The Research On Heat Treatment Of TWIP Steel And Microstructure Change During Stretching

Twining induced plasticity(TWIP)steel is a new type of steel with high strength and excellent ductility,which has broad application prospects in the automobile.However,the study of TWIP steel has just started,and there are many problems to be solved in the annealing process.In the present thesis,the Fe-16Mn-1.4C-1.2Al TWIP steel used to analyze the effects of annealing process on mechanical properties and microstructure,the influences of tensile rate on the curve of mechanical properties,and the changes of microstructure during stretching.The optimum process parameters of hardness and product of strength and plastic are obtained respectively through using optimized analysis methods,which can provides reference for the application of TWIP steel.Firstly,using Design-Expert software to design Box-Behnken(BBD)program with the factor of heating temperature,holding time and cooling method.Response surface model of hardness is constructed,which used to analyze the effect of annealing process factors on the surface hardness and optimize the annealing process parameters.The results show the model of hardness can good fit the test data and effectively predict within the design range.It predicted that the maximum hardness of 255.36 HV can be obtained with heating temperature of 600 ?,holding time of 5 min and cooling method of water cooling.With increasing holding time,the Vickers hardness decreases.It is largely attributed to the grain size increases and the grain boundary area decreases which reduces the ability to resisting plastic deformation by analyzing the metallographic structure.Secondly,the influence of the annealing process factors on the product of strength and plastic is analyzed based on response surface analysis.Appropriate factors and levels are selected,and a regression model is constructed to fit the relationship between process factors and product of strength and plastic.Based on the model of product of strength and plastic,the significance of each factor is evaluated,the influence rules are analyzed,the optimization of process parameters are realized and the product of strength and plastic is able to predict.The results show the model can good fit the test data and effectively predict within the design range.The optimal product of strength and plastic is 59919.87 MPa%when the heating temperature is 759 ?,the holding time is 21 min and the cooling method is furnace cooling.The deviation rate was 1.6 by experiment.For product of strength and plastic,the most significant factor in the single factor is the holding time and in the interaction terms is the interaction between temperature and time.Finally,the effects of tensile rate(6.67×10-4 s-1,6.67×10-3 s-1,and 6.67×10-2 s-1)on the strength,ductility,and work-hardening curves are studied,as well as the regularity of microstructure changes during stretching.The results show that the influence of strain rate begin to increase in the plastic deformation stage,and with the decrease of strain rate,the tensile strength and the elongation showed an upward trend,and the tensile strength reached 870 MPa and the elongation rate reached 72%.It is found that the n-value curve of the test steel is a waveform curve with continuously increasing amplitude,and the waveform instability state occurs when it is far from the breaking point.Using electron back scatter diffraction technique to observe the microstructure,it is found that the grains will gradually change their orientation and turn toward the favorable orientations<111>and<001>,during the deformation process.The larger the misorientation between adjacent grains,the greater the driving force for the rotation,which lead to big misorientation decreasing and small misorientation increasing after stretching.