Effect of Nb on Recrystallization after Hot Deformation in Austenitic Fe-Ni-C

Abstract

In high-strength low-alloy steels, the deformation, restoration and precipitation effects which occur during finish rolling in the austenitic condition are difficult to study because of the transformation of austenite to ferrite and/or martensite on cooling. To overcome this difficulty a series of austenitic Fe-Ni-C alloy were prepared with and without niobium to allow a more detailed study of the phenomena occurring during finish rolling. The effect of niobium on the static recrystallization of austenite after single-pass hot compression has been studied for a strain range of 0.25-0.9 at temperatures in the range of 850-1000°C.The results obtained by optical microscopy and transmission electron microscopy indicated that the retardation of static recrystallization was caused by either niobium in solution and/or strain-induced precipitates. Static recrystallization in the niobium steel was retarded when niobium was in solution by several times compared with the niobium-free steel. However, much stronger retardation in the niobium steel was observed when strain-induced precipitates were formed. It was observed that the dislocations, subgrain boundaries, and grain boundaries are preferential sites for strain-induced NbC precipitation. Small precipitates (<3 nm) were responsible for the retardation of recrystallization by the pinning of subgrain boundaries and grain boundaries.