On the Formation Potential of Acicular Ferrite Microstructure in Different Steel Grades Focusing on the Influence of Carbon Content

摘要

Since the 1970s comprehensive effort was spent on the explanation of the acicular ferrite formation. Acicular ferrite (AF) nucleates intragranularly on non-metallic inclusions. The needle or lenticular shaped plates radiate in various directions and create a chaotic, interlocking microstructure. The growth of acicular ferrite grains is diffusionless, but excess carbon is rejected to the remaining austenite shortly after transition. The carbon enriched austenite transforms during the ongoing cooling process to perlite, bainite or martensite, or remains as retained austenite in the final structure. The created multiphase microstructure provides excellent mechanical properties, most notably toughness, so that acicular ferrite steels are of increasing interest for steel producers. In literature four main nucleation mechanisms are described: destruction of the crystal structure, creation of dislocation arrays, reduction of lattice mismatch and chemical changes in the local matrix. Literature suggests that a combination of at least two effects is responsible for the nucleation of acicular ferrite, but the exact impact of the mechanisms is not completely understood yet.