Study On Microstructural Evolution And Mechanical Properties Of Economical X70 Grade Pipeline Steel

In the next few years,the world’s energy consumption will continuously increase,crude oil and natural gas will account for two-thirds of total energy demand.The pipeline is the most convenient,economic,environmental and safe way to transport crude oil,refined oil,natural gas and other materials over a long distance.Its high-speed construction will guarantee that the oil and gas will be steadily supplied to market.The long-distance pipeline transportation need go through complicated geographical environment,and the pipeline is susceptible to temperature,humidity and other factors.Therefore,the pipeline steel develops toward high strength and toughness,high corrosion resistance.X70,as high-grade pipeline steel,its application is very broad.The traditional X70 pipeline steel need add the expensive alloying element Mo,as a result,the production costs of pipeline steel increased remarkably.In order to reduce resource dependence,lower energy consumption,this paper aims to design X70 grade pipeline steel by means of "reduction of composition"+"high temperature rolling "+"ultra fast cooling",while not adding Mo elements,by using NG-TMCP technology,to achieve reducing production of X70 pipeline steel,it has good application prospects.In this context,the main contents as well as the research results of this paper are as follows:(1)The dynamic recrystallization behavior of experimental steels was studied by single-pass and multi-pass compression experiments with thermal simulation machine.The results showed that the higher deformation temperature is conducive to austenite dynamic recrystallization behavior.In addition,non-recrystallization temperature will be lower along with the increasing of pass interval time.(2)By applying thermal expansion method,the phase transformation during continuous cooling of experimental steel was studied.Draw static and dynamic CCT curve of experimental steel.The results show that the austenite after deformation has higher phase transition temperature,and is more likely to produce high temperature transformation products.Whether deformation or not,with the cooling rate increases,the grain gradually refined.The austenite after deformation is cooled at a high cooling rate,the hardening state of austenite will be retained,which will play the role of grain refinement.(3)The effect of process parameters on the microstructure of the experimental steel was studied by thermal simulation experiment.The results as follows:①With the increase of the deformation,the high temperature transformation products more easily formed;②With the increase of the cooling rate,the low temperature transformation products more likely formed and the grains become finer;③With the decrease of the final cooling temperature after ultra fast cooling in the two-stage cooling condition,the high temperature transformation products gradually decreased and the low temperature transformation products increased;④After ultra fast cooling,when coiling temperature at 700℃,a large amount of polygonal ferrite formed,and polygonal ferrite no longer formed when coiling below 600℃;⑤When coiling at 500℃,The coiling cooling rate has no obvious effect on the microstructure of the experimental steel.In addition,effects of different holding time on the phase transformation and precipitation behavior of ferrite region(600℃)were investigated by the thermal simulation experiments..The results show that the percentage of ferrite increases significantly with the increase of holding time,and the change of microstructure is not obvious when the holding time exceeds 1000s.The number of precipitated particles increases and the size grows slightly.(4)Hot rolling experiments at laboratory were carried out to study the effects of different cooling rate and final cooling temperature on the microstructure and properties of experimental steel.The results show that when the final cooing temperature is the similar,compared with laminar cooling,the ultra fast cooling process can significantly increase the strength of the experimental steel;When the cooling rate is constant,the strength of the experimental steel increases obviously with the decrease of the final cooling temperature.By the application of the NG-TMCP technology,the strength and toughness of X70 pipeline steel can be significantly improved.