1966 Volume 52 Issue 1 Pages 42-54
The effects of niobium on silicon-killed, low carbon steel were investigated. Most of the data relate to 450-1b laboratory melts, forged and rolled to 3/4 in. plate. Data from one heat of commercial steel were also examined.
In the as-rolled condition, it was found that small additions of niobium increased the yield strength and raised the transition temperature markedly, but that larger additions were less effective.In the normalized condition, the effects on yield strength were similar to those in the as-rolled condition but the changes were less marked. The transition temperature was improved with all levels of niobium, the smallest additions (up to 0.04%) being most effective.
Commercial plate of a similar composition to the laboratory steel was procured with niobium contents of approximately zero, 0.010% and 0.023%, in 1/4 in., 3/4in., and 1-1/2 in. thicknesses. The yield strength and transition temperature results obtained on the commercial material qualitatively corroborated the findings of the laboratory steels. Niobium was most effective in increasing the yield strength in the 1/4 in. plate, the effectiveness decreasing as the plate thickness increased. Rotating beam fatigue tests showed that niobium slightly increased the fatigue limit of smooth bars and had no effect on the fatigue limit of notched bars. The fatigue ratio was unaffected by niobium for smooth bars and decreased somewhat for notched bars.
Limited data, based on electrolytically extracted residues, indicated that the major bulk of the niobium added to the steels was partitioned to the carbide phase. Similarly, much of the nitrogen present in the steels was found to occur in the residues.
The evidence indicated that the effects of niobium on the yield strength and notch ductility of normalized carbon steel were qualitatively explicable in terms of grain refinement, and precipitation strengthening. The evidence did not account for the pronounced effects of small quantities of niobium on as-rolled steel.