Processing, Microstructure And Property Of The X100 High Grade Pipeline Steel

With the development of the industry of oil and natural gas, the pipeline steel for transporting oil and natural gas has been making rapid development. In China, it has been proved that pipeline steel is the best economic and effective tool for transporting oil and gas. The country economic makes great progress and expenditure of oil and natural gas expends continuously. In order to improve the benefits of transportation efficiency and to meet the requirement of the continually increasing of the delivery pressure, the properties of pipeline steel must be upgraded.This mainly topic is the fundamental technique researching of X100 pipelinem steel. The main contents are given as follows:(1) Based on the reasonable composition design and microstructure study, we successfully develop a new X100 pipeline steel, contain element Mn and Ni as well as tiny amount of element Nb and Ti. The nominal composition is composed with: C-0.06, Mn-1.28, Si-0.25, Nb-0.053, Mo-0.053, Ni-1.30 and Ti-0.012, and the properties of the steel can reach >695 MPa, >768 MPa, δ>16.6%, CNV>262J.(2) By using Gleeble-3500 thermo-mechanical simulator, the parameters of the thermo-mechanical control process(TMCP), including the heating temperature, the final rolling temperature, the final cooling temperature and the cooling rate, are tested by the orthogonal experiment with four factors and three levels. The obtained optimum parameters are used in the rolling experiment conducted in the rolling line. The microstructure is characterized by optical microscope and electronic microscope and the mechanical properties of the steel are studied. It is shown that, the microstructure of the specimen rolled under the optimum parameters mainly consists of the granular bainite(GB), acicular ferrite(AF)、lower bainite(LB) and M/A when it is heated at 1180℃, rolled at 810℃, andfinally cooled at 400℃ with the rate of 35℃/s.(3) The influence of different finish cooling temperatures(250℃, 350℃,450℃)on microstructure and mechanical properties of the steel are studied by pilot line experiment and orthogonal experiment, the comprehensive performance of the new designed X100 pipeline steel is evelated.(4) Based on the quantitative research on pipeline steel, the precipitates in steel are almost composed of complex precipitates with irregular geometery. The main precipitates are less than 50 nm. According to their components and geometery, these carbonitride precipitates can be divided into two types, one is mainly composed of Nb carbonitrides and a tiny amount of Ti and V carbonitredes with a round or oval geometery, the other is mainly composed of Ti carbonitrides and a small amount of Nb and V carbonitrides with a polygon structure. Meanwhile, these precipitates with a large size(50nm~100nm) also have two types, one is mainly composed of Ti carbonitride with a square shape and the other is mainly composed of Nb carbonitride with a long strip.(5) The behavior of anti-hydrogen induced cracking of X100 pipeline steel in different solution is investigated by using NACE TM 0284-2003 standards. The results show that the pipeline steel in different solution has good performance of anti-hydrogen induced cracking, there is no cracking found inside the steel, micro-cracking on the specimen surface mainly nucleates and expands at the pitting corrosion pits, but the expansion dismisses in granular bainite.