| Fe-Mn-Al-C austenitic low-density steel has a broad application prospect in the fields of transportation,marine vessels and aerospace because of its high specific strength,excellent plastic and toughness as well as good corrosion resistance.With the increasingly demanding service environment and social development needs of the current Fe-Mn-Al-C austenitic low-density steel put forward higher performance requirements,such as higher alloy strength and more excellent plastic toughness.In order to further enhance the comprehensive mechanical properties of Fe-Mn-Al-C austenitic lightweight steel,this paper takes Fe-27Mn-10Al-x C low-density steels as the research basis,and investigates the effects of different C contents,deformation pretreatment,heat treatment process and microalloying（V,Nb）on the microstructure and mechanical properties of the low-density steel and the mechanism of the effect,and successfully prepares a variety of A variety of high-strength low-density austenitic steels were successfully prepared,and the main research contents are as follows:Low-density austenitic steels with different phases were obtained by designing compositional gradients of C content（0.8,1.2 and 1.4 wt.%）in Fe-27Mn-10Al-C alloy.Ingots were obtained by vacuum induction melting,which were subjected to hot rolling deformation and solution treatment.As the carbon content of the steel increased from 0.8 to1.4 wt.%,the phase composition changed from austenite+ferrite duplex（0.8 wt.%C）to austenite（1.2 wt.%C）and austenite+κ-carbide（1.4 wt.%C）,respectively.The increase in C element widened the austenite phase interval and facilitated the precipitation ofκ-carbides.With the increase of C content in the steel,the yield strength increased from 495 MPa to614 MPa and 897 MPa,while maintaining excellent elongation of 50%,53%and 47%.Among them,Fe-27Mn-10Al-1.4C steel has the best combination of strength and plasticity.Taking Fe-27Mn-10Al-1.4C steel as the target of the study,multiple strengthening mechanisms can be well introduced by combining cold rolling pre-deformation and partial recrystallization annealing.After partial recrystallization annealing,the microstructure of steel consists of recrystallized austenite grains and non-recrystallized austenite grains containing dense dislocations.The sizes of the two grains are 1.16μm and 40μm,respectively,and the dislocation density reaches 1.15×1014m-2,while both grains contain nano-sizedκ-carbides inside.Under the effect of multiple strengthening mechanisms,such as dislocation strengthening,precipitation strengthening and grain boundary strengthening,the low-density steel achieves an excellent combination of strength and plasticity,with yield and tensile strengths of 1164 MPa and 1387 MPa,respectively,and also maintains a total elongation of 35%,while the steel also maintains a work hardening rate of 2.9 GPa.On the basis of partially recrystallized steels,the further precipitation ofκ-carbides was promoted by short-time aging to enhance the precipitation strengthening effect.The partially recrystallized Fe-27Mn-10Al-1.4C steel was aged for 10 min,and it was found that the average size ofκ-carbides in the non-recrystallized austenite grains containing dislocation defects was 20 nm,while the average size ofκ-carbides in the recrystallized austenite grains was only 5 nm.The precipitation ofκ-carbides is facilitated by the accelerated diffusion of elements due to the retained dislocation defects in the non-recrystallized grains in partially recrystallized steels.The yield and ultimate tensile strengths of the steel were increased to1304 MPa and 1405 MPa,and an excellent total elongation of 31%was maintained at the ultra-high strength level.The conventionalκ-carbide age-strengthening heat treatment process is usually carried out in fully recrystallized materials.The fully recrystallized Fe-27Mn-10Al-1.4C steel was aged,and the results of the carbide precipitation behavior and mechanical properties showed that the precipitation rate ofκ-carbides in the fully recrystallized austenitic microstructure was slow,and the aging time was as long as 2-100h.Especially after aging for 48 h,the coarse and large-sizedκ-carbides were precipitated along the austenitic grain boundaries.The plasticity of the low-density steel decreases rapidly,and the maximum yield strength it reaches is only 1095 MPa,and the total elongation is as low as 1.5%.V,Nb elements as strong carbide forming elements can effectively improve the strength of steel.The mechanical properties and deformation mechanisms in the annealed Fe-27Mn-10Al-1.4C-0.1V-0.1Nb steel were investigated.It was found that the type of carbides in the steel was transformed,and both（V,Nb）C particles andκ-carbides were precipitated in the steel,which transformed from single precipitation strengthening ofκ-carbides to double nanoparticle precipitation strengthening consisting of（V,Nb）C particles andκ-carbides.Meanwhile,（V,Nb）C particles have strong pinning effect on dislocations,which can simultaneously improve the tensile strength and work-hardening ability of steel and overcome the deficiency ofκ-carbide precipitation strengthening.Under the combined effect of multiple strengthening mechanisms such as precipitation strengthening,grain boundary strengthening and dislocation strengthening,the yield and ultimate tensile strength of the low-density steel reached 1326 MPa and 1498 MPa,respectively,and the total elongation was as high as 21%.In addition,the deformation microstructure of the steel was observed to be planar slip bands and microbands,which clarified that the pinning effect of（V,Nb）C particles on dislocations and the dynamic refinement between the planar slip bands are the main reasons for the steel work hardening.The comprehensive analysis and discussion of the strengthening mechanism and deformation behavior of Fe-27Mn-10Al-x C low-density steel in this paper provide a useful reference for further development of high-strength plastic austenitic low-density steel. |
