Study Of Microstructure,properties And Deform Behavior In 40SiMnCrAlMoNi Carbide Free Bainitic Steel

In this dissertation,the research object is carbide-free bainitic steel.The effect of heat treatments and nickel content on the microstructure and mechanical properties has been explored.The microstructural evolution under cold rolled was studied,and the deformation and strain partation behavior between retained austenite(RA)and bainitic ferrite(BF)during the tensile process has also been investigated respectively through in-situ electron backscattered diffraction(EBSD)method.The results show that the retained austenite tend to be unstable with the increasing austempering,and it’s easy to take place the strain induced martensitic transformation and strengthen the material.The phases boundaries and sheaves boundaries can be analyzed quantitatively through the statistic of their unique misorientation degrees.It will generate more bainitic ferrite because of a high driving force during the process,therefore,enhance the strength and decrease the elongation.Nickel(Ni)addition improves the length of banitic ferrite after austempering.The steel with Ni has a higher strength and lower toughness than that without Ni.When the specimens are deforming under low tensile rate,the stress and strain could get sufficient time to distribute uniformly in bainitic ferrite and retained austenite,and the ductility get improved because more energy was absorbed by the process of martensitic transformation compared with high tensile rate.The amount of retained austenite rapidly decrease before true strain of 0.054 and slowly decrease after 0.054.RA’s surrounded BF with high schmid factor(SF)value accelerate martensitic transformation of RA,but the surrounded BF with low SF value retards the transformation.The kernel average misorientation(KAM)results show that the strain of RA and BF all rapidly increase first and slowing down after critical points of 0.25 and 0.54 respectively because the deformability of RA and BF decrease follow the increasing strain.The calculated results of strain fraction shows that the contribution of RA for the whole plastic deformation gradually decreases at the whole tensile process and that of BF increases under the corporation of RA.At the same strain,the number of retained austenite that occurred martensitic transformation inrolled specimens are less than that in tensile specimens.