Microstructures And Mechanical Properties Of Fe-12Mn-9Al-(1.0～1.6)C Low-density Steel And Cr Alloyed Strips Prepared By Near-rapid Solidification

Fe-Mn-Al-C alloy is a promising candidate for auto lightweight material as it combines high strength and low density.Medium-manganese low-density steel has received more attention because its raw material cost is relatively low and it can avoid some process problems in the preparation of high-manganese steel.However,the decrease of manganese content in low-density steel reduces the austenite content and stability.Austenite content and stability can be increased by increasing the carbon content in medium-manganese low-density steel,but κ-carbides tend to precipitate around the grain boundaries,which will reduce plasticity of medium-manganese lowdensity steel.This work investigates the effect of carbon content in the range of 1.0wt.%～ 1.6wt.% on the microstructure and mechanical properties of medium-manganese lowdensity steel.Furthermore,Cr element is added to restrain formation of grain boundaryκ-carbides for improvement of plasticity.To avoid element segregation and carbide coarsening,a near-rapid solidification centrifugal casting method is utilized to prepare the samples considering its high-content alloying elements.The Fe-12Mn-9Al-(1.0～1.6)C alloy strips show that with the increase of carbon content in the range of 1.0wt.%～1.6wt.%,the austenite content rises from 78 vol.% to95 vol.%,the size of ferrite decreases from 18μm to 3μm,and the size of κ-carbide in austenite slightly increases.The strength of as-cast strip increases from 921 MPa to1216MPa,and the plasticity presents a downward trend.When carbon content reaches1.4wt.%,the yield strength of the as-cast sheet is greatly increased while the plasticity is significantly reduced,because a large number of κ-carbides particles with size about 100 nm begin to appear at the grain boundary.Although an addition of 3wt.% Cr has no significant effect on the microstructure and mechanical properties of the Fe-12Mn-9Al-1.2C strip,it has huge effect on the Fe-12Mn-9Al-1.4C strip.The addition of 3wt.% Cr increases the content and size of ferrite slightly in Fe-12Mn-9Al-1.4C strip,and it inhibits precipitation of κ-carbides around the grain boundaries.The strength and hardness of 1.4C strip are slightly reduced with addition of 3wt.% Cr,while the elongation is increased tremendously from 14.0% to47.8%,which makes product of strength and ductility increases from 14.9 GPa·% to 50.0 GPa·%.After heat treatment at 550℃ for 3h,the strength of strip increases while the plasticity decreases due to increase in fraction and size of the κ-carbides.As a result,the yield strength of the 1.4C strip increases from 1009 MPa to 1301 MPa and the elongation decreases from 14.0% to 4.3%,while the yield strength of the 1.4C3 Cr strip increases from 907 MPa to 1068 MPa and elongation decreased from 47.8% to 28.9%.It can be seen that the plasticity is significantly improved although the strength drops slightly when 3wt.% Cr is added to the Fe-12Mn-9Al-1.4C strip.As cold rolling reduction increases from 0 to 45%,the ferrite in 1.4C3 Cr strip is elongated along the rolling direction and the deformation texture of the austenite matrix is gradually obvious.The yield and tensile strength of the sample increase with the cold rolling reduction,while the elongation decreases accordingly.At 35% rolling reduction,the tensile strength exceeds 1.5GPa and the elongation remains above 10%.The rapid heat treatment at 400℃/30 s after rolling makes the ferrite size increase slightly and the texture in the austenite matrix disappear,and the change in mechanical properties is not obvious.In summary,the increase of C content from 1.0wt.% to 1.6wt.% promotes the increase of austenite content and the precipitation of κ-carbides,and the strength of the strip is significantly improved while the plasticity is clearly reduced when C content is beyond 1.4wt.% due to appearance of inter-granular κ-carbides around grain boundary.Addition of 3wt.% Cr into Fe-12Mn-9Al-1.4C strip inhibits the precipitation of intergranular κ-carbides,improves plasticity and makes the product of strength and ductility exceed 50.0GPa·%.More than 1.5 GPa strength and good plasticity can be obtained after 35% cold rolling of 1.4C3Cr strip.