2019 Volume 105 Issue 1 Pages 76-85
Effect of Nb addition less than 0.05 mass% on the quench and tempering behavior of spheroidized eutectoid steel, which has been usually applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability with brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared to those of Nb free steel. 3) While the effect of Nb addition on the hardness change during low temperature tempering was hardly observed, not only the impact toughness but also the fatigue durability were improved with 0.01 mass% Nb addition. 4) APT (Atom Probe Tomography) analyses indicated that the precipitation of carbon in solution proceeded directly to the ε and/or θ carbides with carbon contents of higher than 25 at% by Nb addition without going through a clustering process up to 10~15at% during low temperature tempering. 5) In spite of the same content of P, the average bulk concentration of P in the martensite phase markedly increased with the addition of Nb up to 0.05 mass%. 6) Regarding the optimum content of 0.01 mass% Nb on the various mechanical properties under the low temperature tempering of martensite, it is considered that they are dominated by the sum of the positive effect for promoting carbide precipitation during low temperature tempering with Nb addition and the negative effect for deteriorating the toughness with increasing bulk concenteration of P in the martensitic phase with addition of Nb higher than 0.02 mass%.