %, with the optimal nitrogen addition recommended to stay below 0.02 %. These findings provide guidance for controlling nitrogen levels to enhance steel properties through microstructural refinement.