Study On The Strengthening And Toughening Mechanism Of Fe-30Mn-8Al-0.8C Austenitic Low Density Steel

Fe-Mn-Al-C low-density steels,as a new type of low-density high-strength steel,are considered to be the preferred structural material for reducing vehicle weight and improving crash resistance in the automotive field.This study took Fe-30Mn-8Al-0.8C austenitic low-density steel as the research object.The thermodynamic calculation and the establishment of the austenite grain growth model were carried out.The microstructure and mechanical properties of the experimental steel after solid solution and aging treatment were systematically studied.On this basis,the deformation mechanism,microstructure evolution and work hardening behavior of the experimental steel during plastic deformation were discussed.Various studies were carried out on Fe-30Mn-8Al-0.8C austenitic low-density steel,which could provide theoretical references for the industrial production and application of austenitic low-density steels,and the following main conclusions were obtained:(1)The stacking fault energy of the experimental steel is about 72 m J/m~2,and the density is about 6.88 g/cm~3.At the same temperature,as the holding time prolongs,the austenite grains grow continuously,but the growth rate decreases continuously.When the holding time is the same,as the isothermal temperature increases,the austenite grain size increases,and the austenite grain growth rate also increases significantly with the increase of temperature.The function relationship between austenite grain size and holding time was simulated by Beck equation,and the Sellars austenite grain growth model was established to fit the function relationship between austenite grain size and holding temperature and time.(2)The microstructure of the experimental steel after solution treatment is composed of a single austenite phase,and there are a large number of annealing twins in the grains.When the solution time is the same,the strength decreases continuously with the increasing solution temperature,while the elongation increases first and then decrease.When the solution temperature is 950℃,the experimental steel has the best strength-ductility product of 44.3 GPa·%.After aging treatment at 550℃,the microstructure is austenite+κ-carbide,and the average grain size and morphology of austenite do not change significantly.The strength of the sample increases continuously with the increase of aging time,while the elongation presents a continuous downward trend.The work hardening curves of the experimental steels present three stages.The work hardening curve of the experimental steel presents three stages,the elastic deformation stage with a sharp decrease in the work hardening rate,the plastic deformation stage with a slow decrease,and the fracture failure stage with a rapid decrease.The observation for the fracture morphology indicates a ductile fracture of the experimental steel.(3)During the deformation process of the experimental steel,as the strain increases,the grain size decreases and the proportion of low-angle grain boundaries increases linearly.The dislocation density and micro-hardness increase and the proportion of twins decrease with strain.The deformation gradually penetrates into the interior of the large grains,which inhibits the strain localization during the deformation process.The deformation mechanism of the plastic deformation of the experimental steel at room temperature is microband-induced plasticity,and no deformation-induced martensite and twins were observed in the deformed microstructure.With the increase of deformation,Taylor lattice and microband planar dislocation structures present in the microstructure.The width of the slip band is gradually refined.Dynamic slip band refinement is the main work hardening mechanism of the experimental steel.