Microstructure and Transformation Characteristics of Acicular Ferrite in High Niobium-Bearing Microalloyed Steel

The transformation behavior and microstructural characteristics of a low carbon high niobium-bearing microalloyed pipeline steel were investigated by deformation dilatometry and microstructure observation. The continuous cooling transformation curves of the test steel were constructed. The results showed that high niobium content and deformation enhanced the formation of acicular ferrite; the microstructures changed from polygonal ferrite, quasi-polygonal ferrite to acicular ferrite with increasing cooling rates from 0. 5 to 50 ℃/s and was dominated by acicular ferrite in a broadened cooling rate higher than 5 ℃/s. The chaotic microstructure consisted of non-equiaxed ferrite and interwoven ferrite laths with high density dislocations and subunits. The results of isothermal holding treatment showed that acicular ferrite microstructure was formed at 550 - 600 ℃ and consisted of highly misoriented plate packets having internal low angle boundaries. With increasing the holding time or temperature, some low misorienta-tion boundaries changed to high misorientation owing to the movement of dislocations and coarsening of grain.