Development Of X65 Pipeline Steel With HIC Resistance

In recent years,with the help of West-east gas transmission project,the construction of oil and gas pipelines in China has entered a new period of rapid development.Now,the pipeline steel with high strength can be produced in China.But there is not too much in-depth research on high strength pipeline steel with excellent H2S corrosion resistance.Studying the corrosion mechanism and various influencing factors of H2Scorrosion,and development of pipeline steel with HIC resistance and independent intellectual property rights is a necessary condition for ensuring national economic development and energy security.In this paper,the effect of steelmaking,continuous casting and rolling process on microstructure and properties of X65 pipeline steel with HIC(Hydrogen-Induced Cracking)resistance.The plate has excellent mechanical properties and HIC resistance.The main contents and results of this paper are as follows:The law of phase change of X65 pipeline steel with HIC resistance was studied by Gleeble 3800 hot-simulation testing machine.The continuous cooling transformation curves were drawn and cooling rates affecting on microstructure of the steel were studied.The results show that the microstructure of the steel was different at different cooling rate.When the cooling rate was 0.5 ?/s,the microstructure is polygonal ferrite(PF)and pearlite(P).When the cooling rate was 2?5?/s,the microstructure is polygonal ferrite,pearlite and acicular ferrite(AF).With the increase of cooling rate,the ratio of PF is decreased and the ratio of AF is increased.When the cooling rate is more than 10?/s,the microstructure is acicular ferrite and granular bainite(GB).It is better when the cooling rate is 15-25?/s.Because the transformation point in this range is not sensitive to cooling rate and the stable acicular ferrite and granular bainite micro structure can be obtained.According to the results of laboratory research,the parameters of controlled rolling and controlled cooling in industrial production are proposed.The HIC resistance properties and corrosion mechanism of the commercial conventional X65 pipeline steel was studied.It is indicated that the main reason for the failure of HIC resistance is the center porosity onslab and the pearlite banded micro structure on steel plate,which needs to be controlled in production.The total oxygen content,inclusion composition,nitrogen content,slab inclusion,slag composition,slab porosity and segregation were studied.Then the main control points of steelmaking and continuous casting process in industrial trial production were clarified.When the final rolling temperature is 840? and the final cooling temperature is 450?500?,the microstructure of the steel is ferrite and fine low carbon bainite.The plate has excellent mechanical properties and HIC resistance.