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 °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° and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15° makes up the crystallographic packet.