Study On Strength And Toughness Optimization Of X70 And X90 Pipeline Steels

Through the product development and design optimization of X70 and X90,it is recognized in this paper that the following two points will be helpful in the development and optimization of higher strength pipeline steel:the grain boundary strengthening effect of niobium and other alloying elements should be fully exploited;grain boundary strengthening,intragranular second phase strengthening and multiphase composition of microstructure are all considered.The goal of this paper is to understand the mechanism of key alloying elements,give full play to the role of each alloying element and use alloying elements most efficiently.Simultaneous technological and economic advances will be very helpful in promoting earlier practical application of higher strength grades of pipeline steel.In this paper,X70M pipeline steels with a maximum thickness of 20.6 mm is developed step by step,X70 pipeline steel from 9.5mm to 20.6mm thickness specification was designed so that the chemical composition remained essentially unchanged as the thickness gradually increased,however,the strength properties of the coils and tubes were increased or maintained at similar levels as much as possible.The chemical composition of 1 7.5mm thickness X70 is 0.060%C,1.55%Mn and 0.065%Nb,the average yield strength and tensile strength of 17.5mm X70 are 535 MPa and 663 MPa respectively,and the average Charpy V shape impact absorbing energy at minus 60? is greater than 350 J.Because of its simple composition,excellent strength and toughness,X70 has become a very good analysis object.Compared with higher strength pipeline steel containing Cr,Mo,Ni,Cu and V,it is more convenient to find out the most effective factors in chemical composition,microstructure,rolling and cooling process.It is helpful to the development and optimization of X90.The above-mentioned reduced design approach was practiced in the development of X90.Increasing the content of alloying elements such as chromium,molybdenum,nickel,copper to develop X90 can effectively improve the strength of the material,but also make the material less tough.By comparing X80 and X90 of the same thickness specification,it was found that the main reason for the lower toughness of X90 was the occurrence of austenite sub-dynamic recrystallization during rolling deformation,which reduced the actual deformation and increased the degree of microstructure inhomogeneity.In the process of subsequent product development,by reducing the chromium,molybdenum,nickel,copper content of X90,optimizing the design of the rolling procedure to avoid sub-dynamic recrystallization,and combining the post-rolling cooling process with rapid cooling and low-temperature coiling,the uniformity of microstructure was improved and the microstructure was refined,and a high toughness X90 plate and coil was developed.The microstructure characteristics of X70,X80 and X90 are systematically studied.The microstructure types of X70,X80 and X90 are similar,the grain size is effectively refined by the partitioning effect of acicular ferrite with different orientations and the restriction effect of the dislocation-rich cellular structure.The differences are in the relative proportions of acicular ferrite and quasi-polygonal ferrite content,grain size,dislocation density,etc.There are not only TiN and Nb(C,N)complex precipitates with different sizes of 10-100 nm,but also elliptical precipitates with sizes of 10 nm and below.The elliptical precipitate with a size of about 5nm observed in X70 steel is confirmed to be niobium carbide.The segregation of niobium,carbon and phosphorus at grain boundary was found in X70 pipeline steel by three-dimensional atom probe tests.The segregation degree of niobium is the largest,followed by carbon and phosphorus.The maximum atomic percentage of niobium at the grain boundary is 0.29 and 0.47 at.%,which is 7.5-11.9 times higher than that in the matrix(0.039 at.%).The results of electron energy loss spectrum show that the charges in the occupancy of 3d state for grain boundary iron is higher than that in the grain,which indicates that the grain boundary cohesion is enhanced.The First Principle calculation suggests that the 3d valence electronic density of the state of Fe in grain boundary moves to a lower energy,which can reduce the total energy of the system and make the grain boundary more stable.Meanwhile,the charges in the occupancy of the 3d state for Fe in the grain boundary increases,providing more electrons for grain boundary bonding.These improve the strength and toughness of the material.