Investigation On Rolling Processing,Microstructure And Property Of Low Carbon Bainite Pipeline Steels

Viewing the point of petroleum and gas production and consumption structure and the western development strategy of China, an inevitable trend as long-distance, large diameter, high pressure, corrosion-resistant for petroleum and gas pipeline has developed. In recent years, the petroleum and gas industry of our country represented by West-East pipeline project has stepped into a new developing stage, and the X70and X80pipeline steels have been wildly used in the first and second lines of the project. However, the microstructures of those pipeline steels are so special that the property is easy to become instability in the production process, which causes low product rate. Additionally, lots of other aspects of those steels, such as crystal characteristics of micro structure evolution, mechanism of H2S corrosion resistance, micro mechanics mechanism of fracture and welding properties, still remain to be elucidated.The work of this dissertation was carried out integrating with projects of "Micromechanics study on D WTT fracture of high grade pipeline steels", which is supported by the Fundamental Research Funds for the Central Universities,"Application study of strain rate vector inner product in rolling work rate variation" and "Study of forming energy rate functional linear integral method and its application in metal forming", both are supported by National Natural Science Fund of China, and "Development of X65-X80pipeline steels for petroleum and gas transportation", which is supported by a Iron and Steel work in China. Microstructure evaluation, fracture behavior, corrosion resistance and welding properties of low carbon bainite pipeline steel were investigated. The main work and research results are involved as follows:(1) Transformation behavior by hot simulation experiments with different deformation parameters was researched. The continuous cooling transformation (CCT) diagrams were conducted by using thermal dilation method combined with microstructure observation. The results show that the increase of cooling rate depresses the bainite transformation start temperature and accelerate the progress of transformation. With0.7true strain, bainite transformation start temperature could be increased by30-80℃than that without deformation. Bainite lath appear as discontinuous linear distribution as cooling rate lower than5℃/s, and it becomes continuous and long strip shape as cooling rate larger than5℃/s. Bainite lath width decreases as increase of cooling rate, and this relationship can be expressed by an equation:d=(-0.00787+0.01937v)-1/4.09515, where d is bainite lath width, v is cooling rate. In the cooling rate range of1-20℃/s, the fraction of high angle grain boundary (HAGB) increases with increase of the cooling rate, while the fraction tends to decrease as cooling rate larger than20℃/s.(2) The main parameters for rolling low carbon bainite pipeline steel determined by multi pass thermal simulation were as follows:finishing temperature790～810℃; end cooling temperature500～580℃, cooling rate15～20℃/s. It was found that the microstructure of the steel thus rolled is mainly composed of fine and uniform acicular ferrite with high fraction of HAGB. The low carbon bainite pipeline steels were rolled via TMCP (thermo-mechanical control process), HTP (high temperature processing) and UFC (ultra fast cooling) in lab, of which all the mechanical properties came up to the standard API Spec5L and GB/T21237-2007. The results indicate that finishing temperature of HTP can raise20～40℃higher than TMCP, and that of UFC can elevate40～60℃. Thus, it’s very advantage to produce high grade pipeline steel via HTP and UFC to reduce mill load, which provides new ways for iron and steel factories with insufficient capacity mill to produce high grade pipeline steels.(3) X70and X80pipeline steels alloyed slabs of150mm thickness with0.1-0.25%molybdenum were rolled to be the plates in a factory in China, of which thickness less than18mm all the mechanical properties came up to the standard GB/T21237-2007and West-East pipeline project, while that thickness more than18mm, it’s difficult to meet the DWTT property requirement. Then, X70pipeline steels alloyed slabs of250mm thickness with0.1-0.2%molybdenum were rolled to be the plates of thickness more than18mm, and all the mechanical properties came up to the standards. From those results, it can be concluded that DWTT property of pipeline steel shows influence of compression ratio and composition effects. Well DWTT property is easily satisfied with compression ratio more than8.33and molybdenum content higher than0.1%. (4) H2S corrosion resistance test results revealed that cracks of hydrogen blistering on the samples surface were induced by inclusions, which were sulfide, calcium oxide and aluminosilicate determined by EDS. Those cracks gradually extended from the sample surface to the internal, after propagating and joining, finally formed step-like crack. The fracture mode was mostly transgranular cracking, and many calcium and aluminum oxides existed at the crack tip. Hydrogen induced crack growth rate is proportional to the square with K1/σs, that means the higher of material’s yield strength, the weaker of the resistance of hydrogen induced crack propagation.(5) Charpy impact experimental results show that the initial cracking load and the maximum impact load are slightly influenced by temperature, but with the decrease of temperature, crack forming, crack propagation and absorbed energies after crack arrested decrease significantly. DWTT results indicate that DWTT property is difficult to meet the requirement with central banded structure grade is larger than1.0, for the center of the sample is the location prone to brittle fracture. Coarse M/A constitution is extremely unfavorable for DWTT property, however, fine and dispersed M/A constitute is favorable for DWTT property. Coarse and irregular shape calcium aluminate and MnS inclusions is unfavorable for DWTT property, while fine and spheroidizing (Ca, Mn)S inclusion has little influence on DWTT property. Corresponding relationship between effective grain size and high angle grain boundary (HAGB) and DWTT property was detected by electron back scatter diffraction (EBSD) analysis. Shear area of DWTT sample increases as effective grain size decrease and the fraction of HAGB increase. Start fracture toughness of X70and X80pipeline steels determined by CTOD are0.205mm and0.230mm, respectively.(6) Analytical solution for equations of mode I fracture crack tip plastic zone (CTPZ) based on "MY criterion" are obtained. The equations are function of fracture toughness Kj and polar angle θ. The numerical simulated size of CTPZ determined by MY criterion is between those by Mises, the smallest, and Tresca, the largest yield criteria; while the zone is almost coincide with that by Mises’one. Critical radius is the same by MY, Mises’and Tresca’s criteria. The size of CTPZ is related to K1/σs. That size decrease with decreasing of K1/σs. For plane strain, the critical radius is independent of K1/σs, but for plane stress, with increasing of K1/σs, the critical fracture radius increase. (7) Single welding thermal-cycles simulation in heat-affected zone (HAZ) was investigated. The results indicate the shear area of fracture decreases as increase of peak temperature (PT) and input linear energy (ILE). When PT is1350℃and ILE are40and50kJ/cm, the shear area of fractures is equal to zero.-20℃Charpy v-notch impact energy decreases as increase of PT and ILE. And vickers hardness decreases with PT decrease and ILE increase. Charpy v-notch impact energy are larger than150J in all HAZs for the tested steel with ILE lowering than35kJ/cm. Heat input in single welding should be less than35kJ/cm to ensure good Charpy impact toughness. EBSD results show that the effective grain sizes of the steels with900℃-30kJ/cm and1350℃-30kJ/cm are measured to be4.0and43.1μm, respectively. The fraction of low angle grain boundary gradually increases as PT increase from900to1350℃. In the steel with PT of1200and1350℃, the fraction of low angle grain boundary is larger than that with PT of900and1050℃.