The Research On Deformation Behavior Of 70Si3Mn Carbide-free Bainitic Steel

In this paper,the 70Si3 Mn carbide-free bainitic steel was used as the test material.The partition law of macroscopic stress and strain in each microscopic compositional phase in which the volume fraction changes was studied using the modified law of mixture and the Swift equation.And the effect of strain rate and temperature on the mechanical properties of the test material was also studied.In the deformation process,the results show that the bainitic ferrite undertakes 80.0-88.3% of the strain and 73.6-93.2% of the stress,the retained austenite undertakes 10.2-19.4% of the strain and no more than 8% of the stress,and the newly generated martensite has a lower strain,but it undertakes 22.7% of stress in the later stage of deformation.Under the unit volume fraction,the deformation of the retained austenite is the largest,and with the increase of macroscopic deformation,although the volume fraction of the retained austenite is greatly reduced,the increase of its deformation is accelerated,which explains the strong deformability of the retained austenite;the stress undertook by martensite is the largest,indicating that the hard and brittle martensite has a stronger ability to undertake stress.Through the discussion and derivation of the calculation results,a model of three-phase co-deformation was established,revealing the law and mechanism of cooperative deformation between phases.As the strain rate increases,the tensile strength,yield strength,and uniform elongation of the test material gradually decrease.The results of the study show that at a lower strain rate,the retained austenite has a lower transformation rate.This results in a higher content of retained austenite in the low strain rate deformed specimen at the same strain amount.A higher content of retained austenite can more effectively exert the TRIP effect and coordinate the deformation of the bainitic ferrite and martensite,which has greatly improved the strength and uniform elongation of the test material.With the increasing of text temperature,the microstructure of the test material transitions from bainite-free to sorbite and finally all to austenite.In the range of RT-600°C,the strength,elongation and uniform elongation of the test material decrease as the test temperature increases.The main reason is that TRIP effect gradually weakens and the test material has a strong thermal activation effect under the effect of high temperature,dislocations can easily slip and climb,which will reduce the strength of the material.As the temperature increases,carbides gradually precipitate,carbides are the hard phase and the crack origin of specimen fracture,and more carbides will reduce the uniform elongation and elongation of the material.