Study On Surface Roughening Mechanism And Friction Behavior Of Pre-strain Dual Phase High Strength Steel

Dual-phase steel is featured by its high elongation and deformation hardening rates.Its strength can be improved through cold processing.it also displays good plasticity and tenacity.Therefore,the material has been widely applied into automobile component manufacturing.However,its surface is prone to coarsening during the plastic forming process,which may affect the friction behavior during its subsequent forming process.Therefore,it’s necessary to research the change rule of relevant properties of plate surface under different deformation states and its influence upon subsequent sliding friction behavior as those understandings have important instructive significance for further understanding of the plastic deformation mechanism of advanced high-strength steel plate and correct grasping of its deformation laws.DP590 high-strength dual-phase steel plates were taken as research object to explore the problem of surface coarsening during the plastic processing.Unidirectional pre-stretching was adopted to simulate the initial processing toward the material.The surface morphology,mechanical properties,micro-structure and friction properties of samples under different pre-strains were studied.A three-dimensional finite element model of crystal plasticity was established for numerical simulation.Thus,the following results were obtained.(1)With the increase of pre-strain level,the roughness of the material surface grows more severe,and the number of convex peaks and concave valleys on the material surface increases significantly.Plastic deformation mainly occurs in ferrite.The martensites distributed around ferrite tend to cluster as the pre-strain level increases.(2)The average grain size of the material increases with the pre-strain level.During the stretching strain,the crystal texture of the material sees drastic changes and the proportion of(112)[1(?)0] orientated grains increases while(111)[(?)2]oriented grains decreases.After the material is stretched along the rolling direction,the <110>//RD texture was formed and the initial {111} orientation of grain plane is rotated to {112}.The dislocation density also significantly increases with strain level.The micro-hardness on the sample surface shows a continuously increasing trend.The hardening trends on the surface layer and the central layer are not consistent with the strong hardening behavior happening on the surface layer while the weak hardening behavior happening in the central layer.(3)The results from crystal plasticity finite element method show that the initial crystal orientation has a very important effect on the development of material surface coarsening during the plastic deformation.The surface coarsening of the model with initial texture is weaker than that of the model with random texture orientation.This indicates that the presence of initial texture can slow down the development of surface coarsening.The discordance of grain orientation results in deformation incompatibility and the change in surface morphology.The coarsening mainly occurs in the(111)[1(?)1] texture region while the(112)[1(?)0] texture region presents an overall depression.(4)Under the dry friction condition,the friction coefficient increases with the pre-strain level.Pre-strains have a small influence on the friction behavior of the material under the lubricating condition.The change in friction coefficient under low contact pressure is significantly different from that under high contact pressure as a higher contact pressure leads to a lower friction coefficient.