The aim of this study is to clarify the high-temperature strengthening mechanism of Nb-bearing ultra-low carbon steel, which is wellknown 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 NbC dipoles, and the resistance of the dislocation motion caused by the NbC(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.

For the purpose of understanding the mechanism of high temperature embrittlement, especially in the heat affected zone of B bearing low carbon alloy steel, the role of B addition is studied in terms of grain boundary segregation and nitride precipitation of B. BN precipitates at prior austenite grain boundary are supposed to be a dominant cause to the embrittlement of the steels when tensile stress is applied at 600°C followed by heat cycle of welding, where the environment of fire is simulated. Nitride formation is changed from intragranular TiN after hot rolling followed by reheating to 600°C for tensile test to intergranular BN at prior austenite grain boundary after reheating to 600°C following heat cycle of welding. Consequently, the grain boundary fracture takes place for the specimens subjected to the heat cycle of welding when tensile stress is applied after reheating to 600°C, because the intergranular BN leads to the formation of cavity along prior austenite grain boundary. This mechanism is experimentally verified by the fact that high temperature embrittlement is able to be prevented by either the addition of Zr or the more addition of Ti which may fix nitrogen as more stable nitride and inhibit the dissolution of nitride during welding heat cycle.

For the purpose of understanding the mechanism of high-temperature embrittlement, especially in the heat-affected zone of B-bearing lowcarbon alloy steel, the role of B addition is studied in terms of grain-boundary segregation and nitride precipitation of B. BN precipitates at the prior austenite grain boundary are supposed to be the dominant cause of the embrittlement of steel when tensile stress is applied at 600°C, followed by heat cycle of welding, where a fire environment is simulated. After hot rolling, followed by reheating to 600°C for a tensile test, intragranular TiN is changed to intergranular BN at the prior austenite grain boundary after reheating to 600°C following the heat cycle of welding. Consequently, the grain-boundary fracture takes place in the specimens that are subjected to the heat cycle of welding when tensile stress is applied after reheating to 600°C because the intergranular BN leads to the formation of cavity along the prior austenite grain boundary. This mechanism is experimentally verified by the fact that high-temperature embrittlement can be prevented by either the addition of Zr or the addition of more Ti, which may fix nitrogen to a more stable nitride state and inhibit the dissolution of nitride during welding heat cycle.