Research On The Effects Of Stress On Bainite Transformation And Microstructure In Ultrafine Bainite Steels

Ultra-fine high strength bainite steel is a new generation of advanced high strength steel with both high strength and good ductility.In recent years,the effect of stress / strain on bainite transformation has been attracted much attention in order to solve the problem of too long phase transformation time of ultra-fine high strength bainitic steel.At present,the study of the effect of strain on bainite transformation is relatively sufficient,and its influence has been basically explored.However,studies on the effect of stress on bainite transformation is less,and many basic theories related to stress affected bainite transformation need to be clarified.In this paper,a Fe-C-Mn-Si low cos ultrafine high strength bainite steel is set as the object and the effect of stress on bainite transformation and microstructure is investigated.The purpose is to clarify the dilatometry of stress-affected bainite transformation and reveal the effect of elastic stress,plastic stress,compressive stress and tensile stress on bainite transformation amount,transformation kinetics,transformation plasticity,bainite morphology,orientation,lath size,and the morphology and stability of retained austenite etc.In addition,the present study will clarify the influencing mechanism of stress on bainite transformation and establish the stress-affected bainite transformation kinetics model.This work can not only perfect the theory system of stress-affected bainite transformation,but also provide theoretical basis for the acceleration of bainite transformation and the development the technology combining plastic forming and heat treatment,as well as the development of high precision heat treatment simulation software.Dilatometry is used to studied the bainite transformation,high temperature laser scanning confocal microscopy is used for in situ observation,SEM and TEM are used for microstructure observation,EBSD is used for quantitatively investigating the orientation of bainite,XRD is sued to measure the amount and carbon content of retained austenite.The main conclusions are given as follows:(1)The dilatometry of stress-affected bainite transformation is different from that of stress-free bainite transformation.For bainite transformation with stress,the strain(the percentage variation in dilatation)of the sample in the single direction cannot quantitatively represent the amount of bainite transformation and the volume strain,instead,should be used to study the bainite transformation amount.For bainite transformation without stress,the strain of the sample in the single direction can quantitatively represent the amount of bainite transformation.(2)The elastic stress provides additional mechanical driving force for bainite transformation,thus accelerating bainite transformation and increasing bainite amount.In higher austenitization temperature,the grain size of parent austenite is larger and its strength is smaller,so that the effect of stress on bainite transformation is more obvious,and the transformation plasticity is larger under stress.The effect of stress on bainite phase transformation depends on the ratio of mechanical driving force to the total drive force,the size and strength of the parent austenite,etc.In addition,elastic decreases the amount of blocky retained austenite,which is beneficial to increasing the stability of retained austenite.EBSD results show that the mechanical driving force for each bainite variant is different under stress.The bainite varient with favorable orientation is promoted,so that the orientation of bainite tends to be uniform and variant selection occurs under the effect of elastic stress.Moreover,the increase in bainite amount is not accompanied by the increase in carbon content in retained austenite under the effect of stress,which is not expected by the bainite transformation theory.The possible reason is that stress hinders the partition of carbon from bainite ferrite to retained austenite.(3)The combined effect of ausforming and stress on bainite transformation is not a simple superposition of both of them.Although single deformation and single stress increases the amount of bainite transformation,combined ausforming and stress cannot further increase the bainite amount.In addition,deformation interacts with stress: in the initial stage of transformation,deformation and stress enhance their effect on bainite transformation,but then the dislocation forest caused by deformation hinders the growth of bainite,and thus reduces the effect of stress on the bainite transformation.For stress-affected bainite transformation,the hinderance of deformation on the growth of bainite is larger than its promotion on nucleation.Moreover,small deformation in low temperature accelerates stress-affected bainite transformation,but it reduces the amount of bainite transformation,which is different from the effect of deformation on stress-free bainite transformation.(4)The effect of plastic stress on bainite transformation is the combination of mechanical driving force and prior deformation.Plastic stress increases the amount of bainite transformation and obviously accelerates bainite transformation kinetics.The acceleration of plastic stress is more obvious at a lower transformation temperature.In addition,EBSD results show that plastic stress reduces the number of bainite variant in a single austenite,resulting in obvious variant selection and making the bainite orientation more uniform,and this effect is more obvious at a lower transformation temperature.TEM results show that as the plastic stress increases,the thickness of bainite lath increases and the length of bainite lath decreases.Moreover,plastic produces obvious transformation plasticity,and the appearance of transformation plasticity conforms to the Magee mechanism.The lower the transformation temperature,the larger the transformation plasticity.(5)As the stress increases,the promotion degree of stress on bainitic transformation amount increases fast firstly,then increases slowly and finally decreases.There are two critical stress values.The first one corresponds to the yield strength of parent austenite and the second one corresponds to the maximum promotion degree of stress.A simple model is proposed to describe above phenomenon.In addition,the separate roles of mechanical driving force and deformation of plastic stress are divided for the first time in this work,and corresponding quantitative models are established.(6)The mechanical driving forces provided by compressive and tensile stresses with the same magnitude are similar,so their effects on bainite transformation kinetics are similar and there are no obvious differences between the two stress states in the bainite morphology and lath thickness.(7)A new kinetics model of bainitic transformation dependent on stress and transformation temperature is established.Stresses below and above the yield strength of prior austenite are both considered in the proposed new model.The model presents a good agreement with experimental results.In addition,the kinetics parameter b in Avrami equation first increases linearly before the critical value of stress(yield strength of austenite)and then increases faster with the stress.Moreover,the acceleration of stress on bainitic transformation includes three stages.First,before the yield strength of austenite,the acceleration effect increases slowly,then there is a jump of the acceleration degree near the yield strength of austenite,and finally the acceleration effect continues to increase,but its increasing rate slows down gradually after the yield strength of austenite.