Dynamic Regulation And Mechanism Of Bainite Transformation Kinetics In Medium Manganese Steel

As one of the research focus of the third generation automotive steel,the optimization of its microstructure and mechanical properties has been a key issue.At present,the development of medium Mn steel is mainly based on the intercritical annealing process by cold rolling process.In this paper,with the idea of introducing bainte to form a multiphase structure under hot rolling process,a new type steel with high yield strength and high product of strength and ductility was proposed.Therefore,it's key issue to regulate the isothermal transformation kinetics of bainite transformation under the medium Mn steel to the subsequent industrial design.This paper explores the feasibility the regulation of bainite transformation kinetics in medium Mn steel system from the aspects of composition,martensite pre-transformation and Ausforming process.(1)Based on the reference system of 0.2C-4Mn-1.6Si,Mn and Co elements were investigated to explore their influence on bainite transformation kinetics.The results show that Mn addition can significantly slow isothermal bainite transformation kinetics,while Co addition can enhance the Gibbs free energy difference of fcc to bcc crystal lattice and promote bainite kinetics.The influence of Mn segregation of medium manganese on bainite transformation kinetics steel was also studied,the phenomenon of element segregation and the difference of matrix microstructure before and after heat treatment were characterized.The results showed that the isothermal bainite transformation had almost completed while rare bainite transformation occurred in the Mn-rich zone due to the relatively high Mn content than Mn-lean legion.(2)The pre-transformation of martensite process was utilized to clarify the influence of martensite on isothermal bainite transformation kinetics under two-step process and one-step process respectively,and the influence of bainite evolution was further explore.The results show that martensite pre-transformation can accelerate the kinetics of isothermal bainite transformation.The increase of pre-transformation martensite volume fasted the time of isothermal bainite transformation,but the final transformation amount of bainite is reduced.The introduction of bainite can reduce the volume of secondary martensite and promote the stability of retained austenite.The results show that this kind of multiphase optimization process can improve steel plasticity and still maintain high yield strength.(3)The influence of pre-deformation temperature and strains on bainite transformation kinetics was investigated based on Ausforming process,and the influence of bainite evolution on microstructure and mechanical properties was further analyzed.The results show that a maximum bainite isothermal transformation was obtained at the same pre-deformation temperature.The formation of bainite is accelerated when the pre-deformation temperature decreases.The introduction of bainite can compensate the strength loss caused by the reduction of martensite and promote the stability of the retained austenite.The mechanical properties show that this kind of mutiphase regulation process can improve the strength and maintain plasticity.