Abstract

Austenite microstructural evolution during flat hot rolling of low carbon steels has been widely investigated, and the role of microalloying elements such as Nb is well established. During hot rolling, Nb retards static softening kinetics, which results in pancaked austenite before phase transformation and in refined room temperature microstructures. However, in bar hot rolling, the carbon contents are usually higher, and during rolling higher strains and strain rates, and much lower interpass times are applied. The role of Nb at these conditions is not so clear. In this work, multipass torsion tests were performed to study the effect of Nb in solid solution or as strain-induced precipitate on the strain accumulation potential under conditions representative of bar hot rolling, with low carbon (0.05%C) and medium carbon steels (0.25%-0.35%C). When the interpass time decreased to 0.5s, dynamic recrystallization was detected in multipass deformation sequences for both Nb and plain carbon steels. Although Nb in solid solution retarded dynamic recrystallization kinetics, strain-induced precipitation did not prevent it. Increasing carbon content did not lead to significant differences in the stress-strain or austenite microstructural evolution mechanisms of the steels. Significant grain refinement of the dynamically recrystallized austenite with Nb microalloying was also observed.

Keywords:Bar hot rolling; Strain-induced precipitation; Strain accumulation; Recrystallization; Austenite microstructural evolution