| Nanoscale bainite steel with ultrahigh strength and toughness has become an outstanding representative of a new generation of advanced high strength steels(AHSS).The optimal combination of nanoscale carbide-free bainite,film-like retained austenite(RA),and martensite determines the superior comprehensive properties of nanoscale bainite steels.However,such an ideal microstructure is generally obtained by austempering at a very low temperature and long time,which is not conducive to be extensively used in industrial production.In recent years,in order to obtain such an ideal bainite microstructure and shorten the transformation time,the effect mechanism of alloying elements and heat treatment processes on nanoscale bainite steels has attracted much attention.It is necessary to further clarify the effect of composition design and heat treatment processes,especially novelty heat treatment processes such as austempering below M_S and two-step isothermal transformation on the transformation kinetics,microstructure,properties and the stability of RA in nanoscale bainite steel.In the present paper,the transformation of isothermal bainite and martensite was observed directly in situ by LSCM.Dilatometry was used to analyze bainite transformation.OM,SEM,EBSD and TEM were used for microstructure observation.XRD technique was utilized to measure the volume fraction and stability of RA.In addition,a tensile tester is use to obtain the tensile properties of samples after heat treatments.The following conclusions are drawn:(1)Dilatometry and metallographic methods are used to study the effect of ausforming amount and temperature on M_S.The saturation strain(?_s)is first observed and defined.The M_S tends to be constant when the strain is larger than?_s.Moreover,the critical strain(?_c)is firstly proposed.The strain smaller than?_c increases the M_S,whereas the M_S decreases at the strain larger than?_c.In addition,the ausforming temperature does not affect the M_S.The critical strain?_c and saturation strain?_s are not affected by the ausforming temperature.(2)The continuous isothermal martensitic transformation in a Fe–C–Mn–Si bainitic steel during austempering below M_S was firstly observed by in situ LSCM.Differing from the previously accepted viewpoint,the martensitic transformation in the medium-carbon bainitic steel does not finish instantaneously but occurred in a drawn-out fashion as time progressed during the austempering process.Moreover,during austempering,the infection point in the dilatation curve against time is not the demarcation point between martensitic and bainite transformations.After the infection point,the martensitic transformation still persisted with time,and thus the dilatation occurring after the infection point could not be used to determine the amount of bainitic transformation.(3)Finer bainite microstructure and the faster bainite transformation kinetics are obtained when samples are austempered below M_S,however,with the decrease of austempering temperature,the increase of the amounts of athermal martensite(AM)and carbide and the decrease of bainite amount lead to the weakness of mechanical properties of low carbon bainitic steels.It means that whether the heat treatment of austempering below M_S can improve the mechanical properties of bainitic steels depends on the composition of steels and austempering temperature.In addition,the effects of austenitizing temperature on martensitic and bainitic transformation,microstructure,and mechanical properties of a low carbon bainitic steel during austempering below M_S were investigated.With the increase of austenitizing temperature,the amount of AM gradually increases,whereas the volume fraction of bainite and RA first increases and then decreases,and the bainite plates become finer first and then coarse,which leads to the initial increase and a following decrease of mechanical properties.It is significantly different from the results of samples austempered above M_S.(4)At the condition that the total transformation time of two-step isothermal process is the same,the comprehensive property of a low carbon bainitic steel is improved with the decrease of the first step transformation time.For two-step isothermal process,the transformation kinetic of the second step is obviously accelerated with the decrease of transformation time in the first step.Compared with the conventional one-step bainite transformation above M_S,the comprehensive property of bainitic steels is not improved by austempering below M_S.(5)The addition of Ni(?1.00wt.%)affects the transformation kinetics,microstructure and properties of high carbon(?0.80wt.%C)high strength bainite steels.Adding Ni not only reduces the driving force for bainite nucleation and growth,retards the bainite transformation kinetics,but also decreases the amounts of bainite and RA.At higher austempering temperature,the tensile properties of high-carbon bainite steels decrease with the addition of Ni,while at lower austempering temperature,the addition of Ni has little effect on the tensile properties of bainite steels.(6)The addition of Si(1.00wt.%?2.00wt.%),Cr(?1.00wt.%)and Al(?0.50wt.%)affects the affects the transformation kinetics,microstructure and properties of low carbon(?0.22wt.%C)high strength bainite steels.With the increase of Si content,although the transformation kinetics of bainite is hindered and the bainite transformation amount is decreased,the more amount in film-like RA and the less amount in carbide are obtained and thus improves the comprehensive properties of low carbon steels.In addition,single Cr addition promotes the bainite transformation and increase the strength of low carbon bainite steels.Single Al addition leads to the decrease of bainite amount because Al addition promotes the ferrite transformation,and finally weakens the tensile properties of low carbon bainite steels.Simultaneous addition of Cr and Al slightly increases the bainite transformation amount and enhances the tensile properties of low carbon steels,but it weakens the promotion function of Cr on bainite transformation.(7)The lengthening rate of bainite sheaves during cooling is in situ observed and measured by LSCM,and the relationship between the bainite transformation amount and the lengthening rate is analyzed in combination with the dilatometry.With the austempering temperature decreasing,the lengthening rate of bainite plates decreases,whereas the final transformation amount of bainite increases.It indicates that higher lengthening rate does not mean more amount of bainite.In addition,at high and medium temperature ranges,the lengthening rate of bainite sheaves nucleated at grain boundary is larger than that of bainite sheaves nucleated within grains,but they have no obvious difference at low temperature ranges. |
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