Mechanism of the Formation of Columnar Structure in Low-carbon Steel Wires during Annealing

Abstract

Mechanism of the formation of columnar structure in low-carbon steel wires annealed at temperatures below A₃-point was studied.
(1) Columnar structure was developed in wires with medium reduction in area by cold drawing and annealed under a decarburizing atmosphere in the temperature range of α-γ two phase region.
(2) Columnar grains were nucleated in the recrystallizaed grains a little below the surface layer of wires, and were characterized by their distinct preferred orientations and directional growth from the beginning of the grain growth.
(3) Columnar structure exhibited nearly the same cylindrical texture as the recrystallized structure.
(4) A mechanism was proposed for the directional growth of the columnar grains during annealing in the α-γ region : Small recrystallized grains with a cylindrical texture are rearranged into small blocks of grains with a same orientation in the course of γ/α transformation which takes place by decarburization. A large growing grain neighboring these blocks continues its growth by coalescence with the blocks. The activation energy for this process is considered to be less than a process in which each small grain coalesces individually into a large grain.