Effect of Niobium on Cold Work-Annealing Process of Low Carbon Steels

Abstract

The effect of small additions of niobium on the cold work-annealing behavior of low carbon steels was investigated to assess feasibility of application of such process to niobium steels. The base composition of test steels was 0.1%C-(0.15, 0.40)%Si-(0.6, 1.2)%Mn and niobium was added in two levels of 0.03% and 0.06%. After cold reduction of 20-50%, isothermal and isochronal annealing was performed at temperatures between 550 and 700°C.The addition of niobium strongly retarded softening and recrystallization, and resulted in an elongated grain structure after recrystallization. A high manganese content remarkably intensified this retardation effect.
The hardening caused by precipitation of Nb (CN) remained same as in as-hot-rolled condition even after recrystallization. From observations of thin film specimens and analysis of acid-insoluble niobium, a mechanism is proposed to account for the retardation in terms of pre-existing Nb (CN), precipitation of niobium originally in solid solution, and also reversion-re-precipitation of Nb (CN) due to introduction of a high density of preferred sites of precipitation (i. e. dislocations).