| Fe-Mn-Al-C steel was first developed to replace Ni-Cr alloys and was often used in different aspects,including cryogenic application,oxidation resistance at high temperature and corrosion resistance application such as in the aircraft and chemical industries.In recent years,this kind of alloy has attracted considerable attention,especially for the automotive industry,since these steel can be used for structural components for lightweight crash-resistant car body structures.Fe-Mn-Al-C steels have a single phase,duplex,or multiphase microstructure with different composition ratios.The characteristics of its low density and corrosion resistance and its good mechanical properties are regarded as the future development direction of automotive lightweight materials.Based on the design principle and performance requirements of lightweight,Fe-Mn-Al-C duplex lightweight steels with high Mn and Al contents and medium Mn and Al contents are designed in this article,respectively.In this paper,based on the effect of alloy elements on the microstructure and mechanical properties of Fe-Mn-Al-C duplex steel,transmission electron microscopy?TEM?,scanning electron microscope?SEM?,Raman spectroscopy?RS?,X-ray diffraction?XRD?and Thermo-Calc thermodynamic calculation software were used to study the influence of C and Mn on the microstructure,mechanical properties at room temperature and the optimum alloy composition was optimized.On this basis,the deformation mechanism,effect of aging treatment on the properties and corrosion behavior were investigated.Also,the effects of carbon content,grain size and hot processing temperature on the high temperature oxidation induced transformation behavior of Fe-Mn-Al-C duplex steel were systematically studied.The main conclusions are shown as follows:?1?The designed cold-rolled Fe-20Mn-8Al-x C?x=0.25?0.45 wt.%?duplex lightweight steels consist of ferrite and austenite after solid solution treatment at800?1100?.With the increase of carbon content,austenitic phase volume increases,the tensile strength and elongation at room temperature increase gradually.The product of strength and ductility of these steels increases from 25.3 GPa%to 34.6GPa%.As the temperature improved,the duplex microstructure grows and room tensile strength decreases.When the carbon content is 0.25 and 0.35 wt.%,the elongation increases first and then decreases.And the carbon content is 0.45 wt.%,the elongation increases.After deformation,there are many dislocation walls,dislocation cells and slip bands in the microstructure,and the deformation mechanism is dislocation glide.After aging,a large amount of?-carbides are precipitated at the phase and grain boundaries,which degrade the strength and ductility of materials.?2?In the Fe-x Mn-5.5Al-0.25C?x=10?16 wt.%?duplex steel,when the Mn content is 10 wt.%,the microstructure is composed of multiphase,and the microstructure consists of ferrite and austenite when the Mn content is 13 and 16wt.%,respectively.Increased Mn content enlarges the austenite phase volume and stabilizes the austenite.At 850?950?,the elongation of steels increases with the elevated temperature,and strength instead.At 1000?,both the elongation and strength decrease.In the Fe-10Mn-5.5Al-0.25C steel,twinning induced plasticity has occurred in the microstructure during tensile,and the optimum product of strength and ductility can reach 50.1 GPa%.B2 phase sustains the strength and has good coordination deformation ability.The interfaces between B2 phase and austenite are easy to cause dislocation accumulation and keep the plasticity.After aging,?-carbides are precipitated in the microstructure reduce the strength and ductility of materials.?3?DuringtheoxidationprocessofFe-20Mn-8Al-0.25Cand Fe-20Mn-8Al-0.35C duplex steels,oxidation resistance decreases with the increase of carbon.Increased carbon augments the area of internal oxidation and improves the stability of austenite.It takes a long time to decarburization to reduce austenitic stability and more Mn diffusion outward to induce the formation of ferrite layer.The oxidation of carbon in the form of CO and CO2 causes porous defects in the oxide scale,and the formation of continuous and compact Al2O3 layer hinders the further oxidation of the steel and heals the porous defects.After the oxidation of the Fe-20Mn-8Al-0.45C duplex lightweight steel with different grain sizes at 1000?,showing that the weight gain per unit area increases with the increased time with a parabolic law.The interface concentration in the fine-grained sample is much higher than that in coarse-grained sample,resulting in a large number of metal elements diffusion outward and reaction with oxygen,which consume much oxygen and reduce the ability of oxygen penetrating into the matrix.It is easy to form metastable metal oxides in the oxide scale,such as MnO,FeO,Mn3O4,Fe3O4.The fine-grained sample can quickly form oxidation induced ferrite layer and the continuous and compact Al2O3 protective layer,which hinders the extension of internal oxidation and improves the oxidation resistance of the material at high temperature.During the oxidation process of Fe-10Mn-5.5Al-0.25C lightweight steel at 950,1050 and 1150?,the kinetic curves of oxidation present parabolic law.The oxidation resistance decreases seriously with the improvement of temperature,and the diffusion mode changes from internal oxidation at relative low temperature into homogeneous oxidation at high temperature.As the temperature elevated,Mn and C diffuse from austenite into ferrite and the austenite transforms into ferrite.The austenite transforms into martensite after it cools down to room temperature.When temperature is 1150?,the microstructure grows with large grain size.Volume diffusion degrades the ability of Al diffusion and enrichment,no protective Al2O3 formation,deteriorating the oxidation resistance of the material at high temperature.?4?Fe-20Mn-8Al-0.45C and Fe-10Mn-5.5Al-0.25C duplex lightweight steels are polarized and immersion test in the different solution environments,respectively,which show the worst corrosion resistance and maximum weight loss in the neutral environment bearing Cl-.The maximum corrosion current density in different PHs is6.26×10-5 A cm-2 and 7.80×10-6 A cm-2,respectively.And in the alkaline environment,these two steels present good corrosion resistance.After immersion test,their corrosion products are basically the same,consisting of AlO?OH?,FeO?OH?,MnO2,Fe2O3,Fe3O4 and Mn3O4.In conclusion,the optimized Fe-20Mn-8Al-0.45C and Fe-10Mn-5.5Al-0.25C lightweight steels have the best product of strength and ductility,and exhibit better corrosion resistance in alkaline environment.Fe-20Mn-8Al-0.45C steel can improve high temperature oxidation resistance by refining grain size after ensuring mechanical properties.The phase constitution of Fe-10Mn-5.5Al-0.25C steel is unstable and is not suitable for heat treatment at high temperature for a long time. |
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