Study On The Effect Of Deformation Process On Bainitic Transformation And Retained Austenite

The microstructure of super bainitic steels consists of very fine bainite lathes,leading to an ultrahigh strength and a good plasticity.The nona-bainite can be obtained by bainitic transforamtion at a low temperature.At low temperatures,however,the bainitic transformation kinetics is slow and the transformaiton period is long in medium-carbon bainitic steels.This makes it hard for nano-bainitic steel to be promoted in practical production,which can be released effectively by ausforming.In the present,many literatures revealed the influence of ausforming on bainitic transformation and retained austenite.Ausforming affects bainitic transformation kinetics,crystallographic orientation and microstructure.However,the present studies on ausforming are not completed.Based on the present studies,the present thesis therefore trys to further investigate the effect of ausforming on bainitic transformation kinetics and microstructure,and propose two novel deformation processes to control microstructure.Fe-C-Mn-Si series bainitic steels are chosen for the present thesis primarily.A thermal simulator,scanning electron microscope,transmission electron microscope,X-ray diffractometer,nano indentation tester and tensile tester were employeed to study the effect of high-temperature ausforming,low-temperature ausforming and strain rate on bainitic transformation,deformation resistance and retained austenite.Besides,two novel deoformation processes were proposed for microstructure controlling.The main conclusions can be drawn as follows:（1）High-temperature ausforming reduced the undercooling dependence of bainitic transformation.The extent to which bainitic transformation amount is affected by high-temperature ausforming is related to transformation temperatures.The retardation caused by high-temperature ausforming becomes more serious with the decreasing transformation temperature.However,the retardation effect can be weakened by higher transformation temperature because of the static recovery.In the non-ausformed sample,the initial bainitic transformation rate becomes larger at lower transformation temperature.Otherwise,the initial transformation rate is larger at higher temperature in ausformed sample.The austenite is mechanically stabilized by high-temperature ausforming,thus increasing the volume fraction of retained austenite.Also,the fraction and size of M/A consitituent increase.The volum fraction of retained austenite is comprehensively affected by the amount of bainitic transformation and martensitic transformation.（2）The bainitic transformation kinetics is accelerated and the microstructure is refined by ausforming at low temperature due to low-temperature ausforming,and the bainite bundles present the characteristics of cross-distribution.The ausformed sample contains more and finer bainitic ferrite compared with non-ausformed sample.In addition,the volume fraction of retained austenite is larger in ausformed sample.The retained austenite can be optimized by ausforming,which is benifitial to TRIP effect.The microstructure optimized by ausforming leads to an obvious increase in the comprehensive mechanical properties.（3）The extent to which bainitic transformation is promoted by ausforming at different strain rates shows a nonlinear law.As the strain rate increases from 0.01 s^-1 to 5.0 s^-1,the degree of promotion increases then decreases,and tends to be constant at last.On the other hand,the strain rates do not have obviouse effect on the morphology of bainite constituent.The volume fraction of retained austenite changes with the amount of bainitic transformation,indicating that the volume fraction of retained austenite depends on the chemical stabilization of untransformed austenite.（4）The deformation resistance increases with strain rate in most steels.However,the effect of strain rate on deformation resistance of undercooled austenite in bainitic steels depends on the carbon content.In a medium-carbon bainitic steel,the deformation resistance increases with the strain rate from 0.01 s^-1 to 5.0 s^-1 due to the increasing work hardening of austenite.In a low-carbon bainitic steel,the deformation resistance reducees as strain rate increases from 0.001s^-1 to 1.0 s^-1 and then increases as strain rate increases from 1.0 s^-1 to 5.0 s^-1.The bainitic transformation during deformation at low strain rate leads to an increase in deformation resistance.（5）A small strain during isothermal holding induces further transform of untransformed austenite into bainite.The microstructure consists of bainite and austenite when applying deformation.The deformation is mainly applied on untransformed austenite,which leads to a mechanical stabilization of austenite and an obvious increase in the volume fraction of retained austenite.As the strain amount increases from 20%to 40%,the amount of bainitic transformation during the isothermal process after deformation and the volume fraction of retained austenite increase.On the other hand,the thickness of film-like retained austenite reduces because of deformation.Besides,compared to the sample deformed before isothermal holding,most of the bainitic ferrite in the sample deformed during isothermal holding forms before deformation,resulting in a thicker bainitic ferrite.（6）As for two-step deformation,when the total strain is the same,two-step defomation cannot promote the amount of bainitic transformation compared with single ausforming.However,two-step deformation increases the volume fraction of retained austenite,which also increases as the second strain amount increases.Besides,two-step deformation leads to a thicker bainite lath.