Formation of acicular ferrite and influence of vanadium alloying

Abstract

The effect of vanadium microalloying in promoting a tough, acicular ferrite microstructure in C-Mn steels has been investigated. The microstructure obtained consisted of fine interlocking ferrite plates and was indistinguishable from acicular ferrite developed in steel weld metals, apparently from the intragranular nucleation of ferrite at inclusions. A number of variables were examined in high purity experimental steels including composition and heat treatment conditions, and related to a metallographic examination of the microstructure by high resolution micro-analytical transmission electron microscopy and surface analysis. A comprehensive study of the inclusions in the steels, containing different ratios of oxygen and nitrogen concentration, did not find any significant evidence that inclusion assisted nucleation was the sole determining factor in producing acicular ferrite. Moreover, no evidence could be found to relate vanadium alloying to significant vanadium nitride precipitation, either separately, or associated with the inclusions. Thus, in the present steels, any possible alternative influence of vanadium on intragranular ferrite nucleation is not obscured by effects associated with the inclusion population. The vanadium concentration appeared to be the most important influence in developing an acicular ferrite microstructure in these experimental steels, and this is not inconsistent with previous reports in the literature of a beneficial ‘vanadium effect’. Evidence for vanadium segregation in the microstructure was found, which may be related to the effect of vanadium in encouraging the formation of acicular ferrite. Even when there is good evidence that inclusions are responsible for intragranular ferrite nucleation (as, for example, in steel weld metals), a 1 :1 inclusion-ferrite relationship has been difficult to establish. Thus, even an inclusion activated nucleation theory is likely to require additional intragranular ferrite formation without inclusion assistance, such as sympathetic or autocatalytic nucleation, and this could be reflected in the present study by vanadium atom clustering facilitating an alternative intragranular ferrite nucleation mechanism.