Compositional and microstructural changes which attend reheating and grain coarsening in steels containing niobium

This work describes the effect of temperature on both the microstructure and composition of microalloyed steel austenite in the as-reheated condition. Four laboratory steels of similar C levels were analyzed in this investigation. Three steels had different Nb concentrations at constant N levels, and the fourth exhibited a difference in N concentration. The average prior-austenite grain size was determined using quantitative metallographic techniques as a function of reheat temperature. The corresponding amount of Nb in solution in austenite was determined from atom probe analysis. Results from this investigation indicate that at elevated temperatures, representative of typical reheating practice, a smaller amount of Nb is soluble in austenite than what would have been predicted from any existing solubility relation for NbCx in austenite. The solubility of Nb and C in austenite for the low-N steels is described by the relation: Log [Nb][C] = 2.06 - 6700/T. Additionally, it is shown that undissolved NbCx particles are present in austenite approximately 125 degree C above the grain-coarsening temperature