Microstructure Evolution And Dynamic Deformation Behavior Of Medium C And Low Mn Ultra-fine Bainite Steel

Ultra-fine bainite steel has excellent strength-plastic matching,and become a research hotspot.But its long phase transformation time restricts industrial development.In this paper,displacement growth mechanism was used to calculate the transformation kinetics of medium C and low Mn ultra-fine bainite steel under different heat treatment processes,and the mechanism of austenitizing temperature and primary martensite to accelerate the ultra-fine bainite transformation were expounded.SEM,EBSD,TEM,tensile testing machine,impact testing machine and other technical methods were used to study the influence of different heat treatment processes on the microstructure and properties,to seek a ultra-fine bainite steel with the best phase transformation time and performance matching.The mapping relationship between strain rate and changes in structure properties was revealed combined with dynamic deformation tests.The main conclusions are as follows:（1）As the austenitizing temperature decreases,the activation energy of bainite transformation is reduced.And the increase of the density of the original austenite grain boundaries can increase the number of bainite nucleation sites and nucleation rate,shorten the incubation period,and reduce the completion time of the ultra-fine bainite transformation.（2）The primary martensite introducing by the two-stage isothermal transformation process can cause distortions inside the austenite,reduce the activation energy of the ultrafine bainite transformation and eliminate the incubation period of bainite transformation.And the increase of nucleation position can increase the nucleation rate and accelerate ultrafine bainite transformation.The direct isothermal transformation process of ultrafine bainite region has an incubation period of about 600 s and bainite transformation completes at 2000 s.When the primary martensite was introduced,the bainite transformation has no incubation period and phase transformation completed in about 1000 s.（3）The formation of primary martensite refines the width of the bainite lath,Si element inhibits the precipitation of carbides in martensite,and the process of carbon atoms diffuse from martensite to retained austenite dominates,resulting in the content of retained austenite increases.And the distortion caused by the martensite transformation causes the orientation relationship between bainite and austenite to change from K-S to N-W.As the bainite isothermal temperature increases,the width of the bainite ferrite lath increases,and the thickness of the austenite film will increase and gradually change to a block shape,and the content of residual austenite increases.（4）As the austenitizing temperature decreases,the strength and hardness increase,but the elongation and impact toughness decrease.The introduction of primary martensite increases the strength and hardness of ultra-fine bainite steel,and the ductility and impact toughness decrease.When the content of primary martensite is 35%,the best matching of strength and toughness is obtained.The tensile strength reaches 1760MPa,the elongation reaches 9%,and the impact toughness is greater than 34J/cm².When the bainite isothermal temperature increases to 320°C,the tensile strength reduces to 1583 MPa,the elongation increases to 16%,and the impact toughness is greater than 48 J/cm².（5）The results of dynamic deformation studied at high strain rates showed that in the range of500-5000s^-1 strain strains,1050-DIT specimens,QBT235-280 specimens,and QBT235-320specimens all have strain hardening behavior,strain rate sensitivity,and adiabatic shear sensitivity.The QBT235-320 specimens have the lowest adiabatic shear sensitivity,while the 1050-DIT specimens have the highest adiabatic shear sensitivity.During the dynamic deformation of the three test steels,as the strain rate increases,the microstructure is significantly deformed,and the TRIP effect occurs.As the strain rate increases,the TRIP effect weakens.