Research On Microstructure And Properties Of Acicular Ferrite Pipeline Steel

With the rapid increasing of the demand for oil/gas, the research of pipelinesteels have gained unprecedentedly tremendous attention. As the exploitation of fossilfuel resources have been shifted to adverse environment, the properties of pipelinesteels need to be considerably improved. The addtion of micro-alloy elements andoptimizing production process are the most efficient way to acquire pipeline steelswith overall quality. The addition of Nb can improve the properties of pipeline steel,and the Thermo Mechanical Controlling Processing (TMCP) can refine microstrcture.So the effect of Nb and TMCP process on the properties of pipeline steels should besystematicly studied.With respect to the situation mentioned above, the present study was focused onX65pipeline steel. The effect of Nb-alloy element on the phase transformation andTMCP process on the properties of the steel were investigated by using Gleeble1500.The main conclusions were achieved as follows:(i) With the increase of austenitizingtemperature, the microstructures changed from acicular ferrite to bainite ferrite.(ii)Nb(CN) precipitates can promote acicular ferrite transformation by providingnucleation sites while solute Nb can promote the bainite ferrite transformation bysolute dragging effect.(iii) The propertion of acicular ferrite and bainite ferrite couldbe controlled by the state of Nb (Nb in solution or Nb(CN) as precipitates).(iv)Rolling in the Austenitic non-recrystallization zone can increase the transformationstart and finish temperature, and promote the acicular ferrite transformation, extendthe acicular ferrite zone in the CCT diagram.