Influence of Nb and V on the Proeutectoid Ferrite Reaction in Low Carbon High Strength Steels

Abstract

The formation kinetics and the growth rate of proeutectoid ferrite (p.f.) have been examined by means of optical microscopy in commercial low carbon high strength steels containing Nb(0.04-0.08 wt%) and V(0.10 wt%) at temperatures ranging from 640°C to 700°C.
The growth of p.f. immeadiately after solution treatment is remarkably retarded by a small amount of Nb addition. In contrast, V addition rather promotes it. The nucleation is also dramatically supressed by Nb, but not so much by V. The precipitation of Nb(C, N) from austenite increases both the growth and the nucleation rate of p.f.
The growth of p.f. containing Nb and V in solution has been discussed in terms of their carbide forming potential and variation of Ae3 temperature with their additions. The effects of Nb and V can be explained by the "impurity drag" model and "no-partition Ae3" concept, respectively. The acceleration of p.f. reaction resulting from Nb(C, N) precipitation is tentatively ascribed to the reduction of C and Nb atoms in solution. The "pinning" effect of Nb(C, N) particles appears to be relatively small on the growth retardation of p.f.