Study On Behavior And Mechanism Of The Localized Deformation In Medium Manganese TRIP Steel

Due to the excellent strength,elongation and work hardening ability,medium manganese transformation-induced plasticity（TRIP）steels are regarded as the promising lightweight material for automobiles,which has attracted strong interest from academics and industry in recent years.At present,the investigations in element composition design,heat treatment process,and microstructure optimization of medium manganese TRIP steel have made good progress.With the complexity of service conditions and the continuous improvement of structural reliability requirements,researchers began to focus on the issue of its strength failure.The researchers conducted preliminary exploration on the tensile mechanical properties and fracture damage mechanism of medium manganese TRIP steel,and obtained some valuable research results.However,there are still some difficulties in the study of localized deformation characteristics and failure mechanism.Especially for important issues such as the Lüders deformation mechanism and the damage analysis at the tip of fatigue crack.The Lüders localized deformation usually causes high local strain in the tensile deformation of medium manganese TRIP steel,which bring out surface roughening effect during the forming or service process of the material,and resulting in the decline of its mechanical properties.Therefore,this dissertation takes the Lüders localized deformation of medium manganese TRIP steel as the research object,and combined with advanced experimental characterization methods to reveal the micromechanical mechanism of localized deformation.This study has finished some investigation on the micro-scale strain distribution characteristics of Lüders localized deformation,the micromechanical mechanism of Lüders localized deformation,and the effect of microstructure on localized deformation behavior.At the same time,the fatigue mechanical properties and the localized deformation characteristics at crack tip are also studied in this paper.Besides,the important influence of martensitic transformation at the crack tip on the fatigue crack propagation is explored.The following studies are carried out in this dissertation:（1）The Lüders localized deformation characteristics at the micro-scale are investigated.A digital image correlation method based on SEM images is developed,which enables full-field strain measurements at submicron scales based on lithographic nano-speckles.This method is used to analyze the strain distribution characteristics of Lüders localized deformation at the grain scale.Meawhile,the relationship between the Lüders strain and icrostructure are explored by means of EBSD,TEM and other characterizations.It is found that the Lüders strain of medium manganese TRIP steel exhibits a strong non-uniform distribution within and between grains.This distribution characteristics of micro-scale Lüders strain have weak relationship with grain size and crystal orientation,and the distribution of mobile dislocation density inside the grain is the main reason for the distribution of strain.（2）The mechanism of Lüders band formation in medium manganese TRIP steel is studied.The specimen is tensile in stress control mode,and the formation moment of Lüders band is captured by high-speed DIC method.This dissertation studies the micromechanical mechanism of the formation of Lüders bands through Lüders-strain-rate and other macroscopic mechanical parameters.The experimental results show that the Lüders strain rate of medium manganese TRIP steel is as high as 1s^-1,which is three orders of magnitude higher than the average macroscopic strain rate and about 20 times higher than that obtained in a low-carbon steel.In order to find out the formation mechanism of the Lüders band of medium manganese TRIP steel,the res earch is carried out combining theoretical analysis and TEM experimental observation.It is found that the dislocation pinning theory cannot explain the formation of the Lüders band in medium manganese TRIP steel,and the dislocation multiplication mechanism is the main factor leading to the extremely high strain rate of the Lüders band.Furthermore,the influence of martensitic transformation effect on the dislocation multiplication mechanism is investigated under high temperature conditions.（3）Quantitative analysis of dislocation multiplication mechanism during Lüders band formation is conducted.The initial dislocation density and dislocation structure of the sample are changed by the warm-rolling heat treatment process,and the macroscopic mechanical parameters of the Lüders deformation of the sample are obtained based on the high-speed DIC method.The experimental results show that the Lüders strain,Lüders-strain-rate and stress drop have obvious quantitative dependence on the dislocation structure.Higher dislocation multiplication ability usually leads to higher Lüders strain,Lüders-strain-rate and stress drop.Through the analysis of the multiplication process,it is found that the stress drop acts as a driving force to accelerate the movement of movable dislocations,which can increase the Lüders strain and Lüders-strain-rate.In addition,based on the evolution results of the axial strain field,this dissertation analyzes the coexistence of uniform plastic deformation and Lüders localized deformation during the formation stage of the Lüders band,and the competition between the Taylor strain hardening mechanism and the dislocation proliferation mechanism is revealed.（4）The localized deformation at the crack tip during the fatigue crack propagation of medium manganese TRIP steel and the influence of martensitic transformation effect on the fatigue crack propagation are studied.By adjusting the heat treatment process,two materials with different volume fraction and stability of metastable austenite are obtained.A series of important issue such as the fatigue crack growth rate,phase transition zone and plastic zone at the crack tip,crack growth mode,the interaction between crack tip and microstructure have been systematically studied.The experimental results show that in the stage of short fatigue crack of medium manganese TRIP steel,the transformation martensite at the crack tip is retard the propagation of fatigue crack due to its higher strength and hardness.In the long crack stage,due to its relatively brittle properties,the transformed martensite is prone to fracture under the load of local stress,and more secondary formations are formed inside grain or at the grain boundary,thereby accelerating the growth of fatigue cracks.