Interphase Precipitation Behavior In A X90 Pipeline Steel Under Different Process Conditions

With the depletion of oil and natural gas resources increasing,the difficulty of mining becomes bigger.The higher strength,toughness,corrosion resistance and cracking resistance and other good properties of pipeline steel are required urgently.Precipitation strengthening is one of the most important methods for enhancing the mechanical properties of pipeline steel besides fine-grain strengthening,and it is one of the developing directions of microalloyed steels.X90 pipeline steel is researched by different thermal simulation and heat treatment,which is to study the influence of interpass time,deformation temperature,deformation rate and keeping temperature at different time on microstructure and properties of X90 pipeline steel.The main results are summarized as follows:(1)The influence of different interpass time and different deformation temperature on microstructure and hardness of X90 pipeline steel is studied by thermal simulation.The results show that the hardness is high firstly and decrease subsequently with the interpass time and deformation temperature increasing.And the main reason is that the precipitates affect austenite recrystallization and refine the austenitic microstructure.When the deformation temperature reaches 800? and the interpass time keeps 30 s,the hardness is the highest and the microstructure is the best.(2)The effect of different strain rate and deformation time on microstructure and hardness of X90 pipeline steel is studied through thermal simulation.Results show that the hardness of the steel is increasing with the strain rate increasing,because of more lath bainite and M-A islands.The hardness is the highest when it is with in 2min after deformation,which is relevant to the amount and size of precipitates.(3)The steel is deformed and kept at different temperature through the thermal simulation experiment.The effect of deformation and keeping temperature on microstructure and precipitates of X90 pipeline steel is studied.The results show that the hardness of steel is higher after twice deformation than that of once deformation,which is mainly affected by the second phase particles precipitating and the content of bainites.(4)The steel is reheated at different temperature and time in the heat treatment experiment.The effect of precipitates and microstructure of pipeline steel is studied.The results show that the maximal product of tensile strength and total elongation(16675MPa%)was obtained when isothermal temperature was 650? for 30 min.This is attributed to the effect of precipitates phase,microstructure and grain size.