Study On The Structure Design And Properties Of High Strength And Toughness Bainitic Steel In Recession Process

With the continuous development of society and economy,environmental pollution and energy crisis are becoming more and more serious.As one of the industries with the largest energy consumption,reducing the weight of BIW is the current research trend.The third generation of advanced high strength steel could achieve the perfect combination of high strength and high plasticity of steel materials,it has a great significance to achieve low BIW weight,improve vehicle safety,energy saving and environmental protection.In this paper,taking nano-bainitic steel as the prototype and considering the bainite transformation,it is proposed to use Al element instead of Si element to design a new low alloy bainite Fe-C-Mn-Al steel,making it overcome nano-bainitic steel's shortcomings of high carbon content and long bainite isothermal time.The relationship between microstructure and mechanical properties is explored,and the problems of how to solve the block residual austenite and improve the stability of the retained austenite are studied emphatically.The rational bainite-Q&P heat treatment process is designed by using Thermo-Calc,MUCG83 softwares and phase change instrument.The martensitic transformation occurs in the experimental steel after rapid bainite isothermal at 450?,and the carbon enrichment of retained austenite occurs at 400?.The retained austenite content increases at first and then decreases with the extension of time.When the partitioning time is 180s,the residual austenite content reaches the peak of 24.1%,and the comprehensive mechanical properties are the best:the tensile strength is 1202MPa,the elongation is 22.2%,and the product of strength and plasticity is increased by 51%compared with that without partitioning.With the addition of Q&P heat treatment process,the initiation position and propagation mode of crack in experimental steel are changed.The sharply crack appears in the martensite phase before partitioning,while the inactive crack appears in interface of the martensite and ferrite after partitioning.The application of two-step bainite isothermal heat treatment is proposed to solve the problems of unstable large block residual austenite and low retained austenite content after one-step bainite isothermal After two-step bainite isothermal,the size of the block retained austenite is reduced from 6-10um to 1-2um.Compared two-step method at 460-400? with one-step method at 400?,the residual austenite content increases from 15.4%to 23.1%.Compared two-step method at 460-380? with one-step method at 380?,the retained austenite content increases from 11.5%to 18.3%.The mechanical propertie of the experimental steel is improved significantly compared with the one-step method.The best mechanical propertie is obtained under the two-step process of 460-400?:the tensile strength is 911MPa and the elongation is 40.7%,the product of strength and plasticity is increased by 108%compared with one-step method..The stage tensile test of bainite-Q&P and two-step bainite isothermal process shows that the residual austenite content and work hardening index decline sharply at first and then decline gently with the strain increases.The block retained austenite undergoes TRIP effect during the work-hardening phase of the material,then the film retained austenite undergoes the TRIP effect during the uniform extension of the material.In the two-step method,the work-hardening section of the tensile curve is greatly reduced,the reduction range of residual austenite content decreases at the early stage of strain and the small variation of the work-hardening index show that the two-step bainite isothermal process can significantly reduce the size of block retained austenite.