| Energy consumption,environmental pressure and requirements for vehicle safety have promoted the rapid development of lightweight car body materials and designs.Especially,for the new energy vehicles,reducing the total weight can significantly increase the endurance.Currently,the main way to achieve lightweight is the application of high-strength steels to reduce the thickness of the steel plates.However,there is a limitation to thin the thickness of the body parts with stiffness requirement,which limits the considerable weight reduction.For this reason,reducing the density of high-strength steels is another new way to reduce weight of automobiles.Due to its superior properties such as lightweight,high strength,excellent strain hardening ability and collision absorption capability,Fe-Mn-Al-C low-density steels with highMn and high-Al content are expected to become the next generation of high-quality automotive lightweight materials.Moreover,such low-density steels also have high corrosion resistance and high temperature oxidation resistance,thus they also have wide allocation prospects in the fields of cryogenic storage tanks,aerospace,military and chemical engineering.However,unlike the conventional iron-based alloys,the physical metallurgy of Fe-M-Al-C alloys is more complicated.There are still many problems need to be solved,and in particular,there are fewer reports on the control of the microstructure and mechanical properties of high-Mn high-Al lowdensity steels produced by thin strip casting technology.In this paper,Fe-30Mn-8Al-1.2C low-density steel(abbreviated as 8A1 steel)is the main research subject,and the mechanisms of microstructure control,strengthening and toughening during recrystallization annealing and aging process of the austenitic low-density steel produced by thin strip casting have been investigated systematically,to provide theoretical guidance for the process design of Fe-Mn-Al-C austenitic low-density steels.The main conclusions are as follows:(1)The thermophysical properties,equilibrium phase diagrams and precipitation kinetics of κ phase for Fe-30Mn-xAl-1.2C steels were studied,respectively,to provide a theoretical basis for the composition design,recrystallization annealing temperature and aging annealing temperature of high-Al steels.The results indicate that the increase of A1 content from 6.5 wt.%to 11 wt.%in Fe-30Mn-xAl-1.2C steel increases the weight reduction from 12.60%to 18.45%,and elastic modulus decreases from 167.03 GPa to 144.09 GPa,and the related stacking fault energy increases from 83.13 mJ/m2 to 105.17 mJ/m2.Their deformation mechanisms are dislocation glide.Additionally,the precipitation-temperature-time curve of 8A1 steel was plotted according to the change of isothermal expansion with time,and the precipitation nose temperature of k phase in 8A1 steel is about 800℃.(2)The 8A1 steel produced by the strip casting technology was subjected to cold rolling and recrystallization annealing,to achieve good combination of strength and toughness by adjusting the austenite grain size.The results show that with the increase of recrystallization annealing temperature,the austenite grain size gradually increases,the strength,elastic modulus and strain hardening rate decrease,and the elongation and toughness increase.After annealing at 800℃ for 20 min,partial recrystallization occurs and three types of κ phase precipitates also appear simultaneously,and the tensile fracture is a ductile-brittle mixing mechanism.After annealing above 900℃ for 20 min,fully recrystallized single austenite is obtained,and the related fracture is ductile.The non-aged austenitic low-density steel with 10 μm recrystallized austenite grains has good combination of mechanical properties with yield strength of 574 MPa,tensile strength of 965 MPa,elastic modulus of 151 GPa,elongation of 48.3%,strengthductility balance of 46.61 GPa·%and toughness of 400 MJ/m3.(3)The precipitation behavior of nano-sized κ phase in 8A1 steel during isothermal aging annealing was investigated,as well as its effects on the strength,toughness and strain hardening behavior.The results indicate that with the increase of the aging time,the second phase grows from the initial nano-sized short-range ordering phase gradually along the<001>directions to nano-sized κ phase with modulated structures.The κ phase has a high thermal stability and maintains good coherency with the austenite matrix.The intragranular nano-sized κ phase significantly enhances the yield strength and elastic modulus,and reduces strain hardening rate.After long period of aging for 8 h,the intergranular κ phase formed and seriously deteriorated the elongation and toughness.Aging at 600℃ for 1 h obtained an excellent comprehensive mechanical properties for yield strength of 910 MPa,tensile strength of 987 MPa,elastic modulus of 140 GPa,elongation of 32.5%,strength-ductility balance of 32.08 GPa-%and toughness of 315 MJ/m3 through nano-sized κ precipitation strengthening.(4)The strengthening and toughening mechanism and strain hardening behavior were investigated under the synergistic control of grain refinement and precipitation strengthening,to improve the strength and toughness of aged 8A1 steel.The results show that the synergistic effect of grain refinement and second phase precipitation strengthening increased strength;when the second phase is short-range ordered phase,it has little influence on elongation and toughness;when the second phase is nano-sized κ phase,it declines the elongation and toughness dramatically.The refinement of austenite grain size promotes the precipitation of κphase and intergranular κ phase.The 8A1 steel with refined austenite grain size of 44 μm aged at 600℃ for 1 min,has excellent mechanical properties,i.e.yield strength of 744 MPa,tensile strength of 915 MPa,elastic modulus of 156 GPa,elongation of 50%,strength-ductility balance of 45.75 GPa·%and toughness of 424 MJ/m3.(5)Based on the previous experimental results,the optimization of two-step cold rolling process and aging annealing process was conducted to further improve the strength and toughness of 8A1 steel.The results reveal that two-step cold rolling refines austenite grain size to 36 μm.Coherent κ phase with an average size of 3.5 nm precipitates within the matrix when aged at 550℃ for 60 min,and the lattice misfit between κ phase and austenite matrix is only about 0.13%.Particularly,the formation of intergranular κ phase is avoided.The interface between nano-sized κ phase and austenite matrix is irregular and there is a gradual atomic concentration gradient adjacent the interface,which is the evidence of the κ phase morphology in the early growth period and discovered for the first time using APT technology.Ultimately,high-Mn high-Al austenitic lightweight steel with extraordinary comprehensive properties for density of 6.75 g/cm3,yield strength of 928 MPa,tensile strength of 1026 MPa,elastic modulus of 148 GPa,elongation of 43.7%,strength-ductility balance of 44.84 GPa%and toughness of 434 MJ/m3 are obtained.The toughness is significantly improved while aaintailing high strength,excellent uniform deformation ability and strain hardening ability.(6)The hot stamping and warm stamping technologies were coupled with the microstructure control processes,to evaluate the forming properties of the 8A1 steel with high strength and high ductility.The results show that the combination of hot stamping and recrystallization annealing process makes the U-shaped workpiece with tensile strength of 918 MPa and elongation of 61%,and the combination of warm stamping and aging process makes the U-shaped workpiece with tensile strength of 970 MPa and elongation of 39%.The formation of abundant nano-sized κ phase enhanced the strength of warm stamping U-shaped workpiece,while also maintaining excellent plasticity.The maximum thickness reduction rate of the hot stamped U-shaped workpiece and warm stamped U-shaped workpiece is about 10.71%and 9.82%,respectively.The springback angle of each stamped workpiece for investigated steel is smaller than the conventional Q235 steel cold stamped U-shaped part.It indicates that these two stamped workpieces have good dimensional accuracy and uniform plastic forming ability.Thus,the stamping process coupled with microstructure control process develops a highstrength and high-ductility austenitic lightweight steel,which provides a new idea for the forming process of low density steels. |
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