COMPOSITIONAL AND MICROSTRUCTURAL CHANGES WHICH ATTEND REHEATING AND GRAIN COARSENING IN STEELS CONTAINING NIOBIUM

This work describes the effect of temperature on both the microstructu re 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 c onstant N levels, and the fourth exhibited a difference in N concentra tion. The average prior-austenite grain size was determined using quan titative metallographic techniques as a function of reheat temperature . The corresponding amount of Nb in solution in austenite was determin ed from atom probe analysis. Results from this investigation indicate that at elevated temperatures, representative of typical reheating pra ctice, a smaller amount of Nb is soluble in austenite than what would have been predicted from any existing solubility relation for NbC(x) i n austenite. The solubility of Nb and C in austenite for the low-N ste els is described by the relation: Log [Nb][C] = 2.06 - 6700/T. Additio nally, it is shown that undissolved NbC(x) particles are present in au stenite approximately 125-degrees-C above the grain-coarsening tempera ture.