Research And Development Of X80 Pipeline Steel Seamless Pipe Based On Oxide Metallurgy

Under the conditions of high transportation pressure and extreme cold environment,highgrade seamless pipeline has become one of the development directions of pipeline steel,and high-grade seamless pipeline in China needs to be developed urgently.In view of the research and development status of domestic high-grade seamless pipes,this paper adopts an oxide metallurgy process that introduces fine inclusions into steel to develop X80-grade pipeline steel seamless pipes with high toughness and resistance to hydrogen sulfide corrosion.The main research work and results of this paper are as follows:(1)Based on the performance requirements and application characteristics of X80 grade seamless pipe,based on the composition requirements of API Spec 5L,combined with the influence of oxide metallurgy process on the microstructure and properties of pipeline steel,summarized the previous work experience.The chemical constituents of X80 seamless pipe with two composition systems were formulated,which were divided into X80-Mo and X80-Cr.(2)The continuous cooling transformation behavior of tested steel in two kinds of composition systems was studied,the critical transition temperature was determined,the influence of cooling process on the structure was studied,and the static CCT curve was drawn.With the increase of cooling rate,the microstructure undergoes a transition from F and P to B and eventually to M.Since Cr can significantly reduce the diffusion of carbon in austenite and slow down the formation of austenite,the austenite phase region of X80-Cr steel is slightly narrower than the phase region of X80-Mo.(3)By studying the variation of microstructure and properties of tested steel under different Q-T processes,a reasonable Q-T process was determined.The off-line quenching temperature was 920℃,and the tempering temperatures of X80-Mo and X80-Cr steels were 600℃ and 630℃ respectively.The tested steels are tempered sorbite structure,and the impact and tensile properties are improved due to the dispersion strengthening of micro-inclusions.The fracture surfaces are ductile fracture.The inclusions at the bottom of dimples are all composite inclusions of TiO2-ZrO2-MgO-MnS.(4)The welding thermal simulation experiments under different heat input rates were carried out.The results show that the HAZ impact energy of the tested steel is higher than 30 J at-40℃,and a certain amount of AF appears.The impact energy of X80-Mo steel is slightly higher than that of X80-Cr steel,and the type of inclusions promoting AF nucleation is TiO2Al2O3-CaS-ZrO2-MnS composite inclusion.(5)HIC sensitivity test and hydrogen permeation test were carried out on tested steel.The results show that,due to the oxide metallurgy technology makes the inclusions diffuse and refine,avoiding the formation of oxide clusters and long strips of MnS.Among them,the formation of TiO2-Al2O3-CaS-ZrO2-MnS composite inclusions with weak hydrogen atom adsorption,the better control of the mass fraction of P,S,Mn,Ni and other elements in steel,and the Cr and Cu which are beneficial to the improvement of corrosion resistance.Reasonable addition of elements,as well as tempered sorbite with good resistance to H2S corrosion after tempering,make the test steel under both component systems have good resistance to hydrogen sulfide corrosion.