Role of Combined Addition of Niobium and Boron and of Molyb- denum and Boron on Hardnenability in Low Carbon Steels

Effects of the combined addition of niobium (Nb) and boron (B) and of molybdenum (Mo) and B on hard-enability were investigated using low carbon steels. Strength synergically increases due to the combined addition of Nb and B and that of Mo and B. It is thought that strength increases due to these combined additions because austenite (*g) to ferrite (*a) transformation is retarded and bainite transformation is promoted due to the increase in the segregated B along the *g grain boundary before *g to *a transformation. The mechanism for the increase in the segregated boron along the *g grain boundary by these combined additions is considered below. Fe23(C, B)6 precipitates formed along the *g grain boundary are suppressed by these com-bined additions because of the suppression of C diffusion towards the *g grain boundary due to the precipitation of the fine dispersive niobium-titanium carbonnitride (Nb, Ti)(C, N) or titanium-molybdenum carbonnitride (Ti, Mo)(C, N) and the formation of C clusters of Nb and Mo during rolling or during cooling after rolling. Therefore, the segregated B along the *g grain boundary increases and *g to *a transformation is retarded. The combined addition of Nb and B or that of Mo and B in low C bainitic steel is effective for increasing strength without deteriorating low temperature toughness. It is clarified that the increments of hardenability by the combined addition of Nb and B is different from that of Mo and B due to the difference of the amount of carbide precipitates.