Experimental Study On Mechanical Behavior And Damage Mechanism Of Banded Microstructure In Microalloyed Steel

The mechanical properties and damage failure of microalloyed steel are the basic scientific issues,which have attracted much attention in various fields such as pressure vessels,ships,automobiles,oil and gas pipelines,etc.Meanwhile,it is a multi-scale and complex proposition combining material science and damage mechanics.The banded structure is an inevitable mircostructure in alloy steel,and its formation is related to composition segregation,improper thermal processing,slow cooling rate after rolling,and non-uniform composition.The banded structure increases anisotropy in mechanical properties,which also affects the deformation process and fracture behavior of the material.Considering the problems of mechanical properties,interface strength and local deformation characteristics of adjacent bands,traditional mechanical testing methods are difficult to understand the deformation damage mechanism of the different banded structure.In this study,the experimental research on the mechanical behavior and damage mechanism of banded structure in microalloyed steel was carried out.The characteristics of the mechanical field of different banding regions were analyzed.Meanwhile,the relationship between the banded structure and the mechanical response was revealed,which provides theoretical support for the regulation and evaluation of the microstructure of the microalloyed steel.It is also the basis for the analysis of structural damage failure of materials containing banding.Firstly,the mechanical behavior of the coarse banded structure consisting of polygonal ferrite and hard banding containing the hard second phases is studied on the macroscopic scale.The evolution law of the full-field deformation of the banded structure under the uniaxial tension condition was obtained in the rolling direction(RD)and the transverse direction(TD).The results show that the damage in RD tensile is diffusely distributed in mircrostucture,and the hard banding shows obvious hindrance to the deformation and expansion in the soft banding.Meanwhile,the TD exhibits a higher sensitivity to the local deformation.According to the intrinsic correlation between banding direction and local deformation,the mechanical behavior differences of 0°,45°and 90°under the same content of banding were quantitatively studied.Based on the statistical analysis of DIC full-field deformation measurement and experimental data,the parameters describing the degree of deformation strengthening K_f and the strengthening rate I_f were proposed and used to describe the inhomogenious and anisotropic behavior caused by the banded structure.The experimental results show that the deformation strengthening shows a nonlinear growth after entering the yielding stage.The strengthening rate of different banding directions is different.The degree of deformation in the 45°direction under the same stress is the highest,and the strengthening rate of the material is stable when K_f reaches 0.5.In addition,the effects of soft and hard banding content and banding width on flow stress under tensile load were analyzed.The results show that the flow stress increases with the increase of the hard banding content and decreases with the increase of the banding width.Secondly,the mechanical behavior of the banded structure formed by different acicular ferrite grains was studied on the mesoscale,and the local deformation characteristics of the banded structure under RD and TD tensile were obtained.The results show that both of the coarse and fine acicular ferrite bands have a certain degree of yield behavior at the beginning of the plastic deformation and exhibit shear banding.The fine-grained banding is gradually strengthened with the increase of deformation,and the strain localization behavior corresponding to the coarse-grain banding is observed under TD tensile condition.Meanwhile,the full-field strain distribution of the0°,45°and 90°direction of the coarse/fine grain banding under continuous deformation is given.The results show that K_f of the 45°is still the largest under the same stress.The strengthening rate of the material tends to be stable when K_f reaches 0.4,which is smaller than the banded structure consisting of polygonal ferrite and hard banding mentioned above.These indicate a higher strengthening rate and a larger deformation resistance.The difference in grain size still affects the damage evolution and anisotropy,and the degree of influence is directly related to the difference in mechanical properties of the constituent bands.Thirdly,considering the texture characteristics in the original structure of pipeline steel,the SEM-DIC method and Electron Backscattered Diffraction technique were combined to study the acicular ferrite/granular bainite(AF/GB)banded structure at the microscopic scale.The damage failure and grain orientation under different loads were obtained and analyzed.The results show that the ductility of GB is poor and the mechanical properties of banding interface are weaker.The larger grain size and single orientation of the GB reduces the resistance to deformation damage and increases the rate of damage propagation at higher stress intensity levels.The coarse GB should be avoided as much as possible in the production of materials.Finally,the residual stress of microstructure in different banding was measured based on the ring-core method using FIB-DIC technology.The natural speckle model was designed,and the measurement accuracy was analyzed.The elastic modulus of the soft/hard phase was measured and analyzed by nanoindentation technique.The results show that the distribution of carbide in the material could cause a difference in hardness,but has little effect on the elastic modulus.Meanwhile,the residual stress of the microstructure in the hard banding is significantly larger than that of the soft banding.The banding inhomogeneity caused by component segregation will affect the distribution of micro-scale residual stress.Large residual compressive stress could reduce the rate of fatigue crack growth in the hard banding and enhance the fatigue properties of the material.