Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels

Transformation behavior and morphological characteristics of acicular ferrite in linepipe steels were investigated through the use of a dilatometer and EBSD. The results show that the volume fraction of acicular ferrite increased with an increase in the amount of hot-deformation in the austenite non-recrystallization region because acicular ferrite is formed at nucleation sites such as dislocations within austenite grains. This study found that acicular ferrite is formed at approximately 600 degrees C, where the transformation behavior can be characterized as a two-stage reaction: (i) nucleation of acicular ferrite and (ii) formation of polygonal ferrite between acicular ferrite grains. EBSD analyses show that an acicular ferrite grain consists of several sub-units misoriented by 1-2 degrees and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15 degrees makes up the crystallographic packet. (c) 2007 Elsevier B.V. All rights reserved.