| As the developing of our national economy and change of energy structure, the need of the pipeline steel for transporting oil and natural gas is uprising. At the same time, the requirement of pipeline steel properties is improving as well. X80 pipeline steel is a new type of high strength pipeline steel which is to be satisfied for the demand of high toughening and economy of transportation. But the original microstructure had changed when the pipe fitting were reheating, refining and precipitation strengthening were decreased, which decreased the properties of the pipe fitting. In order to resolve this problem, X80 pipeline steel which was treated by many times heating were performed in this paper. In order to raise properties of the pipe fitting, the regulation of effect of heat treatment process on microstructure and properties of X80 steel were systematic researched. On this basis, the optimal heat treatment scheme was determined.The heat treatment experiment for X80 steel is carried out in this paper. The change of microstructure of the steel which was treated by heat treatment were observed by OM,SEM and TEM. At the same time, the change of mechanical property of the steel were analyzed by tensile test,impact test and hardness test. The effect of different process conditions on microstructure and properties of the steel was analyzed by the synthesis of the results of the test. The optimum heat treatment parameter is proposed for X80 steel.And the research results were as follows.Microstructure obtained when X80 steel was treated by many times heating was mainly granular bainite. Matrix microstructure is mostly massive ferrite which has big size of effective grain and a little lath-like ferrite. M/A island and micro-alloyed carbon nitrogen compound were distributed in the matrix. Twinned martensite is in M/A island. The strength and toughness were all low because of the microstructure in the X80 steel.The tension strength and the yield strength increased with the increase of the quenching temperature. The yield strength could reach the API standard when the quenching temperature is 1010℃. The toughness have notably improvement when the steel was treated by heat treatment. The toughness decreased with the increase of the quenching temperature. The toughness decreased rapidly when the quenching temperature is 1100℃. X80 steel bring excellent strength and toughness when the quenching temperature ranges is 1010℃～1070℃.Microstructure obtained when X80 steel was treated by water quenching was granular ferrite and lath-like ferrite, which ensure excellent strength and toughness of the steel. So water is the best quenching medium.Tempering temperature had less effect on the tensile strength. The yield strength decreased with the increase of the tempering temperature. Impact value increased slightly with the increase of the tempering temperature. The change of the toughness became slower when the tempering temperature is 550℃. In a word, the change of tempering temperature had less effect on the strength and toughness. It explained that X80 steel had good tempering stability. X80 steel bring excellent strength and toughness when the tempering temperature ranges is 550℃～600℃.The optimum heat treatment process scheme is that the quenching temperature ranges is 1010℃～1070℃followed by water quenching and the tempering temperature ranges is 550℃～600℃.
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