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 (γ) to ferrite (α) transformation is retarded and bainite transformation is promoted due to the increase in the segregated B along the γ grain boundary before γ to α transformation. The mechanism for the increase in the segregated boron along the γ grain boundary by these combined additions is considered below. Fe23(C, B)6 precipitates formed along the γ grain boundary are suppressed by these combined additions because of the suppression of C diffusion towards the γ 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 γ grain boundary increases and γ to α 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.
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