Investigation of Strain-Induced Precipitation of Niobium Carbide in Niobium Micro-Alloyed Steels at Elevated Temperatures

Abstract

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s(-1) strain rate at 900, 925, 950, and 1000 degrees C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 degrees C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 degrees C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 degrees C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 degrees C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 degrees C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 degrees C.