Microstructure And Mechanical Properties Study On A Ferrite/Bainite Dual Phase Steel

Ferrite/bainite dual phase steel has been a new type of high strength low alloy steel with high strength,high elongation and excellent stretch-flangeability,which is suitable for the manufacture of product with complex shape.When the steel was used for fabricating automotive parts,it could reduce vehicle weight,as a result,it could deduce fuel consumption and improve the safety level of automotive.Research on the production technology of ferrite/bainite dual phase steel,providing an experimental and theoretical foundation for industrializing mass production.My major work was to combine the CCT curve,to design the technology of the TMCP(Thermo-Mechanical Control Processing)and to study the influence of TMCP process on the microstructures and mechanical properties of the ferrite/bainite steel.The influence factors of stretch-flangeability and the factors which affect crack propagation mechanism were studied.The conclusion include the following parts:(1)The continuous cooling transformation curves of three kinds of experimental steels were studied by means of thermal simulation experiments.The deformation increases the ferrite transformation start temperature(Ar₃),expands the ferrite and pearlite transformation areas,and reduces the bainite transformation area.Nb makes the Ar₃ of the Ti bearing low carbon steel down with the prior deformation of austenite.Ar₃ rises with the increase in Si with deformation.(2)The experimental steel containing Nb,Ti were developed by TMCP technology.When the grain size of ferrite is about 8.2?m,and bainite content is around 30%,the tensile strength is 628MPa,the elongation is more than 20%,and hole expansion ratio is up to 96%.The mechanical propertis and stretch-flangeability of experimental steels are both satisfied.(3)In term of TMCP parameters which effect on the microstructures and mechanical properties,a better process route is determined:The reheated temperature is 1200?,the start temperature of rough roll is 1100?,the start temperature of fine roll is between 950?1000?,the finish roll temperature is between 850?880?,the start cooling temperature is about 800?,the cooling rate is about 25?/s,the coiling temperature is 500?.(4)The crack propagation mechanism and surrounding microstructure were researched by means of SEM observation.The crack propagation of FBDP steels belongs to microviod coalescence fracture.When the crack meets the bainite,it chiefly propagates along the phase interface of ferrite and bainite.When the crack propagates to ferrite,a great deformation of ferrite will help to reduce the stress concentration near the crack.As a result,the crack propagation is restrained.