Effect of Dissolution and Precipitation of Nb on Phase Transformation, Microstructure, and Microhardness of Two High-Nb Pipeline Steels

Niobium (Nb) as an important microalloying element is widely applied in high strength pipeline steels. In this work, the continuous cooling transformation diagrams of two high-Nb steels with and without hot deformation were studied using a Gleeble 3500 thermal simulator. The amounts of dissolved Nb, undissolved Nb, and precipitated Nb were determined by inductively coupled plasma-atomic emission spectrometry. Results show that the increasing of Nb content in the high-Nb steels can restrain the prior austenite grain growth, dynamic, and/or static recrystallization; moreover, it can suppress polygonal ferrite transformation and promote acicular ferrite and bainite transformation, refining the microstructure and increasing the microhardness as a consequence. Nevertheless, the amplified Nb content in steels escalates trends of strain-induced Nb(C,N) precipitation. The increase in the amount of Nb(C,N) precipitates promote the polygonal ferrite and acicular ferrite transformation, while also decrease the microhardness. The results from this work show that the higher Nb content of 0.13% in the tested steel is unnecessary.