Unrecrystallized Area Controlled Rolling And Controlled Cooling Impact Study On The Nb Steel Organization

With the quick development of niobium microalloyed technology in the world,its products have gradually moved towards to low carbon high niobium steel.Niobium microalloyed steel basically all need to roll in austenite recrystallization zone and non-recrystallization zone.As the content of niobium in steel increased significantly will inhibit strongly the deformed austenite to recrystallize.Using above technology to low carbon high niobium steel can cause the atructure and texture inhomogeneity appearing mixed crystal organization.Aiming at the characteristics,this article exploringly adopts the controlled rolling technology in the non-recrystallization zone of niobium microalloyed steel which makes the whole process in austenite non-recrystallization zone. The rolling experiments were evaluated to study evolvement regularity of recrystallization and deformation band and phase transformation,mechanics properities and strengthen mechanism under the condition of controlled rolling and controlled cooling in non-recrystallization zone.Thermal simulation experiments were used for investigation of original austenite grain size with soaking temperature of low carbon high niobium steel and X70 pipeline steel.The large number of undissolved Nb(C,N) and other particles pinning austenite grain boundaries led to produce more uniform austenite before 1100℃.As the Nb(C,N) and other dissolved particles could not pin grain boundaries effectively after 1250℃.So austenite grain size increased rapidly.The recrystallization evolvement regularity of low carbon high niobium steel had been investigated deeply.The low carbon high niobium steel was easy to promote dynamic recrystallization in the case of high deformation temperature and low strain rate.However,large deformation,high deformation temperature,high strain rate and small grain size was easy to promote static recrystallization.The results showed that steel had been rolled in non-recrystallization zone at the initial stage which deformed 40% with strain rate of 2s-1 in 1000℃.Using different controlled rolling process in the austenite non-recrystallization zone the results showed that steel niobium microalloyed was easy to produce the uniform organization of deformation band in the case of large deformation,high strain rate,low deformation temperature,and small grain size. Using different controlled cooling process in the austenite non-recrystallization zone the results showed that with decreasing the temperature and increasing strain rate promoted ferrites to nucleate.However,increasing the temperature promoted bainite transformation. With the increasing the deformation,the amount of proeutectoid ferrite increased obviously and the hardness decreased largely.With increasing the deformation of austenite inhibited lath bainite tonucleate and grow making the morphology of bainite gradually from the lathy into granular.The final microstructure was mainly of ferrite and some pearlite when cooling rate was slow.when cooling rate was more than 5℃/s,the final microstructure was mainly of small granular bainite.With increasing the cooling rate,the longitudinal growing rate of bainitic ferrite was more than lateral growing which promoted to produce lath bainite.The hardness also became high when the cooling rate increased largely.Rolling tests obtained the fine and uniform organization and eliminated mixed crystal problem of niobium microalloyed steel.With increasing the amount of the pass,the Nb(C,N) precipitates were bacame more small and dispersed and dispersed and played a very good precipitate strengthened role.From the mechanical data of low carbon high niobium steel,minimum yield strength,elongation and Rp0.2/R-m were 643MPa,556MPa and 0.83 respectively,and the total elongation was 22～25%.The strength and plasticity of low carbon high niobium steel approached the requirements of senior pipeline steel.