| Energy and environmental issues are becoming increasingly prominent throughout the world.Due to these concerns,the research of lightweight steels in various industrial fields is getting widespread interest.Fe-Mn-Al-C austenitic steel,which can be applied in automobiles,metallurgy,offshore platforms and other fields,has attracted sufficient attention owing to its low density and remarkable comprehensive mechanical properties.With the increasingly harsh service conditions,improving the mechanical properties,service life,and corrosion resistance of Fe-Mn-Al-C austenitic steel has become a key issue.Microalloying elements have the effects of solid solution strengthening,fine-grained strengthening,and precipitation strengthening in steel.Therefore,Nb-V microalloyed Fe-Mn-Al-C austenitic steels were designed in present work,and conventional Fe-Mn-Al-C austenitic steel was selected for comparasion.The effect of Nb-V microalloying on hot deformation characteristics of Fe-Mn-Al-C austenitic steel was investigated.The influences of Nb-V microalloying and thermo-mechanical processes on the microstructures,mechanical properties and corrosion resistence of Fe-Mn-Al-C austenitic steel were analyzed at length.Meanwhile,the microstructure evolution and deformation mechanisms of the test steels under different deformation conditions were revealed.The results indicate that the flow softening phenomenon is obviously observed in the0.16 wt.%Nb-0.16 wt.%V microalloyed Fe-Mn-Al-C austenitic steel at low temperatures and low strain rates during the hot compression processes.The hot deformation activation energy of the Fe-Mn-Al-C steel is slightly affected by the addition of Nb-V.The uniformly distributed（Nb,V）C particles of nano-size,which precipitate in the Nb-V microalloyed Fe-Mn-Al-C steel,act as the nucleation sites for dynamic recrystallization.These particles promote the dynamic recrystallization and pin the grain boundaries to restrain the growth of recrystallized grains.Nb-V microalloying renders a positive influence on the hot deformation stability of the Fe-Mn-Al-C steel and broadens the optimum hot processing window.Based on the processing map and microstructure,the optimum hot processing windows of the Nb-V microalloyed Fe-Mn-Al-C steel at the compression strain of 0.7 are determined as 894-1025°C/0.01-0.14 s-1 and 1050-1200°C/0.03-0.95 s-1.Equiaxed austenitic grains are obtained in all of Fe-Mn-Al-C steels subjected to solid solution,solid solution+aging,hot rolling and hot rolling+aging processes,respectively.Hot rolling process reduces the grain size significantly.Compared with the solid solution treated specimens,both the strength and hardness of the hot-rolled or aged specimens are improved,while the impact energy is reduced.The increased content of Nb and V microalloy elements in the steels can achieve better grain refinement effect under the same thermo-mechanical process.The grain size of 0.16 wt.%Nb-0.16 wt.%V microalloyed Fe-Mn-Al-C steel is decreased by over 40%compared to the conventional one.The size and content of the Nb C and（Nb,V）C hardly change during high temperature and hot rolling treatment because of their high thermal stability.The strength and hardness of the Fe-Mn-Al-C steel with Nb-V microalloying are increased due to the precipitation and fine grain strengthening effects,while the elongation and impact energy are decreased.The 0.16wt.%Nb-0.16 wt.%V microalloyed Fe-Mn-Al-C steel obtains the maximum strength,with the yield strength,tensile strength and micro Vickers hardness of 669 MPa,1001 MPa and263.7±2.7 HV,respectively,after hot rolling+aging treatment.The strengthening mechanisms that contribute significantly to the yield strength are solid solution strengthening and dislocation strengthening.Meanwhile,the segregation of carbon atoms after aging treatment also improves the yield strength to a relatively high extent.Further,it is revealed that the deformation mechanism is microband induced plasticity for the selected Fe-Mn-Al-C steels.The microstructures of the 0.16 wt.%Nb-0.16 wt.%V microalloyed and conventional Fe-Mn-Al-C steels after low-cycle fatigue test show parallel deformation bands,which are composed of dislocations with obvious planar slip characteristics.The spacing of the dislocation slip bands decreases gradually with the increase of total strain amplitude.Due to the Nb-V microalloying,the spacing of dislocation slip bands in the Nb-V microalloyed Fe-Mn-Al-C steel is narrower than that of the conventional one at the same total strain amplitude.The initial cyclic deformation behavior of the test steels is cyclic softening at various total strain amplitudes.The fatigue life of the Nb-V microalloyed steel is higher at low and medium total strain amplitudes（0.4%and 0.6%）,but lower at high total strain amplitude（0.8%）than that of the conventional one.In the periodical immersed corrosion test,relatively dense oxide passivation films are formed on the surface of the test steels,regardless of the application of Nb-V microalloying.The corrosion product is detected to beγ-Fe OOH at the initial corrosion stage,while Fe3O4and Al2O3 become dominant at the last stage.The weight loss rate and average corrosion rate of the Nb-V microalloyed Fe-Mn-Al-C steel are higher,and the polarization resistance and low-frequency impedance mode values are lower than those of the conventional one at the early stage.With further increasing the corrosion time,the weight loss rate and average corrosion rate of the Nb-V microalloyed Fe-Mn-Al-C steel decrease,showing an improved corrosion resistance.It is suggested that the corrosion resistance of the test steels depends on the grain size at initial corrosion stage,while it is mainly determined by the carbon content in the matrix and the structure of the rust layer at the last corrosion stage. |
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