| The continuing growth of vehicle holding is the trend of the development of the society.With the increasing demand for high-performance materials in the automotive industry,a new type of transformation-induced plasticity(TRIP)steel with increased Mn content,the so-called Fe-Mn-Al-C TRIP steel,is being actively studied.In the present study,the microstructure evolution,the mechanical properties and the deformation behavior of Fe-Mn-Al-C TRIP steels was investigated after hot-rolling,cold-rolling,heat treatment and tensile deformation.The investigated steel has a composition of Fe-8Mn-8Al-0.8C(wt.%).The density of the steel was measured to be 7.19 g/cm3,which is 8%lower in comparison with pure Fe.The equilibrium phase diagram and the phase transition temperature were calculated using the Thermo-Calc and the Thermal expansion experiment.The intercirtical temperature ranges from 700? to 1340? and the Ms temperature is 171.2?.The hot-rolled sheets,were quenched at first and then cooled in the air to suppress the precipitation of ?-carbides,leading to the improvement of mechanical properties.The properties of hot-rolled Fe-8Mn-8Al-0.8C steels were further improved by quenching-tempering(Q-T)treatment.And the Q-T steels consisted of coarse?-ferrite regions and fine-grained(FG){austenite+martensite} regions.After an optimal quenching treatment at 900? for 60 min and tempering treatment at 200? for 60 min,the steel exhibited ultrahigh ultimate tensile strength(UTS)of 1410 MPa,total elongation(TE)of 29%and product of strength and elongation(PSE)of 41 GPa·%.The effect of Q-T treatment on the austenite stability and the microstructure evolution and deformation mechanism was investigated.Tempering process promoted the uniform distribution of carbon element in austenite,resulting in an elimination of the serrated behavior in the strain hardening rate curve.The element partitioning from the supersaturated martensite and ?-ferrite to austenite during tempering,as well as its contributing effects to austenite stability,was also discussed.The intercritical annealing(IA)treatment was also used to improve the properties of hot-rolled Fe-8Mn-8Al-0.8C steels.The steel exhibited a ferrite-austenite duplex microstructure.After an optimal annealing treatment at 700?,the steel exhibited a good combination of UTS of 1133 MPa,TE of 31.5%and PSE of 36GPa·%.Strain rate effect on microstructural evolution and deformation behavior was studied.The difference in serrated behavior at various strain rates is deduced to be directly related to the dynamic strain aging(DSA)factor.The strain rate of 10-1 s-1,the critical strain rate of adiabatic heating,used here can readily result in a temperature rise.No significant DSA was observed in the specimen deformed at 10-1 s-1 whereas the serration became more drastic with decreasing strain rate in the range of 10-2?10-4 s-1.According to multiscale analysis through XRD,EBSD and TEM,the strain induced martensitic transformation proved to be improved by increasing the strain rate in the range of 10-4?10-2 s-1.The positive effect of increasing shear bands played a dominating role as the result of considerably high austenite fraction.The hot-rolled steel which was annealed at 700? for 60 min was further cold-rolled.Effect of quenching temperature and tempering time on the cold-rolled steel was investigated.The steel exhibited an austenite+ferrite duplex microstructure while the ferrite consisted of ?-ferrite and ?-ferrite.After an optimal quenching treatment at 900? for 60 min and tempering treatment at 200? for 60 min,the steel exhibited UTS of 988 MPa,TE of 35.8%and PSE of 35 GPa·%.The yield point elongation of the cold-rolled steel was eliminated and the yield ratio was reduced through the Q-T treatment.The multi-scale control theory of microstructural evolution and mechanical properties through various rolling process and heat treatment were utilized to obtain the high strength and ductility steel which exhibits a tensile strength of 1000?1400 MPa with one composition,so called "one composition for multilevel strength". |
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