Growth Behavior And Phase Transformation Kinetics Of Ultra-High Strength Nano Carbide-Free Bainite

Nanoscale bainite steel with ultra-high strength and toughness commonly composes of nano-sized carbide-free bainite,retained austenite and martensite.Nano bainite is usually transformed at a low temperature for a relative long time,which is not conducive to industrial production.Ausforming and isothermal bainite transformation temperatures are the key factors that affect the nano-bainite transformation kinetics.Thus the influence of ausforming and isothermal bainite transformation temperatures on the growth of bainite should be investigated to improve production efficiency and meet the needs of various industries towards nano-bainite steel.The effect of ausforming on the bainite microstructure and the behavior of bainite growth during isothermal transformation in ultra-high strength nano-carbide-free bainite steel was studied in this study.The isothermal bainite transformation was observed directly in situ by LSCM.Dilatometry was used to analyze bainite transformation.OM,SEM,EBSD and TEM were used for microstructure observation.The author aimed to clarify the effect of ausforming on bainitic transformation.The results could provide theoretic reference for industrial production of ultra-high strength carbide free nano-bainitic steel.The main conclusions can be drawn as follows:(1)Ausforming process affects the transformation kinetics of bainite steel(0.83C-2.03Mn-1.84Si-1.12Cr-1.09Al-1.67Co).The high-temperature ausforming promotes the phase transformation rate at the initial isothermal stage,however,decreases the final amount of bainite.The low-temperature ausforming prolongs the incubation period of bainite transformation,but promotes the phase transformation rate in the isothermal later stage,and increases the total amount of bainite transformation.For the tested steel,the maximum amount of bainite was obtained using 400℃deformation and then isothermal transformation at 300℃.(2)The austenite growth and bainite growth were dynamically observed using LSCM.The austenite grow process can be promoted by two ways.One is the migration of grain boundary.Another is that one grain is first portioned into pieces and then joins in a new grain.The bainite grow process can be promoted by four ways,which includes the nucleation at grain boundary,the nucleation at original austenite twins,the nucleation inside grain and the nucleation at growing bainite laths.(3)Bainite growth has the features of interlocking and impingement of bainitic platelets,some platelets stop growth,and the others continue growth till meet the grain boundary.The platelets growth rate is affected by the orientation relationship.The growth rate of lath with the same orientation and similar nucleation time is relatively close.The growth rate is also related to the isothermal temperature.The higher the isothermal temperature,the greater the growth rate of bainite laths.(4)The deformation below the martensite start temperature(M_s)retards the kinetics of the bainitic transformation,leading to prolong incubation period.When the isothermal transformation temperature is lower than the M_s,the orientation relationship between bainitic/ferrite and the parent phase austenite is close to the K-S type.Although the deformed sample has obtained a part of pre-martensite which can increase the bainite nucleation rate,some nuclei are invalid due to the effect of orientation.Deformation changes the orientation of the parent phase austenite and destroys the original orientation relationship of bainite,reducing the valid number of bainitic nuclei.