Effect of element Nb and V alloying on austenite grain growth behavior in 45Mn5Al4 steel

ABSTRACT A new martensitic low-density steel was designed, and the behavior of austenite grain growth, along with the influence of niobium Nb and V microalloying, was investigated. The results indicated that the austenite grain growth rate in the new martensitic low-density steel was rapid, the austenite grains gradually increased in size with rising austenitizing temperatures, reaching a coarsening temperature of approximately 1000°C. Nb microalloying significantly refined the austenite grain structure and elevated the coarsening temperature to about 1200°C. Nb microalloying resulted in the formation of two size ranges of NbC precipitates: those smaller than 0.3 μm and those larger than 2 μm. The formation of NbC precipitates was the primary reason for the reduced grain growth rate in the Nb microalloyed experimental steel. The NbC precipitate phase hindered the migration of austenite grain boundaries, thereby slowing down the grain growth rate. The NbC precipitates smaller than 0.3 μm dissolved in large quantities when heated above 1200°C, weakening the nailing effect on grain boundaries, which was the main reason for the rapid grain growth observed in the experimental steel. Conversely, the NbC precipitates larger than 2 μm were difficult to eliminate, even when subjected to prolonged heating at 1250°C. The austenite grain growth model for the newly designed martensitic low-density steel was calculated as: D = 1.47 × 103 · t0.097 · e−52423/RT, (900–1150°C) D = 3.45 × 1014 · t0.108 · e−367331/RT, (1200–1250°C).