| Over the past fifty years,the Ni-containing cryogenic steels have been widely used to prepare various cryogenic container.In recent years,"Ni-reduce"and "Ni-free" become the inevitable development trend of the Ni-containing cryogenic steels due to the scarce resource and high price of Ni,internationally.In 2015,the high Mn austenitic cryogenic steel developed by Posco has been successfully used in LNG storage tanks.In comparison,there are still many problems for the"Ni-free"cryogenic steel domestically developed by our country,such as the poor yield strength and cryogenic toughness,and the anisotropy of mechanical properties.In this study,a Fe-Mn-Al light-weight cryogenic steel with high strength,high plasticity,and good cryogenic toughness was developed based on the composition design ideas of ultra-low C,high Mn,and medium Al.The microstructure and mechanical properties control techniques of this tested steel were mastered by researches on hot deformation behavior and heat treatment technology.The plastic deformation mechanism and strengthening-toughening mechanism under room temperature and-196? of the tested steel were revealed.The results are as follows:Two kinds of light-weight cryogenic steel,named Fe-24.4Mn-4.04Al-0.057C and Fe-26.0Mn-6.20Al-0.050C were designed and prepared by considering the effects of alloying elements on phase composition,deformation mechanism,mechanical properties and density of steel.The phase transformation point,density and stacking fault energy of the two tested steels were calculated and measured.The results show that:microstructure of the Fe-24.4Mn-4.04Al-0.057C steel is single austenite,with the uniphase austenite area ranging from 560.5? to 1238.9?.The density of this austenitic steel is 7.19g/cm3,8.6%lower than that of pure iron.The microstructure of the Fe-26.0Mn-6.2Al-0.05C steel is composed of 80.9%austenite and 19.1%?-ferrite,with the dual-phase area ranging from 490.5? to 1302.3? The density of this duplex steel is 6.98g/cm3,11.3%lower than that of pure iron.The stacking fault energies of austenite in these two steels are both in the range of 25-55mJ·m-2.Compared to Fe-Mn-Al austenitic steel,the Fe-Mn-Al duplex steel exhibits a series of abnormal hot deformation behavior.During the whole hot deformation period,both work hardening and dynamic softening behavior occur earlier in?-ferrite.The prior dynamic recovery in ?-ferrite causes the yield-like behavior,and the uneven strain partitioning between austenite and ?-ferrite results to the serration on the flow curves.The continuous dynamic recrystallization of ?-ferrite at temperature>900? causes a"Dynamic recovery type"flow curves,and the discontinuous dynamic recrystallization of austenite at temperature<900? lead to a"Dynamic recrystallization type"flow curves.Based on an improved hyperbolic sine equation and the exponential saturation work-hardening law,a constitutive model was developed to predict the transient stress of the Fe-Mn-Al duplex steel by considering the influence of microstructural evolution on the flow curves.And this model was verified to be remarkably accurate.The tensile strength and yield strength of the hot-rolled Fe-Mn-Al duplex steel are 76.7MPa and 119.4MPa higher than that of Fe-Mn-Al austenitic steel.But due to the existence of the banded structure,the Fe-Mn-Al duplex steel exhibits a poor plasticity and toughness,as well as the anisotropy of mechanical properties.After being solution treated at 1000? for lh,the Fe-Mn-Al duplex steel obtains the optimal comprehensive mechanical properties,with the tensile strength and yield strength of 574.9MPa and 362.4MPa,elongation of 57.55%,and impact energy at-196? of 129.7J.cm-2.The high distortion energy retained in austenite increases the driving force for grain boundary migration,which results to the discretization of the banded 5-ferrite during solution treatment.The strength of the as-solutionized Fe-Mn-Al duplex steel is associated with phase proportion and the varying microhardness.The excellent ductility and toughness are attributed to microstructure homogenizing and TWIP effect in austenite.In the process of tensile deformation at room temperature,?-ferrite shows a series of cross-slip characteristics.While austenite exhibits the typical plane-slip characteristics and dominates the main deformation at the early stage of plastic deformation.At the later period,TWIP is the main deformation mechanism of austenite,and strain is transferred from austenite to ?-ferrite through the high-density dislocation wall resulting in a coordinated deformation of the dual-phase structure.The stacking fault caused by plane-slip of partial dislocation and stress concentration resulted from dislocation plug-up are two requisite preconditions for the formation of mechanical twin in austenite.The good strength and plasticity of the Fe-Mn-Al duplex steel at room temperature is attributed to the multistage work hardening behavior caused by the interaction among dislocation,mechanical twin and grain boundary.In cryogenic environment,the suppression of atomic activity increases the critical stress required for dislocation slip,while the critical stress required for the formation of mechanical twin decreases because of the decrease of stacking fault energy.Therefore,the dislocation motion is inhibited during tensile deformation at-196?.The predominant TWIP effect in austenite enhances the tensile strength and yield strength of the Fe-Mn-Al duplex steel to 945.0MPa and 585.0MPa,and increases the elongation to 64.5%.In case of impact deformation at-196?,stacking fault is more likely to occur than dislocation slip,so the stress concentration caused by external load makes the twin deformation mechanism dominated by nucleation.The good cryogenic toughness of the Fe-Mn-Al duplex steel is attributed to the TWIP effect caused by nano-twin in austenite and the ?/?phase boundary cleaning effect caused by ultra-low carbon design. |
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