Effect Of Cu Element On Microstructures And Mechanical Properties Of Fe-28Mn-9Al-0.8C Austenitic Matrix Lightweight Steel

Fe-Mn-Al-C steels have got much attention because of their good mechanical properties and low density.Among them,the austenitic matrix lightweight steels have excellent plasticity,but the strength is not enough to be applied in some industry field.Based on the advantage that nano-scale Cu-rich particles can be precipitated in the steel without relying on C and N elements.In this work,the co-precipitation of nano-scale Cu-rich particles and ?-carbides in the matrix was used to improve the mechanical properties of austenitic matrix lightweight steel by adding Cu element.Due to high content of Mn and Al in the austenitic matrix light-weight steel,elemental segregation is prone to occur,and cracks easily happen during the rolling process in the traditional production process.To avoid the above problems,near-rapid solidification centrifugal casting method was used to prepare near net-shape strips.The Fe-28Mn-9Al-0.8C-(0,1,3,5)Cu(wt.%)steel strips were prepared,the effects of Cu on microstructure and properties of strips was studied.According to the results,an addition of 1wt.%Cu had no obvious effect on the microstructure of the Fe-28Mn-9Al-0.8C steel strip.But with increasing the amount of Cu element,the ferrite in the steel strips was changed from irregular block to thin bar,and when it increased to 5wt.%Cu,the ferrite became dendritic.Moreover the addition of Cu inhibited the ordering of ferrite.In terms of mechanical properties,the addition of Cu slightly reduced the strength of the steel strips prepared under near-rapid solidification.The reason is that no large number of nano-scale Cu-rich particles were precipitated in the cast steel strips,and the addition of Cu also reduced the ferrite content in the steel strips and increased the stacking fault energy of the matrix.When Fe-28Mn-9Al-0.8C-(0,3)Cu steel strips were heated at 500 ?-700 ? for 3 h,intragranular ?-carbides were precipitated in the two steel strips.And the yield strength of the steel strips was improved during the heat-treating at 500 ?.When they were heat-treated at 600 ?,the plasticity of two steel strips began to decrease due to precipitation of intergranular ?-carbides,but the yield strength and plasticity of the Cu-containing steel strip were clearly higher than that of the Cu-free steel strip due to the formation of a large number of the Cu-rich particles and the relatively less precipitation of intergranular ?-carbides.When they were heat-treated at 700 ?,the plasticity of the steel strip was deteriorated because of a large amount of coarse intergranular ?-carbides precipitated.When the heat-treating time was extended to 6 h and 20 h at 500 ?,more nano-scale Cu-rich particles precipitated from the austenite and ferrite in the 3Cu and 5Cu steel strips.After heat-treating at 500 ? for 20 h,the size of Cu-rich particles in the austenite matrix was about 30-40 nm.According to the results of nano-beam diffraction,it was found that the crystal structure of Cu-rich particle was FCC structure being same as that of the pure Cu,and the precipitation of Cu-rich particles also promoted the precipitation of intragranular ?-carbides in the austenitic matrix.After heat-treating at 500 ? for 20 h,the yield strength and tensile strength of 5Cu steel strip reached 808±5 MPa and 1008±12 MPa,respectively.They were clearly higher than that of the Cu-free steel strip,and the plasticity had no clear effect.It was concluded that when 3-5wt.%Cu element was added into austenitic matrix lightweight steels,nano-scale Cu-rich particles and ?-carbides were precipitated from austenitic matrix after heat-treating at 500 ?-600 ?.The increment in yield strength caused by co-precipitation of nano-scale Cu-rich particles and ?-carbides was about 289 MPa,and the yield strength was clearly higher than that of the Cu-free steel strips,at the same time the steel strips still have good plasticity.