Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400–600 °C in the static CCT diagrams to 450–700 °C in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation.