Effect Of Second Phase Precipitations On Transformation,Microstructures And Mechanical Properties Of Carbide-free Bainitic Steels

Carbide-free bainitic steel has excellent comprehensive properties and its research is of great significance in application.In this dissertation,the objects of the research are medium carbon and high carbon carbide-free bainitic steels.In order to accelerate the bainitic transformation and improve the properties,the second phase precipitations were introduced to the steels through microalloying.The effect of second phase precipitations on bainitic transformation,microstructure,mechanical properties and fatigue properties was investigated.The bainitic transformation of medium carbon carbide-free bainitic steel was innovatively observed by in situ transmission to discuss the mechanism of microstructure evolution during the nucleation and growth processes.The heat treatment process was adjusted and the influence of heat treatment parameters on bainitic transformation,microstructure and mechanical properties was studied.Carbide-free bainitic steels were microalloyed with N and Al or N and V,and the second precipitations of AlN or VN were induced by heat treatment process.It was interesting to find that AlN or VN particles possessing a low mismatch with bainitic ferrite provided the heterogeneous nucleation sites and effectively accelerated the bainitic transformation.At the same time,the increase of nucleation points resulted in refined microstructure and improvement of the strength and hardness.The effect of AlN particles on bainitic transformation in medium carbon carbide-free bainitic steel was observed in situ by high temperature confocal microscope.AlN particles could provide effective nucleation points of bainitic ferrite.Growth rate of bainitic ferrite varied at different nucleation sites,and the growth rate of bainitic ferrite nucleated within the grain was higher.With increasing isothermal time,AlN particles induced more secondary nucleation,then the nucleation position at the phase boundary gradually took the place of the nucleation position at the grain boundary and played a leading role in the nucleation process.With the increase of tempering temperatures,the thickness of bainitic ferrite plates in the high carbon bainitic steel increased and the volume fraction of retained austenite decreased.The steel at low tempering temperature possessed optimum comprehensive mechanical properties.Different cyclic deformation behaviors were observed for the steel at different tempering temperatures.The steel tempered at 160 °C had a high cycle hardening capacity,and almost no cyclic softening was discovered after instantaneous cyclic hardening.During cyclic deformation,the retained austenite was transformed into straininduced martensite,and the deformation twin was generated in the retained austenite.The effect of ausforming with different strains on bainitic transformation and microstructure in medium carbon carbide-free steel was analyzed.Ausforming at 300 °C before isothermal transformation could dramatically shorten the incubation period and greatly promote the early stage of bainitic transformation.Most of the bainitic transformation could be completed within 30 s.The degree of bainitic transformation was affected by the ausforming strain.The bainitic microstructure after ausforming was refined and the number of short bainitic plates increased.The transformation of nano-sized bainitic ferrite was in-situ observed systematically and precisely by means of the spherical differential environmental transmission electron microscope.The nucleation and growth of subunits and ultrafine subunits repeated during the growth process.The subunits of bainitic tranformation were mostly quadrilateral strips.The evolution of dislocations during the nucleation and growth process of bainitic ferrite was observed,and dislocations were accompanied by movement of the phase boundary of bainitic ferrite.The results of bainitic transformation kinetics by using the three methods,i.e.thermal dilatometer,high-temperature laser confocal microscope and environmental spherical transmission electron microscopy,were different due to the ratio of specimen thickness to grain size.