Study On Fracture Mechanism And Fatigue Properties Of A Medium Manganese Steel

The combination of high strength and high toughness make the third generation of automotive steel has great potential for application in the automotive industry.Medium manganese steel uses TRIP effect to improve its mechanical properties,a major microstructure of TRIP steel is a small amount of retained austenite and a large amount of ferrite.This metastable austenite will induce martensitic transformation during plastic deformation,increase the work hardening rate and improve its formability.In this paper,the following research is carried out around medium manganese steel:（1）Fracture mechanism of medium manganese steel:The fracture theory is introduced and the fracture toughness of medium manganese steel is characterized by several methods.The quasi-static tensile test of medium manganese steel at high temperature and normal temperature were carried out by in-situ mechanical test system.The crack propagation behavior of medium manganese steel during tensile process was observed.Martensitic transformation occurs during tensile process and a large number of secondary cracks are thus produced.Fracture morphology of the specimens after fracture was analyzed.Judgment from a micro perspective,the fracture mechanism of the specimens was a mixed mode of micro-void coalescence ductile fracture and quasi-cleavage brittle fracture.The real-time change of crack tip of medium manganese steel during quasi-static tension is recorded by SEM,and the fracture properties of medium manganese steel at room temperature is preliminarily characterized by CTOA.DIC is combined with EWF to determine the essential fracture work strength of me dium manganese steel at normal temperature and high temperature.The fracture property is better at high temperatures due to the softening of the material and the absence of martensite phase formed by phase transformation.（2）Fatigue property of medium manganese steel:In-situ mechanical testing system was used to test the fatigue crack growth of medium manganese steel at high temperature and normal temperature.The real-time fatigue crack growth behavior of medium manganese steel at different temperature was observed,and the fatigue crack growth rate of medium manganese steel at different temperatures was measured.Experiments show that crack branching and sub-crack propagation occur in medium manganese steel under fatigue load.Since the martensitic transformation occurs at normal temperature,the yield strength of the medium manganese steel is increased,and the residual compressive stress generated by the martensitic transformation counteracts the partial energy required for crack propagation,the fatigue crack growth rate at room temperature is lower than that at high temperature,and theΔK_th of fatigue crack growth rate at room temperature is larger.With the increase of stress intensity factor,the influence of residual compressive stress introduced by martensitic transformation on fatigue performance is gradually weakened.