The effect of Nb addition of less than 0.05 mass% on the quenching and tempering behavior of spheroidized eutectoid steel, which has usually being applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability upon 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 with those of Nb-free steel. 3) While hardly any effect of Nb addition on the hardness was observed during low- temperature tempering, not only the impact toughness but also the fatigue durability was improved by 0.01 mass% Nb addition. 4) Atom probe tomography (APT) analyses revealed that the precipitation of carbon in solution directly resulted in the formation of ε - and/or θ -carbides with carbon contents of around 25 at% without the formation of clusters with 10–15 at% C upon the addition of a trace amount of Nb. 5) For the same P content, the average bulk concentration of P in the martensite phase markedly increased with the addition of up to 0.05 mass% Nb. 6) Regarding the optimum Nb content of 0.01 mass% for various mechanical properties upon the low-temperature tempering of martensite, it is considered that the mechanical properties are dominated by the balance between the positive effect of promoting carbide precipitation during low-temperature tempering by Nb addition and the negative effect of deteriorating the toughness with increasing bulk concentration of P in the martensitic phase upon the addition of more than 0.02 mass% Nb.
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