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

Year: 2025

Type: article

Abstract: This study investigates 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 advanced characterization techniques (SEM-EDS, OM, HT-CLSM, EPMA). The results show that the nitrogen content in the steel rises with prolonged nitrogen blowing, reaching saturation after approximately 35 min. As nitrogen content increases, the types and sizes of nitrides in the steel expand, accompanied by the formation of more complex composite inclusions. 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 (1480~1580 °C), BN (1290~1430 °C), and AlN (1070~1470 °C). The maximum quantity of nitrides increases with nitrogen content, with nitrogen preferentially binding to boron, thereby inhibiting AlN formation. BN and AlN formation involve nitrogen extraction from TiN, reducing the quantity of TiN. Additionally, Lattice mismatch analysis indicates that TiN, BN, and AlN, with mismatch degrees below 12%, effectively induce austenite nucleation and form abundantly at grain boundaries. 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.

Source: Journal of Materials Research and Technology

DOI: 10.1016/j.jmrt.2025.03.090