Study On Relationship Between Microstructure And Strain Hardening Index Of X70 Pipeline Steel

At present,the demand for deep-sea petroleum gas at home and abroad is increasing day by day.At the same time,the performance requirements of pipeline steel for deep-sea petroleum gas transportation are also increasing.As one of the pipeline steels for deep-sea oil and gas transportation,X70 needs to serve in a complex deep-sea environment.It requires pipeline steel to have good plasticity and toughness,high strength,corrosion resistance,impact resistance and other properties,therefore,we introduce the n value to study.The n value,also known as the strain hardening index,is an important mechanical property parameter calculated by the plastic stage of the stress-strain curve of the tensile test,which can reflect the ability of metal materials to resist uniform plastic deformation.The higher the n value,the smaller the deformation of the pipeline steel,and the more stable the performance of the pipeline steel.However,the research on the n value is common in thin plates,and there are few studies on the calculation and rationality of the n value of medium and thick plates.How to introduce the n value into the evaluation of medium and thick plates for deep-sea pipeline steel is of great significance.Therefore,in this thesis,the medium-thick plate for X70 deep-sea pipeline steel is taken as the research object,and the relationship model between microstructure and macroscopic material properties is constructed by combining experiment and simulation.The relationship between microstructure morphology distribution,volume fraction,grain size and n value is explored,and the failure mechanism of plate is predicted.In this thesis,some properties of X70 pipeline steel were tested,such as plasticity,toughness,strength,hardness,composition and microstructure.Then the relationship between macroscopic strain hardening exponent（n value）and microstructure of X70pipeline steel was studied by means of representative volume elements（RVE）finite element method,and the correctness of the simulation method was verified by experimental results.Finally,the relationship between microstructure and n value was explored by adjusting the parameters such as morphology distribution,volume fraction and grain size.The results found:1.X70 pipeline steel is composed of C,Si,Mn,P,S,Al,Cr,Ti,Nb and other chemical components.Through experiments,the yield strength of X70 pipeline steel is 515 MPa,the tensile strength is 594 MPa,the n value is 0.155,the hardness HV₁₀is197 HV,and the impact energy is 387/J.It has good strength and toughness and low temperature toughness.Its microstructure includes bainite,granular bainite,acicular ferrite,polygonal ferrite,M-A islands and other structures.2.Combined with the tensile test results,the correctness and feasibility of the simulation method are verified.By comparing and analyzing a variety of n-value calculation methods,the P_17.5/P_7.5two-point method is more accurate,simple and efficient.3.Under the ideal boundary conditions,the distribution of bainite structure has little effect on the n value;with the increase of bainite content,the macroscopic mechanical properties of the material are continuously improved and the n value is continuously reduced.4.The stress is mainly concentrated in bainite,and the plastic deformation is mainly concentrated in ferrite.During the tensile deformation process,the slender bainite and the two-phase interface are more likely to initiate cracks,and transgranular and intergranular fracture occur.