Investigation On The Continuous Cooling Transformation Of The High Strength Pipeline Steel

The need for oil product has become the bottle-neck of development. It's necessary tosearch and exploder nature gas from outlying area. How to thansport the new exploderednature gas to the east area where is developed and populous is an emergency problem.Therefor, the high properties pipeline steel is the key of the problem.Now, the aim to produce high properties pipeline steel is to get lower bainite. Theexplodering of high strength pipeline steel is through optimizing the microstructure ofpipeline steel to get the lower bainite, so optimizing the chemical composition of pipelinesteel and the process of heat treatment is the key.The continuous cooling tranformation curve was measured by Formast-F thermalexpansion instrument and the microstructure was observed by optical microscopy, then therole of the molybdenum and boron was investigated in the continuous cooling process. Theprecipitated rule of the element Niobium, Vanadium and Titanium was studied by TEMand SEM. Effect of molybdenum, boron and microalloy element on the properties of thetested steel was analysed by microhardness, then theoretical foundation to developing theultra high strength pipeline steel called "X120" was laid.As a result, formation of proeutectoid ferrite and granular bainite was repressed byadding molybdenum and the range of cooling rate for forming lower bainite was enlarged;the grain size was refined most effectively when adding 0.3ï¼…Mo and the optimal valuemicrohardness was achieved. The Mo-B coaction was good for improving hardenabilityand mechanical property. The Mo-B coaction had greater effect than sum of molybdenumand boron. The tested steel which contained 0.3ï¼…Mo and 15ppm B was optimal. Theprecipitation size was associated with the content of molybdenum. The precipitation size oftested steel without molybdenum was 100 nm; that of tested steel with 0.3ï¼…Mo was 40 nm;that of tested steel with 0.5ï¼…Mo was 70 nm.