Research Of Solid-state Phase Transformation Mechanism At Moderate Temperature And Strain Rate Dependency Of Fe-30Mn-9Al-1C Lightweight Steel

Austenitic single-phase Fe-Mn-Al-C steel with high manganese and aluminum contents has the advantages of high-strength,low-density,high-capacity of impact energy absorption,high plastic elongation and the ability of aging strengthening,which has been extensively studied as a candidate of new automotive structural materials.Fe-Mn-Al-C steel can undergo various phase transformation processes during the heat treatment,such as austenite transform into ferrite and the precipitation of?-carbide and?-Mn phase in austenite.The?-carbide has a face-centered cubic perovskite structure.Except for the effect in precipitation strengthening,the precipitation of?-carbide will not significantly reduce the plasticity of the steel because it has a lattice constant close to austenite.While?-Mn is a simple cubic phase,it usually precipitates after prolonged aging,which causes brittleness of steel.At present,most reports on the microstructure transformation during heat treatment of Fe-Mn-Al-C steel focus on the precipitating behavior of?-carbide,however,the precipitation on?-Mn phase has rarely been reported.In addition,the pre-deformation has a significant effect on the phase transformation process of steel during heat treatment.Nevertheless,there is no clear understanding about the influence of pre-deformation on phase transformation in Fe-Mn-Al-C steel.Aiming at this problem,this paper studies the solid-state phase transformation of Fe-30Mn-9Al-1C steel after pre-deformation at medium temperature,and proposes a new mechanism for the rapid precipitation of?-Mn phase in austenite grain interiors.The precipitation kinetics of?-Mn phase at 600? is fitted by the Johnson-Mehl-Avrami model.Furthermore,the mechanical properties of Fe-30Mn-9Al-1C steel at various strain rates are analyzed.It is found that the strain rate dependence of yield strength changes abnormally at high strain rate.This phenomenon is analyzed and explained by the fitting data from X-ray diffraction pattern.Through analysis and discussion in this paper,the following conclusions are drawn:?1?Austenite single-phase structure at room temperature was obtained by induction melting,casting,forging,hot rolling,cold rolling and annealing of Fe-30Mn-9Al-1C steel.Its density was determined to be 6.61 g/cm³,which was15.3%lower than that of pure iron.Based on the Sub-regular Solution Model,the stacking fault energy of Fe-30Mn-9Al-1C steel was calculated to be 92.7m J/m².?2?The?-Mn phase can rapidly be precipitated after annealing in the medium temperature range of 550-750? in Fe-30Mn-9Al-1C steel,and the precipitation positions include austenite grain boundary and transition band.The?-Mn phase precipitated along the transition band has a certain spatial continuity.The intragranular?-carbide precipitated by Spinodal decomposition promotes the precipitation of the?-Mn phase along the transition band without the accompanying precipitation of the ferrite phase.The precipitation mechanism is ?????-Mn+??.According to the thermodynamic model of Fe-Mn-Al-C quaternary alloy system,the Gibbs free energy change of this precipitation reaction is estimated to be-46740.89 J/mol.Up to 55.4%of the strain storage energy may be presented in the deformation band.The critical nucleation radius of the?-Mn phase is 5.54 nm.?3?The strain storage energy concentrates on the deformation band and the boundary of deformed austenite grain,resulting in non-uniform nucleation of the?-Mn phase during the heat treatment.The Avrami exponent of the?-Mn phase is less than 1 fitted by the Johnson-Mehl-Avrami equation.?4?When the strain rate increases in the range of 10^-4-10³ s^-1,the yield and tensile strength of Fe-30Mn-9Al-1C steel increase gradually,and then decrease when it reaches 10¹ s^-1.The strain rate sensitivity coefficient decreased from0.049 to-0.048.The reason is that dynamic strain aging occurs when the strain rate reaches 10¹ s^-1,which causes an intermittent obstructive relationship between solute atoms and dislocations.According to the MAUD full-spectrum fitting results,the dislocation density of the steel near the fracture surface reached the order of 10¹⁵ m^-2.According to the fitting result by the modified Williamson-Hall plot method,the proportion of screw dislocations increases at high strain rate;at the same time,the M value of the steel increases in the anomalous region of elongation values,which reflects that the correlation among the dislocations is weak at this stage and the dislocations are more randomly distributed.Therefore,the cracking caused by the stress concentration during the deformation process is delayed.At high strain rates,the adiabatic heating of the steel can be more than 100?.The above reasons together cause the tensile elongation of Fe-30Mn-9Al-1C steel to increase at high strain rate.