High Temperature Tensile Properties and Its Mechanism in Low-Carbon Nb-Bearing Steel

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 degrees 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 degrees C within 10 s and then held for 600s. Further, compared with Nb-free steel or 0.1% Nb-bearing steel, 0.3% Nb-bearing steel has considerably reduced ductility at 600 degrees 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 degrees 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.