THE INFLUENCE OF NIOBIUM SUPERSATURATION IN AUSTENITE ON THE STATIC RECRYSTALLIZATION BEHAVIOR OF LOW-CARBON MICROALLOYED STEELS

This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during a n isothermal holding period following deformation. Four low carbon ste els, microalloyed with Nb, were used in this investigation Three of th e steels had variations in Nb levels at constant C and N concentration s. Two steels had different N levels at constant C and Nb concentratio ns. The results from the isothermal deformation experiments and the su bsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (T-RXN) increases with inc reasing Nb supersaturation in austenite. Quantitative transmission ele ctron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 time s larger than Nb(CN) volume fractions found within the grain interiors . This high, localized volume fraction of Nb(CN) subsequently led to h igh values for the precipitate pinning force (F-PIN). These values for F-PIN were much higher than what would have been predicted from equil ibrium thermodynamics describing the solution behavior of Nb in austen ite.