Study On Recrystallization And Continuous Cooling Transformation Of Hot Deformed Austensite Of X120Pipeline Steel

To meet the needs of large-diameter and high-pressure transmission of modernpipelines, the development of high-level pipeline has been paid more and more attention.The research and development of X120pipeline steel needs to combine alloy design andcontrolled rolling and cooling processes reasonably. Uniaxial hot compression tests wereperformed on Gleeble3500Thermo-mechanical simulator to simulate controlled rollingand cooling processes of X120pipeline steel. Recrystallization behavior of the test steelwas investigated. The corresponding continuous cooling transformation (CCT) diagramwas constructed. The effect of cooling rate after hot deformation on the microstructure andmechanical properties of X120pipeline steel was investigated.Under the condition of high temperature and low strain rate, dynamicrecrystallization occurred easily. When strain rate was10s-1, the dynamic recrystallizationdidn't occur in the deformation process. By studying the strain-stress curves andobservations recrystallization grains growth process, the Zener-Hollomon parametricequation was established. The Qd(activation energy) of the X120pipeline steel was498.29kJ/mol.The static recrystallization fraction under different deformed temperatures wasestimated by means of compensating. The effect of deformation temperature andinter-pass time was significant. The activation energy(Q) of static recrystallization of theX120pipeline steel was401.56kJ/mol. The precipitation-time-temperature curve of theX120pipeline steel was also obtained.The resultant microstructures ranged from polygonal ferrite (PF) and acicular ferrite(AF) at slow cooling rates to lath bainite (LB) at fast cooling rates. The results show thatwith the increasing cooling rate, the bainite plate was refined and the volume fraction ofgrain with high-angle boundary increased. Mechanical properties test results showed thatharden-ability, yield strength and ultimate tensile strength were enhanced by increasingcooling rate, which were corresponding with the increase of fine lath bainite andhigh-angle boundary, respectively. It is also indicated that the formation ofhomogeneously distributed fine precipitates was promoted by a high cooling rate, and the precipition strengthening effect is better.The physical simulation results of controlled rolling and cooling process had areference value on the development ultra-high strength X120pipeline steel. Test resultsprovided basis for formulating rolling technology window.