2014 Volume 55 Issue 6 Pages 899-906
The aim of this study is to clarify the high-temperature strengthening mechanism of Nb-bearing ultra-low carbon steel, which is well-known as superior steel for high-temperature applications. Observations by Three-dimensional Atom Probe (3DAP) suggested that the Nb atoms are either distributed in a solid solution within the grain or segregated at the grain boundary after hot-rolling. The strength at 600°C increases significantly upon addition of Nb, and the corresponding dominant strengthening mechanism is considered to consist of the following: the resistance for the dislocation gliding motion due to solute Nb, the retardation of the dislocation climbing-up motion due to solute Nb and Nb–C dipoles, and the resistance of the dislocation motion caused by the Nb–C(N) clusters formed when the materials are heated up to 600°C within 10 s and then held for 600 s. Further, compared with Nb-free steel or 0.1% Nb-bearing steel, 0.3% Nb-bearing steel has considerably reduced ductility at 600°C. This is attributed to the retardation of recovery due to the Nb addition. TEM observations imply that the dynamic recovery takes place easily during the tensile deformation at 600°C in Nb-free steel or 0.1% Nb-bearing steel, whereas the tensile stress increases significantly because of the work hardening presumably caused by the retardation of the restoration process by further addition of Nb. Hence, a rupture followed by necking is thought to occur easily. Moreover, there is a possibility that the segregated Nb at the ferrite grain boundary might affect the dislocation behavior resulting in an increase in the steel strength at a high temperature and a retardation of the recovery process. This possibility will be investigated in a future work.