On the Fractal Nature of Acicular Ferrite, and V(C,N) Precipitation in Medium Carbon Microalloyed Steels

摘要

Acicular ferrite (AF) is a highly sub-structured non-equiaxed ferrite which is obtained from austenite by isothermal transformation or under continuous cooling. It grows by a mixed diffusion-shear transformation at a temperature slightly higher than upper bainite range. The AF plates nucleate first on non-metallic inclusions before growing autocatalytically in a chaotic interlocked arrangement. The misoriented disposition of plates produces an optimal strength-toughness combination.rnTo better understand the particular space array of AF plates, fractal analysis has been conducted on medium carbon vanadium microalloyed steels. The determination of fractal parameters makes possible the characterisation of AF irregularity from both qualitative and quantitative point of view.rnA fine precipitation inside the AF plates has been evidenced and analysed with the help of transmission electron microscopy (TEM). This kind of precipitation seems to be close related to the influence of alloy elements like vanadium in AF transformation.