Effect Of Rare Earth On The Tensile Properties And Microstructure Of X80Pipeline Steel

The solubilityof Rare earth in the steel is extremely low, when the quantity of rare earth insteel changes, micro structure and performance of steel will have significant change. At thesame time, the quantity of rare earth elements dissolved in steel affects the dissolution andprecipitation of micro element as well as the recrystallization process, as a result, the mechanicalpropertyof steel varies.This article is based on X80pipeline steel and the influence of rare earth on microstructure,tensile properties and micro orientation are studied systematically. Structure variation inthickness direction under tension is studied with optical microscope (OM) and scanning electronmicroscope. Ansys simulation software is used to describing the influence of rare earth intension and compared with the experiments results. With the help of electron backscattereddiffraction (EBSD), the macroscopic texture of fracture under the condition of tension isanalyzed. The relationship between fracture microstructure, macro texture and microorientations and high grade pipeline steel mechanical properties is obtained which provide athoretical and experimental basis for engineering application.Microstructure observation shows that the microstructure of X80pipeline steel underrolling state changes from axial ferrite transforms to small uniformly distributed flake bainitestructure and the structure is obviously refined. The Image-Pro plus statistical results show thatthe grains refine from13.26μm to12.94μm. Combining the results of tension experiments,conclusion can be drown that the mechanical property gets a notable improvement with theadding of rare earth elements, the tensile strength gets734MPa which is13%higher than thesteel without rare earth elements. The simulations show that the distance that equivalent stressdecline gently becomes larger form2.31mm to2.45mm. As the increase of this distance,equivalent stress declines and near the center of fracture, the largest equivalent stains are0.834263、1.02744.The observation of fracture shows that crack extension edge becomes serrated with rareearth elements, path change significantly. Fracture analysis shows that the addition of rare earth can improve the fracture morphology of the pipeline steel, the nests become more uniform.Simulated measurement analysis of the nests shows that the nests size of the fracture with rareearth is0.453μm, less than0.836μm of without rare earths. After rolling, steel precipitatedphase of Nb(C, N), TiN, Fe3N,(TiV)(C, N) is given priority to, and after adding rare earthelements Nb (C, N) strong peak appeared.EBSD micro region analysis shows that after adding rare earth,{112}<110> textureincrease form2.61%to23.8%, this is good for the improvement of X80pipeline steeltoughness. Pipeline steel after adding rare earth within small Angle grain boundary increases,when the crack extension block effect; At the same time, low energy heavy lattice quantityincreases,which can reduce or avoid cracks; In addition, adding rare earth make effective grainsize of pipeline steel by5.097μm down to3.0053μm microns.