Effect Of Pulse Current On Mechanical Properties And Microstructure Of Ultrahigh Strength Steel

Ultrahigh strength steels(UHSS)are important materials to realize the lightweight of automobiles.However,the high flow stress and high yield strength of UHSS lead to their high forming force and large residual stress after forming.These features are easy to induce fracture and springback.The traditional forming process is difficult to meet the forming requirements of UHSS.By using pulse current to control the microstructure of materials and the application of electrically assisted forming(EAF)processes are currently hot research topics.Pulse current has been proved to improve the microstructure of a variety of materials.EAF has also significantly improved the forming effect of many materials,such as titanium alloy,magnesium alloy and other alloys which are hard to form.But few studies involve the effect of pulse current on ultrahigh strength steels.This work studies the effect of pulse current on the microstructure and mechanical properties of UHSS.It focuses on whether the pulse current can improve the deformation and forming effect of UHSS.The following work was performed:(1)Different methods of pulse current processing experiments were performed on MS1300 and QP980,the resistance of two UHSS to pulse current was investigated,and the effect of pulse current on their microstructure and mechanical properties was studied.The results show that the microstructure and mechanical properties of the two UHSS remain almost unchanged when the current density is relatively low.When the current density is higher,the martensite and residual austenite in the UHSS are decomposed.It results in deterioration of the mechanical properties at room temperature.The effect of the pulse current on the material mainly results from the joule heating effect.But its mechanism and effect are significantly different from traditional heating methods.(2)The effect of pulse current on the deformation process and fracture behavior of two UHSS was studied through electrically assisted tensile experiments.Under the appropriate current density,the elongation of the two steels can be increased while the microstructure and morphology of the two steels do not change significantly.For MS1300 steel,the elongation after breaking can be increased from 8.3%at room temperature to 12.4%under the current density of 6.58 A/mm~2.The peak stress of the QP980 is reduced to 900MPa under the current density of 7.46 A/mm~2,and the elongation is increased from 22.3%at room temperature to 38.4%.Further increasing the effective current density,the elongation and flow stress of the two UHSS decrease significantly.With the increase of current density,the brittle fracture tendency of both UHSS exists.(3)Based on the finite element simulation software ABAQUS,the multi field coupling analysis of current processing and electrically assisted bending of MS1300 were conducted.The interaction and evolution law of the temperature field,electric field,and stress-strain field of MS1300 steel during the current processing were explored.In the simulation of electrically assisted bending of MS1300 steel,the current load applied during bending deformation was adjusted to observe the effect of current on its forming process.It was found that the proper setting of the current loading method and the current intensity can reduce the forming force and springback of the MS1300 sheet.It is helpful to improve the forming performance.