| With the vigorous development of liquefied natural gas(LNG)industry and the uneven distribution of natural gas resources,the storage and transportation of liquefied natural gas(LNG)has become one of the key problems to be solved.High manganese steel has both high strength and high toughness at low temperature,and its production cost is low.It has broad application prospects in LNG storage and operation industry.Two kinds of Fe-Mn-Al-Si-C high manganese steels have been designed in this paper.By adding microalloying elements,combining rolling process and aging treatment process,the yield strength of laboratory steel has been greatly improved.In addition,the welding performance of microalloyed high manganese steel was studied by combining the welding thermal simulation with the actual welding process.The main research results are as follows:(1)Taking into account the performance index of each alloy system of high manganese steel and the role of alloy elements in the steel,the Fe-Mn-Al-Si-C high manganese steel was selected as the research object,and the low carbon design of the experimental steel was carried out.The composition of the experimental steel was Fe-26Mn-1 Al-0.5Si-0.2C and Fe-26Mn-lAl-0.5Si-0.06Ti-0.02V,respectively.(2)The effects of finishing rolling temperature on the microstructural analysis and mechanical properties of two kinds of experimental steels were studied.The results show that the microstructures and mechanical properties of the experimental steels with finishing rolling temperature of 1030? and 930? are significantly different.When the finishing rolling temperature is 930?,the average grain size of the experimental steel decreases(the two steels decrease by 10 and 11?m,respectively),and the yield strength increases(the two steels increase by 32MPa and 53MPa,respectively).The fine grain strengthening and precipitation strengthening of microalloying elements V and Ti have greatly improved the mechanical properties of the experimental steel.The yield strength of the microalloyed experimental steel rolled at low temperature is 33 7MPa.(3)The effect of aging treatment on the structure and properties of microalloyed experimental steel was studied.The results showed that a large number of nano-sized dispersed carbides were precipitated during aging treatment,which further enhanced the yield strength of the experimental steel.Compared with hot rolled sheet,the low temperature toughness of the experimental steel after aging for 5 hours has not changed much,but the yield strength has been greatly improved,reaching about 390MPa.(4)The microstructures and properties of HAZ were analyzed by welding thermal simulation experiments.The results show that with the increase of peak temperature,the average grain size of experimental steel increases,and the carbides of V and Ti dissolve,grow and precipitate in varying degrees.The combined effect of grain size and carbide makes the hardness of HAZ of experimental steel present a " W" shape change trend,while the change trend of impact energy at low temperature is opposite.During the welding thermal cycle,HAZ of experimental steel softens.Therefore,the heat input should be reduced to prevent or reduce HAZ softening.(5)Tungsten Argon Arc Welding(TIG)was used to conduct experimental study on the welding of experimental steel.The results show that the weld structure is mainly composed of coarse columnar grains and a few fine equiaxed grains.The hardness of the weld zone is higher than that of the base metal,but the impact toughness at low temperature is lower than that of the base metal.The impact energy at-196? is only about 70J.The obvious coarsening of HAZ grains leads to poor tensile properties of welded joints,and their yield strength is only 296MPa. |
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