Research On Precipitation And Microstructure Of X80Pipeline Steel In Hot Rolling Process

In recent decades, with the transport distance and pressure increasing, pipeline steelis developing to the high strength, high toughness, good weldability and corrosionresistance of high performance. X80pipeline steel is the preferred and important steel forpipeline transportation, grain refinement is the most effective way to improve the strengthand toughness. The main micro-alloy Nb and Ti in X80pipeline steel play an importantrole on enhancing the strength and toughness, combined with precipitation theory andcontrolled rolling technique to obtain high strength steels are important research fields.This paper takes X80pipeline steel containing Nb and Ti as the research object. Thestatic recrystallization behavior is investigated on Gleeble3500thermo-mechanicalsimulator. Discussing the stress relaxation curves at different deformation temperature anddrawing PTT curve to analysis the relationships between precipitation, temperature andtime of precipitation by stress relaxation experiment; The microstructure of hot deformedX80pipeline steel is observed by OM, SEM and TEM, the austenitic grains size atdifferent deformation temperature and different inter-pass time has been analyzed. Theacicular ferrite and M-A islands structure of X80have been studied. The precipitationmorphology have been observed.According to stress-strain curve under two-stage interrupted compression experiment,the static recrystallization softening rate and activation energy of X80pipeline steel havebeen calculated, the activation energy is486.5kJ/mol; The nose temperature is950℃obtained by PTT curve.The precipation theory and TEM observation show that TiNinhibits grain growth at high temperature,and Niobium carbonitride mainly precipates inthermal deformation process.The results shows that, by reasonably controlling deformation temperature andinter-pass time, the effect that Niobium and Titanium carbon inhibit austenite grain growthcould be enhanced, refined acicular ferrite and dispersively distributed M-A islandsstructure could be obtained, the strength and toughness could be improved greatly.