| The development of offshore platform steel with high heat input weldability is the trend of China's offshore engineering equipment development and marine market needs, this work had summarised the research on Coarse Grain Heat Affected Zone(CGHAZ) of offshore platform microallyed steel in both China and overseas, and based on the previous work, a comparative study was conducted on the CGHAZ of E36 grade V-N-Ti and Nb-V-Ti microalloyed offshore platform steels, and the effect of weld heat inputs, especially high heat inputs on the microstructure evolution and toughness variation in the CGHAZ of V-N-Ti and Nb-V-Ti microalloyed steels were investigated systematically. Using Gleeble 3800 thermal mechanical simulator, the welding thermal circle was simulated; the simulated heat affected zone continuous cooling transformation diagram(SHCCT) and the microstructure evolution of two experimental steels were conducted and comparatively analysed; the mechanical properties, combined with microstructure evolution, were tested out and analysed; using TEM, Thermo-Calc software and prior austenite grain degree test, the pin effect of precipitates on the prior austenite grain had been investigated; the quantitative statistics on high angle grain boundary and effective grain size had been work out, and with the result of cleavage fracture and secondary crack observation, the effect of precipitates on the formation of ferrite and grain refinement had been analysed; through quantitative study on MA constituent and pearlite colony and critical cleavage crack size calculation, the effect of heat input on cleavage crack initiation of CGHAZ had been studied.The results of this work indicate: in the condition of different heat inputs, the CGHAZ of V-N-Ti microalloyed steel has better toughness than that of Nb-V-Ti steel, and when t8/5 is 30s(heat input of 103 k J/cm), the impact toughness at-40? reaches peak at 56.67 J. And this is because the CGHAZ of V-N-Ti microalloyed steel have highest crack initiation and propagation energy. Under different heat input, the crack propagation energy is influenced by the combined effect of MA contituent and pearlite conloy, and the size of fracture cleavage facet reflects the variation of crack initiation energyAs for the CGHAZ of V-N-Ti steel, from the aspect of microstructure evolution, the CGHAZ has finer microstructure of lath bainite, granular bainite, polygonal ferrite, and small amounts of pearlite; In the view of precipitate particles, fine and evenly distributed precipitate particles have the pin effect on the prior austenite grain boundary, and also promote the polygonal ferrite formation by providing ferrite formation nucleus, so the finer ferrite furtherly refine the grain size; From the aspect of fracture mechanism, the MA constituent and pearlite colony is the nucleus of cleavage fracture microcrack in the CGHAZ, and their combined area fraction in t8/5=30s is the lowest compared with area fraction in other t8/5s, which means it needs more crack initiation energy to consume during the fracture process, and the effective grain size is finer when t8/5 is 30 s, which brings a higher cleavage fracture propagation energy, therefore, when t8/5 is 30 s, the fracture in CGHAZ of V-N-Ti steel needs more energy to absorb.With the extension of t8/5,the microstructure evolution in CGHAZ of Nb-V-Ti steel turned from martensite, lath bainite, granular bainte into lath bainite and granular bainite; the prior austenite grain size is coarsening, and MA constituents are often distributed alongside the prior austenite grain boundary; also worth metioned is the oversized effective grain size. Therefore, the impact toughness in the CGHAZ of Nb-V-Ti microalloyed steel is deteriorated. |
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