EFFECT OF Nb SOLUTE DRAG AND NbC PRECIPITATE PINNING ON THE RECRYSTALLIZATION GRAIN GROWTH IN LOW CARBON Nb-MICROACLOYED STEEL

A kinetic equation for austenite grain growth has been derived concerning the mutual effect of NbC and Nb solute in low carbon Nb-microalloyed steels. It is shown that both solute drag of Nb in solid solution and pinning of NbC particles inhibit the grain boundary migration during grain growth after recrystallization in low carbon Nb-microalloyed steels. At high temperatures the NbC pinning plays a dominate role for retarding the austenite grain growth with less Nb solute drag effect. An obvious Nb solute drag restraint was, however, observed at relatively low temperatures. Also, the theoretical calculations are in good agreement with experimental results. The effectiveness of drag effect of soluble atoms and pinning effect of precipitates can be characterized by a p factor. The pinning of precipitates and solute drag of soluble atoms are more effective for suppressing grain growth as p > 0 and p < 0, respectively. And the ratio of Nb in solute and Nb in precipitate as p = 0 reaches the priority and most effectively retards the grain growth. In the traditional hot rolling or austenitizing temperature range, a strong suppression for grain growth after recrystallization could be obtained due to those fine NbC particles smaller than 10 nm in Nb-microalloyed steels.