Orientation Distribution of Proeutectoid Ferrite Nucleated at Prior Austenite Grain Boundaries in Vanadium-added Steel.

Abstract

Recently, an improvement in the properties and performance of steels by refining the ferrite grain size down to the one micrometer range has been actively pursued. Even though the heavy deformation of super-cooled austenite followed by accelerated cooling has been generally accepted as an effective way of producing ultrafine grained steels, other approaches that exploit the fine precipitates dispersed at both the grain boundaries and in the matrix as ferrite nucleation sites have also been attempted. Ishikawa et al. showed that vanadium nitride (VN) particles, which precipitated at the austenite grain boundaries, enhance ferrite nucleation and induce a random distribution of the ferrite orientation. Furuhara et al. also showed that the orientation of the intragranular ferrites, which nucleated at the compound precipitates composed of manganese sulfide and vanadium nitride, deviates from the Kurdjumov– Sachs (K-S) orientation relationship. They suggested a method to evaluate the orientation characteristics of the intragranular ferrite grains by comparing the theoretically possible misfit angles between the ferrite grains with the measured values, which can be calculated from the K-S relationship. Recently, Suh et al. proposed a method to evaluate the deviation angle of ferrite from the K-S relationship by an electron backscattered diffraction (EBSD) technique. By measuring the orientation of the ferrite grains that formed at prior austenite grain boundaries and that of the adjacent martensite packets that formed within the austenite grains, the orientation characteristics of the ferrite grains can be evaluated in terms of the deviation angle of the ferrite grains from the K-S relationship. In this paper, this method was applied to compare the orientation distribution of the ferrite grains nucleated along the austenite grain boundaries in a V-containing steel to that of base steel. The difference in the orientation distribution between the two steels is explained by the nucleation behavior of the ferrites observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and EBSD.